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ARMORTHANE PROTECTING YOUR WORLD

PROTECTING YOUR WORLD

ARMORTHANE PROTECTING YOUR WORLD

Planter Box
Liners:
What Works & Why

What to line a planter box with, based on what it’s made of and what you grow, plus drainage done right and sourced food-safety answers from the people who make polyurea.
Planter Lining Quotes

Built-in or commercial planters? Get a quote from a certified applicator.

Tell us what the planters are made of, how many there are and roughly how big. We'll connect you with an ArmorThane-certified applicator who will quote a seamless sprayed lining, thickness and product in writing.

  • Built-in concrete, block and brick planters
  • Rooftop, plaza and courtyard planters
  • NSF 61 lining for food crops and potable contact
  • Contractors: ask about spray rigs and training
Rather talk now? Call (417) 831-5090
Step 1 of 3
1 in.
How far copper moved from treated boards
Oregon State Extension, raised beds
30%
Wood moisture where serious rot begins
USDA Forest Products Laboratory
$0.92
Per sq ft for 45-mil pond-grade EPDM
PondGard 5 x 25 ft, Sept 2026
0 in.
Gravel to put in the bottom
WSU: it holds water instead of draining it

A planter box liner has one job: keep wet soil off the box without drowning the roots. Get that balance wrong in one direction and a pine box rots in a few seasons. Get it wrong in the other and you've built a bathtub.

Quick answer

Line wooden boxes on the walls and floor, then let the water out. Use 30–60 mil HDPE sheet or 45-mil pond-grade EPDM for a liner that lasts, or heavy 6-mil poly on a budget. Staple it about an inch below the rim, cut drain holes that line up with the box's own, and fill with one soil mix. No gravel layer. On a patio, use landscape fabric instead of plastic so water can pass. Molded inserts are easiest for window boxes. Built-in concrete and commercial planters need a real waterproofing membrane. Jump to the liner picker or the size calculator.

Most of what ranks for this topic is a product page or a forum thread, and the advice contradicts itself. Plastic "leaches chemicals." Plastic is "food safe." Gravel "helps drainage." Pressure-treated wood "poisons vegetables." Some of that is myth, some is outdated, and some depends on what your planter is made of.

We make spray-applied polyurea linings in Springfield, Missouri, and we've spent 35 years around things that hold water and dirt. So we'll say it early: for a cedar box on the porch, you don't need us. A sheet of HDPE and a staple gun will do. Where a sprayed lining earns its money is built-in concrete, rooftop and commercial planters, and we'll show you where that line falls.

What's in this guide
  1. 1Do planter boxes need a liner?
  2. 2Planter box liner materials compared
  3. 3Which liner should you use? (picker)
  4. 4How to line a planter box, step by step
  5. 5What to put in the bottom of a planter box
  6. 6Liner size and cost calculator
  7. 7Is a planter box liner food safe?
  8. 8How to line a raised garden bed
  9. 9How much soil to fill it (calculator)
  10. 10Concrete and commercial planters (spray estimator)
  11. 11Planter liner mistakes
  12. 12Hire an applicator, or become one
  13. 13Planter box liner FAQ
Start here

Do planter boxes need a liner?

Wooden planter boxes made of pine, spruce or fir do. Cedar, redwood, galvanized steel, fiberglass and plastic mostly don't, though a liner can still add years or ease a food-safety worry. Built-in concrete and masonry planters need waterproofing rather than a loose liner. The deciding question is what happens when that material sits against wet soil for years.

PlanterLiner?WhyWhat we'd use
Untreated pine, spruce or firYesPines, spruces and true firs rate slightly or nonresistant to decay; Douglas-fir only moderately (USDA FPL)HDPE or pond-grade EPDM, walls and floor, with drain holes
Cedar or redwoodOptionalHeartwood rates resistant, but less so in ground contact and humid climates; sapwood rots like pineWalls only, or walls plus a perforated floor
New pressure-treated (ground contact)OptionalCopper rose only within 1 in. of the boards, not in the vegetables (Oregon State)Line the walls if you'd rather have the barrier
Old lumber, pre-2004 treated, railroad tiesLine it or skip ediblesCCA and creosote concerns (EPA, MU Extension)Heavy plastic between wood and soil, per MU Extension
Galvanized steelOptionalZinc leaches only in very acidic soil, pH below 5 (UMN)Walls lined if the maker is unknown and you grow food
Weathering or painted steelUsuallyWet soil keeps bare steel dampSheet liner or sprayed coating, plus drain holes
Aluminum, fiberglass, plasticNoAlready waterproofDrain holes; patch or insert if cracked
Built-in concrete, block, brickWaterproof itMasonry passes water and shows it as leaks and efflorescenceSealed sheet membrane or sprayed polyurea
Window boxInsertEasiest to replant, easy to swapMolded polypropylene liner, $9–$24
Sources: USDA Forest Products Laboratory Wood Handbook ch. 14; Oregon State, MU and UMN Extension; EPA; Root & Vessel and Windowbox.com prices (Sept 2026).

Wood: it's about keeping it below fiber saturation

Wood doesn't rot because it's outdoors. It rots when it stays wet. The USDA Forest Products Laboratory puts it plainly: serious decay happens only when wood moisture is above the fiber saturation point, about 30%, and air-dried wood at 20% or less "should provide a reasonable margin of safety." Wood pressed against damp potting soil never gets to dry out. That's the whole case for a liner.

Species matters. The same handbook rates cedar heartwood (western redcedar, eastern redcedar and others) and old-growth redwood as resistant, Douglas-fir as moderately resistant, and most pines, spruces and true firs as slightly or nonresistant. It also says untreated sapwood of "essentially all species" has low resistance, and resistance drops in ground contact and in warm, humid climates. A cedar box with pale sapwood streaks is part cedar, part pine.

Treated wood is a food question, not a rot question

Modern ground-contact lumber won't rot fast either way. People line it because of what might move into the soil. We cover the research in the food-safety section; the short version is that current copper-based treatments tested fine in an Oregon State study, and lumber from before 2004 is a different story.

Metal, fiberglass and plastic

These don't rot, so a liner is about soil chemistry, heat or looks. University of Minnesota Extension says zinc from galvanized beds can leach only in highly acidic soils (pH below 5), and warns that some prefab metal beds sold online have been found to contain lead. Fiberglass and plastic just need enough holes.

Concrete and masonry

A concrete or block planter is a sponge with a hard shell. Water moves through it and carries salts to the outside face, which is where white efflorescence comes from, and in a built-in planter it can end up in the wall or slab next door. These need a continuous membrane. More in concrete and commercial planters.

Compare

Planter box liner materials compared

For a wooden box, the best planter box liner is a heavy polyethylene sheet: 30–60 mil HDPE or 45-mil pond-grade EPDM if you want it to outlast the box, or 6-mil poly if you're on a budget. Landscape fabric is for holding soil where water must pass. Molded inserts win in window boxes. Sprayed polyurea is for concrete and commercial work.

MaterialWaterproof?Drains through?Food-garden notesPublished priceBest for
6-mil polyethylene sheetingYesNo; punch holesPolyethylene; not certified food-grade unless labeledNot verified; cheapest per sq ftBudget wood boxes; double it
HDPE sheet, 30–60 milYesNo; cut holesPolyethylene; no food claims on the pages we checked$1.30–$2.30/sq ft in 80- to 100-ft rolls; smallest roll $242.12Long-life wood boxes, lots of planters
EPDM pond liner, 45 mil (pond grade)YesNo; cut holesSold as fish-safe$0.92–$1.20/sq ft; 5 x 5 ft for $23One or two boxes, odd shapes
EPDM roofing membraneYesNoA pond supplier says most contains algaecides or fire retardantsn/aSkip it for edibles
PVC or vinyl sheetYesNoPhthalate plasticizers are used in PVC (CDC)n/aWe'd avoid it for food crops
Landscape fabricNoYesWoven or spun syntheticNot verifiedOpen-bottom beds on patios; over drain holes
Molded polypropylene insertUntil you open the holesPre-scored holesPolypropylene$9–$18 (Root & Vessel); $12.97–$23.97 (Windowbox.com)Window boxes
Coco coir linerNoYesCoconut fiber and natural latex (Lowe's listing)Not verifiedWire baskets and troughs
Cardboard or newspaperNoYesBreaks down in the bedFreeSmothering weeds under open-bottom beds
Hardware cloth, ½–¾ in.NoYesGalvanized or stainless (UC IPM)Not verifiedGophers and voles, under the soil
Brush-on oil or sealerSlows watern/aCheck the label for food or plant usen/aOutside faces; not a soil-contact barrier
Liquid rubber (brush or can)If thick enoughNoDepends on the product's approvalsn/aSmall repairs, odd shapes
Spray polyurea (applicator)Yes, seamlessNo; detailed drainsAquaShield is ANSI/NSF 61 approved for potable waterQuoted per jobBuilt-in concrete, steel, rooftop, commercial runs
Prices: Pro Fabric Supply HDPE rolls, pondliner.com PondGard 45-mil EPDM, Root & Vessel and Windowbox.com, all checked Sept 2026. Per-square-foot figures are our math. "Not verified" means we couldn't confirm a current price for this page.

Plastic sheeting (poly)

The cheapest waterproof option and the one most people mean by "line it with plastic." It works. MU Extension's advice for questionable lumber is literally "a heavy plastic liner" between wood and soil, with a warning not to tear it. Thin sheet tears on staples and trowels, so double it. Keep it out of the sun, since UV is what breaks polyethylene down; that's why HDPE makers sell UV-stabilized grades.

HDPE sheet: the long-life version

Thick, stiff, puncture-resistant polyethylene sold in 30- to 100-mil rolls. One maker estimates a 100-year life "when not exposed to direct sunlight," which is a claim, not a test we've seen, but it tells you sunlight is the enemy. Price is fine per square foot ($1.30–$2.30) and awkward per purchase: the smallest roll we found is $242.12. Great if you're lining a dozen boxes. Heavy for one.

EPDM pond liner

Soft, stretchy rubber that folds into corners without fighting you, and it's sold by the piece: a 5 x 5 ft sheet of 45-mil PondGard was $23 in September 2026. Buy pond-grade, sold as fish-safe. A pond supplier notes that most EPDM roofing membranes contain algaecides or fire retardants, which is fine on a roof and not what you want next to carrots. If you're building a pond too, our koi pond liner guide covers EPDM in depth.

Landscape fabric

Not a waterproof liner at all. It holds soil in and lets water out, which is exactly what UMN Extension recommends under a raised bed that sits on a patio. It won't keep a wood box dry, but a square of it over each drain hole in a waterproof liner stops soil from washing out.

Liquid rubber and brush-on coatings

Paint-on and spray-can rubber coatings can seal a small planter or a crack, if you build enough thickness and let them cure fully before soil goes in. Check the label for plant or potable-water use. Our spray-on rubber coating guide compares the can products with professionally sprayed polyurea.

Interactive tool 1 of 4

Which planter box liner should you use?

Tell the picker what the planter is made of, where it sits and what you're growing. It returns a liner, a drainage plan and the cautions that apply, built from extension research and manufacturer data, not guesses.

Liner picker

What should I line my planter box with?

Pick the closest match. Results update as you change inputs.

Growing
Doing it
Line it: sides and floorUntreated pine, spruce or fir
Liner

What to use

  • Best: 30–60 mil HDPE or 45-mil pond-grade EPDM, one piece, stapled 1 in. below the rim.
  • Budget: 6-mil poly sheeting, doubled.
  • Why: the USDA Forest Products Lab rates most pines, spruces and true firs slightly or nonresistant to decay (Douglas-fir moderately), and untreated sapwood of nearly every species has low resistance.
Drainage

Let it drain

  • Drain holes in the liner, lined up with the box’s own holes. About ½ in. on a 6 in. grid is our rule of thumb.
  • No gravel layer. WSU’s Linda Chalker-Scott shows it raises the saturated zone instead of draining it. Same soil top to bottom.
  • On a wood deck: raise it on feet or spacers. Planters parked in one spot trap moisture against the boards.
Food crops

Materials for edibles

  • Stick to polyethylene (poly, HDPE) or polypropylene. FDA lists those olefin polymers for food-contact articles (21 CFR 177.1520), though a garden sheet isn’t certified food-grade unless the maker says so.
  • Skip PVC. Phthalates are used in PVC plastics such as garden hoses (CDC).
  • EPDM: buy pond-grade, sold as fish-safe. A pond supplier notes most roofing EPDM contains algaecides or fire retardants.
Watch for

Before you build

  • The liner protects the inside only. Oil, stain or paint the outside faces, and keep the bottom off wet ground and decking.
  • Stop the liner’s top edge about an inch below the rim, where the lip shades it, and fill soil to just below that edge.

How it decides: decay ratings from the USDA Forest Products Lab Wood Handbook; treated-wood guidance from MU, Oregon State and UMN Extension and EPA; patio and metal-bed guidance from UMN; drainage from WSU; gopher mesh from UC IPM; plastics from FDA 21 CFR 177.1520 and CDC; EPDM grades from a pond supplier. The 6-in. drain-hole grid is our rule of thumb.

Step by step

How to line a planter box, step by step

Cut one sheet big enough to cover the floor and run up every wall, fold the corners instead of cutting them, and staple the top edge about an inch below the rim. Then cut drain holes that line up with the box's own, cover them with a scrap of fabric, and fill with one soil mix. It takes about as long as a trip to the hardware store.

1 2 3 4 5 6 7 BOX WALLDECK
  1. Liner in one piece, folded at the corners, tight to the walls.
  2. Top edge stapled about 1 in. below the rim, out of sight and sun.
  3. Headspace: soil stops about an inch below the liner edge.
  4. Drain holes in the liner, lined up with the box's holes or floor gaps.
  5. Fabric or screen over each hole to keep soil in.
  6. Feet or spacers so water leaves and the deck dries.
  7. One soil mix, top to bottom. No gravel layer.

  1. Check the box first

    The box needs drainage: holes in the floor or gaps between floor boards. If it has neither, drill ½ in. holes before you line it. Brush out old soil and let the wood dry.

  2. Measure the inside

    Inside length, width and depth. The size calculator turns those into a cut size. For a one-piece liner the sheet is the length plus twice the wall height, by the width plus twice the wall height.

  3. Cut one sheet

    One piece beats five panels: no seams to leak or split. Lay it in, press it into the floor, and push it into the corners.

  4. Fold the corners

    Pinch each corner into a triangle fold and lay it flat against a wall, like wrapping a present. Don't cut the corners out; that's where cut liners fail.

  5. Staple the top edge

    Fold the top edge over once for strength and staple it about 1 in. below the rim every 4 in. or so, with stainless or galvanized staples or plastic cap nails. Below the rim, the lip and the plants shade it from UV, and the edge still sits above the soil line.

  6. Cut drain holes

    Press down where the box's holes are and cut through the liner at each one. Where the floor is solid, ½ in. holes on a 6 in. grid is our rule of thumb. A liner without holes is a bathtub.

  7. Cover the holes

    A square of landscape fabric or window screen over each hole keeps soil from washing out. Not gravel; the next section explains why.

  8. Fill and water

    Fill with one mix to about an inch below the liner edge, water it in, and top up after it settles. Set the box on feet if it sits on a deck or patio.

Window boxes

Skip sheet liners and buy a molded insert. Polypropylene window-box liners run $9–$18 from Root & Vessel and $12.97–$23.97 from Windowbox.com for 18–36 in. lengths, with drill-out or pre-scored drain holes. Windowbox.com notes they stay watertight indoors if you leave the holes closed. Measure the inside of the box, because inserts come in fixed lengths. Replanting becomes lifting one liner out and dropping another in.

Metal planters

Galvanized stock-tank and corrugated planters are usually fine unlined; see UMN's zinc and lead notes above. Bare or painted steel stays damp behind wet soil, so a sheet liner or sprayed coating on the inside helps, as long as the drain holes stay open. Sun-baked metal gets hot; a liner doesn't change that much, a light color and some shade do.

Planters on wood decks

The liner protects the box. The deck needs its own protection. As our wood deck coatings page puts it, pots and planters that sit in one spot trap moisture for long periods. Put the box on feet or spacers so air moves underneath, and move it now and then.

Drainage

What to put in the bottom of a planter box

Put drain holes in the bottom and nothing else. No gravel, rocks, pot shards or packing peanuts. A coarse layer under fine soil doesn't drain the soil; it holds a saturated zone above it until the soil is soaking wet. Use the same soil mix top to bottom, fabric or screen over the holes, and feet under the box.

Why gravel makes drainage worse

Washington State University horticulturist Linda Chalker-Scott explains it in "The Myth of Drainage Material in Container Plantings": water "will not move from a finely soil texture into a coarser material until the finer soil is saturated." So gravel "will induce the very state it is intended to prevent." Her advice is the same soil throughout, and drainage holes. The water table that forms at the bottom of a container just moves up to sit on top of the gravel, closer to the roots.

The liner version of the same mistakeA plastic liner with no holes, sitting on a solid box floor, creates the same saturated layer, only permanent. If you're lining a box that has no holes of its own, drill the box first, then cut the liner to match.

Deep planters: raise the floor, don't fill it with junk

A 30-in.-tall planter holding shallow-rooted flowers doesn't need 30 in. of soil. The common fix is filling the bottom with bottles or rubble, which puts a coarse layer under fine soil again. A cleaner fix is a false floor: a slatted shelf on cleats partway down, with its own drain path, and the liner on top of it. The soil stays one continuous column.

Holes, fabric and feet

  • Holes: at every low point, lined up with the box's holes. We use ½ in. on a roughly 6 in. grid across a solid floor.
  • Fabric or screen: a square over each hole keeps soil in and lets water out.
  • Feet: an inch of air under the box so water can leave and the surface underneath dries.
  • Trays: only where runoff would stain or damage something, and empty them.
  • Indoors: keep the liner intact, grow in nursery pots set inside it, and empty standing water.
Interactive tool 2 of 4

Planter box liner size and cost calculator

A one-piece liner for a box is the inside length plus twice the wall height, by the inside width plus twice the wall height. A 36 x 12 x 12 in. box takes a 60 x 36 in. sheet, 15 sq ft, which is about $20–$25 of 30-mil HDPE or $14–$18 of pond-grade EPDM at September 2026 prices.

Size & cost calculator

How big a liner do I need, and what does it cost?

Inside dimensions in inches. The sheet runs up each wall to your stop line, plus a fold for stapling.

Planter shape
0 = use published prices
60 in. x 36 in.
Cut size, one piece
15.0 sq ft
Per planter
15.0 sq ft
Total to buy
$20–$25
At published prices

One sheet 60 in. x 36 in., corners folded, not cut. Liner height on the wall: 12 in. (12 in. deep, stopping 1 in. below the rim, plus 1 in. to fold and staple). Cut-and-tape panels instead would take 11.0 sq ft per planter, but every seam is a place to leak or tear. $1.30–$1.69 per sq ft in 48- and 24-in. x 100-ft rolls (Pro Fabric Supply, Sept 2026). The smallest roll, 12 in. x 100 ft, is $242.12.

Cut & install
  • Drain holes: about 12 per planter at ½ in., on a 6 in. grid, lined up with the box’s own holes or gaps (our rule of thumb).
  • Fasteners: about 24 stainless or galvanized staples (or plastic cap nails) at 4 in. spacing along the top edge.

How we calculated this: liner wall height h = depth − stop below rim + fold. Box: (L + 2h) x (W + 2h). Round: a square (D + 2h) on a side. Sides only: 2(L + W) + 6 in. overlap, by h + a 2 in. buried toe. Prices are published September 2026 roll and piece prices converted to square feet; roll minimums apply.

Common planter sizes

Planter (inside)One-piece cut sizeSq ft30-mil HDPE45-mil EPDM
Window box, 30 x 8 x 8 in.46 x 24 in.7.7$10–$13$7–$9
Planter box, 36 x 12 x 12 in.60 x 36 in.15.0$20–$25$14–$18
Deep planter, 48 x 24 x 18 in.84 x 60 in.35.0$46–$59$32–$42
Round, 24 in. across x 18 in. deep60 x 60 in.25.0$33–$42$23–$30
Raised bed 4 x 8 ft, sides only, 12 in.24 ft 6 in. x 14 in. strip28.6$37–$48$26–$34
Our calculation, 1 in. stop below the rim and 1 in. fold. Material only, before roll minimums: HDPE's smallest roll we found is $242.12, while EPDM sells by the piece from $23 (5 x 5 ft). A window box is usually better served by a $9–$24 molded insert.
Vegetables and herbs

Is a planter box liner food safe?

Polyethylene (poly, HDPE) and polypropylene are the safest common choices; FDA lists both among the olefin polymers allowed in food-contact articles. Avoid PVC and roofing-grade EPDM around food crops. For a sprayed lining, ask for ANSI/NSF 61 approval. And the wood you're lining against is usually less of a threat than the internet suggests.

Pressure-treated wood: what the research says

Arsenic-based CCA is the reason treated wood got its reputation. EPA says manufacturers voluntarily stopped making CCA-treated wood for homeowner uses in December 2003. Today's residential lumber uses copper-based preservatives such as ACQ and copper azole.

  • Oregon State Extension (rev. April 2026): beds built from treated lumber raised soil copper "only within 1 inch of the bed edge," by about 20–68 ppm, inside the natural range for Willamette Valley soils, with "no increase in copper concentration" in the vegetables and herbs grown in them.
  • MU Extension: ACQ has no arsenic or chromium but contains copper that can leach; a 2007 study found copper exposure from ACQ-treated wood isn't expected to harm adults or children. Leftover CCA stock "probably should not be used in raised beds for growing food crops," and creosote ties that still ooze can injure plants.
  • UMN Extension: use lumber rated for ground contact, avoid old lumber, and consider lining the edges to reduce copper leaching into the soil.
  • EPA, for existing CCA structures: a penetrating oil- or water-based stain applied regularly "may reduce the leaching of chemicals." Never burn treated wood.

Our read: with new ground-contact lumber, a liner is optional insurance. With old or unknown lumber, line it or grow flowers in it. MU's instruction is simple: "place a heavy plastic liner between the treated lumber and the soil."

Which plastics belong next to food crops

  • Polyethylene and polypropylene: FDA's 21 CFR 177.1520 says the listed olefin polymers "may be safely used as articles or components of articles intended for use in contact with food," under that section's conditions. A roll of garden sheet isn't certified food-grade unless the maker says so, but the base resin is the right family.
  • PVC and vinyl: CDC notes phthalates are used in PVC plastics such as garden hoses. We'd leave PVC for ornamental plantings.
  • EPDM: pond-grade sheet sold as fish-safe, not roofing membrane. A pond supplier notes that most roofing EPDM contains algaecides or fire retardants.
  • Fabric grow bags and landscape fabric: UMN says research is limited on whether plastic fabric bags shed microplastics into soil. Unknown, not proven harmful.

What NSF 61 means, and doesn't

ANSI/NSF 61 is the health-effects standard for products that touch drinking water. As our aquatic coatings guide explains, cured samples are soaked and the water is analyzed for contaminants against drinking-water limits. Our AquaShield polyurea is ANSI/NSF 61 approved for direct contact with potable water, and its page says it meets USDA criteria.

Two honest caveats. NSF 61 is a drinking-water standard, not a "garden soil" certification; it's simply the strictest leaching test widely available for a coating. And it applies to the specific product, so "polyurea" in general isn't NSF 61. Ask for the approval on the product being sprayed.

Our positionFor a food garden in a wooden box: HDPE or pond-grade EPDM, top edge above the soil line, drain holes open. Skip PVC, roofing rubber, and anything that smells like solvent. If it's a sprayed lining, NSF 61 paperwork for that exact product.
Raised beds

How to line a raised garden bed

Line the sides of a raised bed, not the bottom. An open bottom lets roots go deeper and water drain into the ground. Add hardware cloth underneath if gophers or voles are a problem, landscape fabric if the bed sits on a patio, and cardboard if you're smothering grass. A plastic floor under a bed on soil only traps water.

On soil

Sides only, open bottom

Run a strip of HDPE or EPDM around the inside walls, stapled near the top, with a couple of inches tucked into the ground at the bottom. Leave the floor open. MU Extension says most plants need at least a 6–12 in. rooting zone, "but deeper would be better."

On a patio

Fabric, not plastic, on the floor

UMN Extension: when a bed sits on a patio, a bottom liner "may also help keep soil in the bed," and you should choose one "that allows water to pass through, such as landscape fabric, rather than nonpermeable plastic."

Gophers, voles

Hardware cloth underneath

UC IPM: "underlay the soil with mesh wire," ½- to ¾-in. mesh, galvanized or stainless for longer life. UMN recommends hardware cloth at the bottom against mice and chipmunks too. Lap it up the sides a few inches.

Weeds, sod

Cardboard first

Plain cardboard over grass or weeds smothers them and breaks down into the soil. It's a one-season barrier, which is all a new bed usually needs.

Metal beds

Usually fine unlined

UMN: zinc leaching needs soil below pH 5, and most garden soils are neutral to alkaline. The bigger warning is that some prefab beds sold online have been found to contain lead. Unknown maker? Line the walls or buy elsewhere.

Treated lumber

Line the edges if you like

Copper from ground-contact lumber stayed within an inch of the boards in Oregon State's testing. UMN suggests lining the edges to reduce leaching. It's optional with new lumber and prudent with old.

Sizing the bed itself

MU Extension suggests 4 ft wide so you can reach the middle from either side, or 3 ft if you can only reach from one side, and notes that beds taller than 18–24 in. need retaining walls with foundations. UMN puts a comfortable height for wheelchair users at about 27 in. Lining doesn't change any of that, but a taller bed holds more wet soil against its walls, which makes the liner more worthwhile.

Lined vs. unlined wood: how long it lasts

We couldn't find a controlled study comparing lined and unlined bed life, and the figures online contradict each other (plastic "degrades in 2 years" on one site, "5–10 years" on another). What's solid is the mechanism: keep the wood below about 20% moisture and decay can't take hold (USDA FPL). A liner that keeps wet soil off the wood, with its top edge shaded, does exactly that. VegTrug, for what it's worth, suggests replacing the liner in its raised planters every two years, when you refresh the soil.

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How much soil to fill a planter box

Multiply inside length by width by soil depth in inches and divide by 1,728 for cubic feet. A 36 x 12 in. box filled 11 in. deep holds about 2.75 cu ft, roughly two 2-cu-ft bags once you allow for settling. For raised beds, UMN Extension suggests about half to two-thirds topsoil and the rest compost.

Soil fill calculator

How many bags, or how many yards?

Inside dimensions in inches. Leave headspace at the top so water doesn't spill over.

3.0 cu ft
Soil to buy
0.11 cu yd
In cubic yards
2 / 3
Bags: 2 cu ft / 1.5 cu ft
78 qt
For quart-sized bags

Soil depth 11 in. (12 in. box, 1 in. left at the top) x 3.00 sq ft of floor = 2.75 cu ft per planter, plus 10% for settling. Containers and planter boxes: a bagged potting mix, the same mix top to bottom, with no gravel layer (WSU). Bags make sense at this size. Bag counts round up; quarts are US dry quarts (25.7 per cu ft).

How we calculated this: volume = floor area x (depth − headspace) ÷ 1,728, times the number of planters, plus the settling allowance (our assumption, not a published figure). 1 cu yd = 27 cu ft; 1 cu ft = 25.7 US dry quarts. Mix ratios from UMN Extension.

One thing the calculator can't do is weigh the result, and we'd rather not guess. Wet soil is heavy and varies a lot by mix. If the planter sits on a balcony, a rooftop or an older deck, get the load checked before you fill anything big. Our rooftop garden page covers the waterproofing side of that job.

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Waterproofing concrete and commercial planters

Built-in concrete, block and brick planters need a continuous membrane on the inside, detailed around every drain. A loose EPDM or HDPE sheet with taped seams works in simple shapes. A spray-applied polyurea membrane bonds to the concrete with no seams, which makes it a strong fit for plazas, rooftops, fountain surrounds and long runs of identical planters.

Where a sprayed liner makes sense

  • Built-in masonry next to a building, a plaza deck or a parking structure, where a leak damages something expensive. Our plaza deck waterproofing page covers the deck side.
  • Large or odd-shaped planters where a sheet would need seams, folds and drain boots.
  • Steel planters. AquaShield's page says it coats carbon or mild steel without a primer.
  • Planters tied to water features, where one applicator can line the pond, fountain and planters in one visit. See polyurea water features and landscape coatings.
  • Runs of commercial planters, where setup cost spreads across many units.

Where it doesn't

One cedar box. Two galvanized troughs. A window box. A spray crew brings a plural-component rig, heated hoses and a job minimum; a $23 piece of pond liner does the same job there. We'd tell you to buy the pond liner.

Polyurea-lined backyard water feature with boulders, bordered by curved brick raised planters
A polyurea-lined water feature running between brick planters: the kind of landscape job where planters get lined in the same visit.
Spray lining estimator

How much sprayed membrane do my planters take?

Inside dimensions in feet. For landscape architects, property managers and applicators planning a bid.

Food crops or potable contact?
90 sq ft
Coated area per planter
540 sq ft
Total coated area
24.2 gal
Material, all planters
4.0 gal
Per planter

Per planter: floor 24 + walls 55 + top of wall 11 = 90 sq ft. Gallons = 540 sq ft x 60 mils ÷ 1,604 x 1.20 for spray loss and profile. Material only: prep, primer, detailing at drains and the applicator’s minimum aren’t in this number.

Product fit

What to ask the applicator for

  • Ornamentals: ArmorFlex UV is aliphatic and color-stable, and lists planters among its typical uses. AquaShield also works where you want the NSF 61 paperwork.
  • Aromatic polyurea (AquaShield is aromatic) can shift shade in sun. Where the lining shows above the soil, ask about an aliphatic, color-stable finish.
Sheet vs spray

Spray starts to make sense

  • A one-piece sheet liner for the same planter takes about 104 sq ft with folded corners: $811–$1,435 of HDPE for all 6 at Sept 2026 roll prices.
  • A single sheet fits this planter without a seam. For one or two planters, sheet is usually the cheaper answer.
  • 6 planters: the more there are, the easier it is to spread the applicator’s setup cost and minimum across one visit.
Thickness

Why 60 mils

  • 60 mils is the thickness our aquatic guide suggests for light-duty water features. It’s a planning number for planters, not a spec.
  • Confirm film thickness against the product data sheet, and ask for a thickness check on the finished job.

Planning estimate. Get a written quote from a certified ArmorThane applicator.

How we calculated this: area = floor + walls + top of wall (perimeter x cap width); gallons = area x mils ÷ 1,604 x (1 + loss). One US gallon covers 1,604 sq ft at 1 mil. The 20% loss allowance and the 60/80/100-mil planning thicknesses come from our aquatic coatings guide, where they apply to water features; for planters treat them as starting points and confirm with the product data sheet.

Which ArmorThane product, and why

ProductChemistryListed substratesWhat its page statesPlanter use
AquaShieldAromatic spray polyurea, 100% solidsConcrete, metalANSI/NSF 61 approved for potable water; meets USDA criteria; applies down to -20°FFood-crop and potable-contact planters in concrete or steel
ArmorFlex UVAliphatic polyurea, 100% solidsConcrete, metal, fiberglass, wood, geotextileColor-stable; lists "Planters, Tunnels & Underground Vaults" among typical usesVisible, sun-exposed linings and ornamental planters
ArmorLinerSpray polyurea liner systemMetals, concrete, fiberglass, woodLow-temperature flexibilityWood or mixed-material planters
WC-111100% solids polyurethane primer/sealerWood, concreteReacts with moisture; ready for ArmorThane topcoatsPrimer under the lining on wood or concrete
From each product page on armorthane.com, September 2026. Aromatic polyurea can shift shade in sunlight, which doesn't affect performance (our aquatic coatings guide). Your applicator picks the system; ask for the data sheet and the NSF listing where it matters.
Avoid these

Planter box liner mistakes

Almost every failed planter liner fails the same few ways: it holds water, it tears, it's the wrong material next to food, or the sun gets to it. Here's the list we'd check before you fill a single box.

  • No drain holes. A sealed liner on a solid floor is a bathtub. Roots drown before the wood ever rots.
  • Gravel in the bottom. It raises the saturated zone instead of draining it (WSU).
  • Plastic under a bed on a patio. UMN says use a liner that passes water there.
  • Liner edge showing above the rim. UV is what degrades polyethylene. Stop the liner about an inch below the rim, still above the soil line.
  • Cut corners. Five panels mean four seams. Fold one sheet instead.
  • Thin plastic, single layer. Staples and trowels tear it. Go heavier or double it.
  • Roofing EPDM or PVC next to food crops. Pond-grade EPDM or polyethylene instead.
  • Old railroad ties or pre-2004 treated wood for vegetables without a liner, against MU Extension's advice.
  • A plastic floor under a bed on soil. Open the bottom; add hardware cloth if pests are the worry.
  • Box sitting flat on a deck. Feet or spacers, so the deck can dry.
  • Paying a spray crew for one small box. Sheet liner is cheaper. Spray is for concrete, steel and volume.
ArmorThane applicators

Sprayed planter liners: hire an applicator, or become one

If the picker said sheet liner, buy the sheet. If you're dealing with built-in concrete planters, a rooftop or plaza, big steel planters, or a run of commercial units, an ArmorThane-certified applicator can spray a seamless polyurea lining and detail every drain. For food crops or potable contact, ask for AquaShield, which is ANSI/NSF 61 approved for direct contact with potable water.

The same rig and crew that sprays a planter can line a pond or a fountain, so one visit can cover all three. More on that side of the work in landscaping polyurea.

Get a planter lining quote →Landscape coatings
Applicator in protective gear spraying a plural-component coating onto a concrete wall
Spray lining goes on with a plural-component rig, one reason it's applicator work, not a weekend project.
NSF 61

AquaShield

ANSI/NSF 61 approved for direct contact with potable water. Concrete and metal.

Aliphatic

ArmorFlex UV

Color-stable, and lists planters among its typical uses. Bonds to concrete, metal, fiberglass and wood.

1,604

Sq ft per gallon at 1 mil

The math behind every estimate: at 60 mils, a gallon covers about 27 sq ft before losses.

For contractors

Planters are detail work. Bundle them.

A single planter is a small job. Planters next to ponds, fountains, plaza decks and pool surrounds are a day's work for one rig. We make the polyurea, build the spray equipment and train your crew at your location. Not a franchise: no franchise fee, no royalties, and startup packages from $25,000.

Become an applicatorFind an applicator

Questions first? Call (417) 831-5090 in the US or 780-444-6200 in Canada.

Straight answers

Planter box liner FAQ

Do planter boxes need a liner?

Wooden boxes made of pine, spruce or fir do; the USDA Forest Products Laboratory rates pines, spruces and true firs slightly or nonresistant to decay, and serious decay starts when wood stays above about 30% moisture. Cedar, redwood, galvanized steel, fiberglass and plastic usually don't need one, though lining the walls adds years to cedar. Built-in concrete planters need waterproofing, not a loose liner. The liner picker sorts it by material.

What is the best thing to line a planter box with?

For a wooden box, heavy polyethylene: 30–60 mil HDPE sheet or 45-mil pond-grade EPDM if you want it to last, or doubled 6-mil poly on a budget. Staple it about an inch below the rim and cut drain holes that line up with the box's own. In September 2026, pond-grade EPDM ran $0.92–$1.20 per sq ft and HDPE about $1.30–$2.30 per sq ft in rolls.

Should I line my planter box with plastic?

Yes for pine and other non-durable wood, as long as the plastic has drain holes. Plastic without holes turns the box into a bathtub and drowns the roots. MU Extension recommends a heavy plastic liner between questionable treated lumber and the soil, and warns not to tear it. Use polyethylene rather than PVC for food crops.

Can I use landscape fabric to line a planter box?

Yes, if your goal is holding soil in rather than keeping the wood dry. Fabric lets water through, so it won't stop rot the way a waterproof sheet does. It's the right choice under an open-bottom bed on a patio, which is what UMN Extension recommends over nonpermeable plastic, and as a patch over drain holes in a plastic liner.

What do I put in the bottom of a planter box for drainage?

Drain holes, and a square of fabric or screen over each one. Not gravel. WSU horticulturist Linda Chalker-Scott shows that water won't move from fine soil into a coarse layer until the soil above is saturated, so gravel keeps the soil wetter. Use one soil mix top to bottom and set the box on feet.

Do cedar planter boxes need a liner?

Not strictly. The USDA Forest Products Laboratory rates cedar heartwood as resistant to decay, but resistance drops in ground contact and warm, humid climates, and pale sapwood has low resistance. Lining the walls, with an open or perforated floor, is a cheap way to add years to a cedar box.

Is it safe to line a raised bed with plastic for vegetables?

Polyethylene is a reasonable choice; FDA lists polyethylene and polypropylene among the olefin polymers allowed in food-contact articles (21 CFR 177.1520). Line the sides, leave the bottom open so the bed drains into the ground, and avoid PVC, which CDC notes uses phthalate plasticizers. On a patio, use landscape fabric on the bottom instead of plastic.

Can I use pond liner to line a planter?

Yes, and it's one of the easiest materials to work with. Buy pond-grade EPDM sold as fish-safe, not roofing membrane; a pond supplier notes most roofing EPDM contains algaecides or fire retardants. A 5 x 5 ft piece of 45-mil PondGard was $23 in September 2026, enough for a 36 x 12 x 12 in. box.

Do you need to line a galvanized metal planter?

Usually not. UMN Extension says zinc leaches from galvanizing only in highly acidic soils, below pH 5, and most garden soils aren't that acidic. UMN also warns some prefab metal beds sold online have been found to contain lead, so line the walls if the maker is unknown and you grow food. Bare or painted steel benefits more from a liner or coating.

How do you waterproof a concrete planter?

With a continuous membrane on the inside, detailed around every drain. A loose EPDM or HDPE sheet with taped seams works in simple shapes. For built-in, rooftop or commercial planters, a sprayed polyurea membrane bonds to the concrete with no seams; for food crops ask for an ANSI/NSF 61 approved product such as AquaShield. The spray estimator sizes the job.

Is pressure-treated wood safe for a vegetable planter?

Modern ground-contact lumber tested well: Oregon State found copper rose only within 1 inch of the boards and not in the vegetables or herbs. Wood treated with CCA, which EPA says left homeowner use in December 2003, is different; MU Extension says it probably shouldn't hold food crops. With old or unknown lumber, use a heavy plastic liner or grow flowers.

How thick should a planter liner be?

Thick enough not to tear when you staple, plant and dig. We'd double 6-mil poly at minimum, and use 30–60 mil HDPE or 45-mil EPDM for a liner meant to last. Sprayed linings are specified in mils too; our aquatic guide uses 60 mils for light-duty water features, a sensible starting point for planters to confirm against the product data sheet.

Can I spray polyurea or bedliner into a planter myself?

Professional polyurea goes on with plural-component, heated spray equipment at a fixed mix ratio, so it's applicator work, not a roller job. Roll-on and spray-can coatings exist, but check the label for plant or potable-water use and let them cure fully. For one small box, a sheet liner is cheaper; our spray-on rubber coating guide compares the options.

AT
ArmorThane Technical Team · Springfield, Missouri
Written from extension research, manufacturer data sheets and 35+ years of making spray-applied linings. Last updated September 26, 2026.

Methodology and sources

Wood decay data: USDA Forest Products Laboratory. Treated-wood, raised-bed and patio guidance: MU, Oregon State, UMN and UMass Extension and EPA. Drainage: WSU. Plastics: FDA 21 CFR 177.1520 and CDC. Prices are published retail and roll prices checked in September 2026, converted to square feet by us. ArmorThane product claims are quoted from the product pages. Calculators are planning tools, not a substitute for a site visit or product data sheets.

  1. USDA Forest Products Laboratory, Wood Handbook ch. 14, biodeterioration of wood (2021, PDF): fpl.fs.usda.gov/documnts/fplgtr/fplgtr282/chapter_14_fpl_gtr282.pdf
  2. University of Missouri Extension G6985, Raised-Bed Gardening (rev. 2017): extension.missouri.edu/publications/g6985
  3. Oregon State University Extension, pressure-treated wood for raised bed construction (rev. April 2026): extension.oregonstate.edu/gardening/soil-compost/pressure-treated-wood
  4. University of Minnesota Extension, raised bed gardens (reviewed 2026): extension.umn.edu/planting-and-growing-guides/raised-bed-gardens
  5. UMass Extension, raised beds (2012): ag.umass.edu/home-lawn-garden/fact-sheets/raised-beds
  6. U.S. EPA, chromated arsenicals (CCA): epa.gov/ingredients-used-pesticide-products/chromated-arsenicals-cca
  7. Washington State University, The Myth of Drainage Material in Container Plantings (Chalker-Scott, PDF): s3.wp.wsu.edu/uploads/sites/403/2015/03/container-drainage.pdf
  8. UC IPM Pest Notes: Pocket Gophers: ipm.ucanr.edu/PMG/PESTNOTES/pn7433.html
  9. eCFR, 21 CFR 177.1520, olefin polymers: ecfr.gov/current/title-21/chapter-I/subchapter-B/part-177/subpart-B/se
  10. CDC National Biomonitoring Program, phthalates factsheet: cdc.gov/biomonitoring/Phthalates_FactSheet.html
  11. Everything Ponds, EPDM rubber pond liner: everything-ponds.com/pond-liners-top-menu/epdm-rubber-liner.html
  12. pondliner.com, PondGard 45 mil EPDM, 5 ft wide (prices Sept 2026): pondliner.com/elevate-pondgard-epdm-45-mil-pond-liner-5-wide
  13. Pro Fabric Supply, HDPE liner for raised beds (prices Sept 2026): profabricsupply.com/products/liner-for-raised-beds
  14. Pro Fabric Supply, cedar planter box liner material: profabricsupply.com/products/cedar-planter-box-liner-material
  15. Pro Fabric Supply, planter liner material HDPE roll: profabricsupply.com/products/planter-liner-material-hdpe-plastic-roll-
  16. Root & Vessel, Poly-Pro flower box liner (Sept 2026): rootandvessel.com/planters/poly-pro-flower-box
  17. Windowbox.com, heavy duty Poly-Pro window box liners (Sept 2026): windowbox.com/heavy-duty-poly-pro-flower-box-liners/
  18. VegTrug, replacement liner for raised planter: us.vegtrug.com/products/raised-planter-liner
  19. Lowe's, Style Selections coconut fiber planter liner: lowes.com/pd/Style-Selections-16-in-Coco-Liner/5001993167
  20. ArmorThane, AquaShield: armorthane.com/products/armorline-products/high-pressure/aquashield/
  21. ArmorThane, ArmorFlex UV: armorthane.com/products/armorline-products/high-pressure/armorflex-uv/
  22. ArmorThane, ArmorLiner polyurea: armorthane.com/products/armorline-products/high-pressure/armorliner-po
  23. ArmorThane, WC-111 primer: armorthane.com/products/armorprime/wc-111/
  24. ArmorThane, aquatic coatings guide: armorthane.com/aquatic-coatings/
  25. ArmorThane, wood deck coatings: armorthane.com/wood-deck-coatings/

latest news & bLOGS

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Line it once, and let it drain.

For a wooden box, a sheet liner and a staple gun do the job. For built-in concrete, rooftop and commercial planters, an ArmorThane-certified applicator can spray a seamless polyurea lining, including AquaShield, which is ANSI/NSF 61 approved for potable water. Contractors: we make the polyurea, build the rigs and train your crew, with no franchise fee.

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