How to Build Planter Boxes

This guide covers building wood planter boxes for a deck or patio, sized for both ornamental plants and edible gardens. The design uses cedar or redwood (both naturally rot-resistant) for the outer shell, drainage holes or a gravel sump in the base, and optional poly liner or pond liner to extend the wood's life in contact with moist soil. A 24×16×16-inch planter box holds enough soil depth for most vegetables (tomatoes need 12 inches minimum, root crops need 18 inches), weighs about 80 lbs when wet, and can be built in 2–3 hours.

Planter boxes placed on a wood deck need felt or rubber pads underneath to prevent trapped moisture from rotting the deck boards. Large planters on a composite deck should have air gaps maintained at all times. Calculate the weight of the filled planter before placing it on a deck: a 36×12×18-inch box weighs approximately 120 lbs when filled with wet soil. Most residential decks are rated for 40–60 PSF live load, so distribution over at least 3–4 deck boards is important for large planters.

Time: 2–3 hours per planter. Cost: $30–$80 per planter depending on size and lumber species. Difficulty: Beginner. Permit: None required.

What You'll Need

Tools

• Miter saw or circular saw with guide
• Drill/driver
• Countersink drill bit
• Tape measure and speed square
• Clamps
• Sandpaper, 120 grit
• Staple gun (for liner, if used)

Materials — One 24×16×16-inch Planter

• Lumber: Western red cedar 1×6 boards (actual 3/4×5.5 in), or redwood 1×6. Both are naturally rot-resistant and require no pressure treatment. Avoid pressure-treated lumber for edible gardens — the copper compounds leach into soil.
• 4 side panels, each requiring two 1×6 boards ripped or used at full width
• 4 corner posts: 2×2 cedar, 15.25 inches each (sets final planter height)
• 4 base cleats: 2×2 cedar, cut to fit inside the bottom frame
• Bottom boards: 1×6 cedar with 1/2-inch gaps between boards for drainage, cut to box width
• Stainless steel or hot-dipped galvanized 1 5/8-inch screws (2 lbs)
• Waterproof wood glue (exterior rated)
• Optional: 6-mil poly liner or pond liner stapled inside to protect the wood
• Optional: 1 inch of pea gravel for the bottom as a drainage sump

Step-by-Step Instructions

Step 1 · Choose and buy the lumber

Western red cedar and redwood are the standard choices for outdoor planters that contact soil. Both resist rot without treatment and are safe for edible gardens. Douglas fir, pine, and spruce will work but require treatment or a poly liner and will still need replacement within 5–8 years depending on climate. At the lumber yard, select the straightest, clearest boards — avoid boards with knots near the edges, which will split at screw locations. Buy 10–15% extra for waste.

Step 2 · Cut all pieces to length

For a 24×16×16-inch planter: two long panels at 24 inches (front and back), two short panels at 14.5 inches (sides — this accounts for the corner post overlap). Corner posts at 15.25 inches produce a finished height of 16 inches when the base is 3/4 inch below the top of the posts. Bottom boards at 14.5 inches spanning the interior width, with 1/2-inch gaps between boards. Cut all pieces before assembly — laying everything out on the floor before drilling reveals any dimension errors before they become permanent.

Step 3 · Pre-drill and countersink all screw holes

Pre-drilling cedar prevents splitting — cedar's grain structure is prone to splitting at screws without a pilot hole, especially within 1 inch of an end. Use a countersink drill bit that drills both the pilot hole and the countersink in one pass. Countersinking allows the screw head to sit flush or below the wood surface, which prevents moisture from pooling around the screw head and speeds the start of rot. All screw holes in the side panels that go into the corner posts need a 1/8-inch pilot and a shallow countersink.

Step 4 · Assemble the sides to the corner posts

The assembly order matters. Each corner is a 2×2 post with two panels meeting at right angles. Start by screwing one long panel to two corner posts — one screw near the top and one near the bottom, with wood glue on the post face before fastening. Then attach a short panel between the remaining two posts the same way. Then connect the two L-shaped assemblies with the remaining two panels. Check square by measuring the diagonals — adjust before the glue sets. A planter out of square will rack over time and the bottom boards will be hard to fit.

Step 5 · Install the base cleats and bottom boards

The base cleats are 2×2 pieces screwed to the inside bottom of each side panel — they create a ledge that supports the bottom boards. The cleats sit 3/4 inch above the bottom edge of the side panels, so the bottom boards sit 3/4 inch above the ground plane when the planter rests on its bottom edge. (Or the planter sits on optional feet — see Step 7.) Cut the bottom boards to span the interior width and lay them across the base cleats with 1/2-inch gaps between boards. These gaps allow drainage without losing all the soil. Screw each bottom board to the cleats from below with two screws per board.

Step 6 · Drill drainage holes (if using a solid bottom)

If the design calls for a solid plywood bottom (for a liner), drill at least six 3/4-inch drainage holes evenly spaced in the bottom. Without drainage, soil stays waterlogged after each rain and kills most plants. Six 3/4-inch holes in a 24×16-inch bottom is a minimum; eight to ten is better. A solid bottom with a poly liner inside and adequate drainage holes is the best longevity option for the wood: the liner keeps the soil from direct contact with the wood, the drainage holes prevent standing water.

Step 7 · Add feet for air circulation

Planter boxes on a wood or composite deck must have feet that keep the bottom off the deck surface. Without air circulation under the planter, the deck boards beneath stay wet, leading to rot, mold, and deck board failure. Cut four 2×4 blocks, 2 inches tall, and screw them to the bottom corners of the planter. For a finished look, chamfer the visible edges of the foot blocks. Self-adhesive rubber or felt pads under each foot protect the deck surface from scratching.

Step 8 · Install liner (optional but recommended for longevity)

A 6-mil poly liner or pond liner stapled inside the planter box (walls only, not the bottom) doubles the life of the wood by keeping moist soil from direct contact with the cedar side panels. Cut the liner oversized, fold it to fit the inside corners, and staple it to the top edge of the side panels with a staple gun. Trim any liner that extends above the top edge. Leave the bottom open or cut the liner above the drainage holes. Do not line the outside of the planter — the wood needs to dry between waterings.

Step 9 · Sand and apply exterior finish

Sand all exposed surfaces to 120 grit. Apply an exterior oil finish (tung oil, danish oil, or a penetrating exterior wood stain) to the outside faces and the top edge of the planter. Do not finish the inside faces if you're lining it — the liner protects the interior. If no liner, apply finish to the inside as well. Re-apply exterior finish every 1–2 years to preserve the natural cedar color; without finish, cedar weathers to a silver-gray in 12–18 months, which is stable but aesthetically different from fresh cedar.

Step 10 · Fill with soil and plant

Add 1 inch of pea gravel to the bottom before soil for additional drainage. Fill with a lightweight potting mix designed for containers — avoid heavy garden soil, which compacts in a planter and suffocates roots. For vegetables: use a mix designed for raised beds, with added perlite for drainage. Fill to 1 inch below the top edge to prevent soil from washing out during watering. Water thoroughly until water drains freely from the bottom, then check that drainage is working before adding plants.

Soil Depth Requirements by Plant Type

• Herbs, lettuce, shallow annuals: 6–8 inches minimum
• Peppers, eggplant, most flowers: 10–12 inches
• Tomatoes, cucumbers, zucchini: 12–18 inches minimum
• Carrots, parsnips, beets: 18 inches minimum
• Dwarf shrubs, perennials: 18–24 inches

Common Mistakes

• Using pressure-treated lumber for edible gardens. Copper from PT leaches into soil and bioaccumulates in edible crops. Use cedar, redwood, or untreated wood with a liner for edible planters.
• No drainage holes. Waterlogged soil kills most plants within one season. Six minimum 3/4-inch holes per box are non-negotiable.
• No feet on a wood deck. Direct contact between a wet planter and deck boards rots both. Always elevate with feet.
• Heavy garden soil in a container. In-ground soil compacts and becomes anaerobic in a planter. Use lightweight potting or raised-bed mix only.
• No liner for longevity. A cedar planter without a liner in wet climates lasts 5–7 years. With a liner, the same planter lasts 15+ years.
• Planters against the house siding. Water evaporating from the planter condenses on adjacent siding. Maintain a 6-inch gap between any planter and wood or composite siding.

When to Call a Pro

Planter boxes are a beginner-accessible project. No professional help is needed. If the intended location is on a rooftop deck or balcony, consult a structural engineer about the combined weight of multiple large planters against the deck's rated load capacity.

Maintenance

Check drainage holes annually — they clog with soil and roots. Clear with a 3/4-inch drill bit or a chopstick. Inspect the bottom boards for rot annually by probing with a screwdriver. Re-apply exterior oil finish every 1–2 years. Replace bottom boards if the screwdriver penetrates more than 1/4 inch. The corner posts and side panels outlast the bottom boards in all cases — build the bottom for removability when planning the original design.

Related Guides

• How to build a raised garden bed — larger-scale outdoor growing spaces
• How to build a pergola — the structure to hang the planters from
• How to build a garden gate
• All build × deck & patio guides
• All build guides

Cedar vs. Redwood vs. Composite: Full Comparison

The lumber species choice for a planter box determines longevity, cost, appearance, and food-safety. Western red cedar is the most common choice for residential planter boxes in the eastern and central United States: naturally rot-resistant due to thujaplicins, light weight (approximately 23 lbs per cubic foot), easy to cut and fasten, widely available, and reliably food-safe with no treatment chemicals. Expected service life in ground contact or with wet soil: 10–20 years with appropriate finish.

Redwood (specifically construction heart redwood or construction common redwood) matches cedar's rot resistance due to naturally occurring extractives and is dimensionally stable across humidity cycles. Redwood is predominantly available in the western United States; shipping costs make it expensive east of the Rockies. Expected service life similar to cedar: 10–20 years.

Composite lumber (Trex, Fiberon, Azek) offers 25+ year longevity with zero rot risk, color stability, and no chemical leaching concerns. The downsides: cost (typically 2–3× cedar), weight (heavier than wood), and appearance (looks like plastic, which many gardeners find objectionable). Cannot be painted. Suitable for large raised bed installations where longevity is prioritized over cost.

Pine or fir: inexpensive and available everywhere, but rot-prone when continuously moist. A pine planter box in direct contact with wet soil may deteriorate in 2–5 years without a liner or sealant. If using pine, line the interior with EPDM rubber or food-safe polyliner, and allow drainage so the exterior can dry between waterings. Pine is acceptable for short-term use or budget builds where appearance is prioritized over longevity.

Pressure-treated lumber (ACQ, AZOLE, or borate treatment): the current generation of pressure treatments (post-2004 formulations) have eliminated arsenic and creosote. Recent university studies (Oregon State, Cornell) have found no meaningful arsenic migration from modern ACQ or CA pressure-treated lumber into garden soil or vegetables grown adjacent to it. However, uncertainty about long-term leaching, especially for root vegetables in direct contact with treated wood surfaces, leads most organic gardeners and certification bodies to exclude pressure-treated lumber from edible plantings. For non-edible ornamental planters, pressure-treated lumber is fully appropriate and will outlast untreated species significantly.

Soil Volume and Planting Depth Reference

The amount of soil required for a planter box is a function of interior volume. For a 24×16×16-inch interior planter: 24 × 16 × 16 ÷ 1,728 = 3.56 cubic feet. Bagged potting mix is sold in cubic feet or gallons (1 cubic foot = 7.48 gallons). A 2-cubic-foot bag fills approximately 56% of this planter; two 2-cubic-foot bags are required. For larger beds — say a 48×24×12-inch interior — volume is 8 cubic feet, requiring four 2-cubic-foot bags.

Rooting depth requirements by crop category:

• Shallow roots (6–8 inches): lettuce, spinach, arugula, radishes, herbs (basil, cilantro, dill), green onions
• Medium roots (10–12 inches): tomatoes (dwarf/patio varieties), peppers, eggplant, cucumber, kale, Swiss chard, beans, peas
• Deep roots (16–24 inches): full-size tomatoes, squash, melons, broccoli, cabbage, root crops (carrots, beets, parsnips)
• Very deep roots (24+ inches): asparagus (also requires 2+ year establishment), large vining squash, full-size potatoes

Soil selection matters: bagged potting mix is lighter than garden soil and drains well, which is critical in a contained planter. Garden or topsoil in a container compacts under repeated watering, reducing drainage and air porosity. Use a potting mix designed for containers, or a Mel's Mix equivalent (1/3 peat or coir, 1/3 compost, 1/3 coarse perlite or vermiculite) for maximum root health.

Drainage Design for Long-Term Health

Drainage failures are the most common cause of planter box plant death. Symptoms: yellowing leaves, stunted growth, root rot. Causes: drainage holes blocked, soil compaction reducing permeability, the container sitting on an impermeable surface that traps water at the hole level. Solutions:

Drainage hole size and quantity: for a 24×16-inch bottom panel, install four 1-inch holes or six 3/4-inch holes, distributed across the bottom surface rather than clustered in one corner. Cover holes with hardware cloth or landscape fabric from the inside before filling with soil — this prevents soil from washing out while allowing water to drain freely.

Feet or stands: raise the planter bottom at least 1 inch off the surface it sits on. On a concrete patio, water cannot drain through the concrete and will pool at the drainage holes, drowning roots even with holes present. Options: glue rubber bumper feet to the planter bottom (easy, inexpensive), attach a 2×2 runner frame to the bottom as a stand (same material as the planter), or mount the planter on a rolling planter caddy (adds mobility). Pressure-treated lumber runners are appropriate for the stand/feet even on a food-grade planter since the treated wood does not contact the soil.

Interior Liner Options and Installation

A liner extends planter life by preventing direct soil and moisture contact with the wood walls. Three liner options:

Polyliner (heavy polyethylene film): cut to fit, staple to the interior walls and floor. Leave drainage holes uncovered. Inexpensive (pool liner material or black poly sheeting from a hardware store). Reduces moisture exposure to the interior wood dramatically. Replace every 3–5 years as the poly degrades under UV exposure at the soil line.

EPDM rubber membrane: the same material used in pond liners and flat roofing. Extremely durable (20+ year lifespan), fully flexible, and food-safe. More expensive than poly liner but essentially permanent. Staple or nail to the interior walls; fold corners cleanly. Cut drainage holes in the EPDM at the bottom panel holes before filling with soil.

Landscape fabric: breathable, allows some moisture transfer, less protective than poly or EPDM but better than no liner. Useful as a secondary liner over hardware cloth covering drainage holes. Not a substitute for a vapor barrier in high-moisture applications.

No liner: acceptable for cedar and redwood planters intended as medium-term (5–10 year) installations. The wood will degrade faster but the planter remains fully functional until structural failure. Staining or sealing the interior with a food-safe sealer (tung oil, raw linseed oil, or beeswax-based sealer) extends unlined cedar or redwood service significantly.

Assembly Hardware for Outdoor Longevity

Hardware choice in a planter box matters more than in indoor furniture because outdoor planters experience continuous moisture, freeze-thaw cycles, and potentially acidic compost. Standard bright zinc screws corrode visibly in 1–2 seasons, staining the wood and weakening the joint. Use:

• Type 316 stainless steel screws: maximum corrosion resistance, no staining, appropriate for coastal or high-humidity environments. Most expensive option
• Type 304 stainless: adequate for non-coastal applications, more widely available and slightly less expensive than 316
• Hot-dipped galvanized (HDG) screws or nails: field-standard for outdoor woodwork; acceptable for most planter box applications. Will show some rust staining over 5+ years but maintain structural integrity for 10–15 years
• Ceramic-coated deck screws (Deck-Mate, GRK, or similar): adequate for dry climates; marginal in continuously moist applications like planters with frequent watering

Corner joinery options: butt joint with screws (easiest, adequate); rabbet joint cut into corner pieces (increases glue surface area, looks more finished); half-lap joint at corners (strongest, requires a table saw or router); finger joint (maximum strength and surface area, requires a jig). For a basic planter box, a glued butt joint with stainless screws provides 10+ years of service if the wood and liner are properly maintained.

Multiple Planter Configuration

A single planter box produces a modest harvest. A productive kitchen garden typically requires 50 to 100 square feet of growing area for a family of four. Planning a multi-planter installation: allow 36 inches of walking access between all planter sides and walls — comfortable working access requires being able to reach the center of a 24-inch wide box from either side, meaning the box cannot be wider than 48 inches (24-inch reach from each side). Standard bed widths are therefore 24 to 48 inches; depth (distance from front to back) is limited by reach, not by the box's structural length.

For a modular multi-planter system, build all boxes to the same height and depth, varying only the length. This allows reconfiguration by moving boxes and adding new lengths. Connect adjacent boxes with removable 2×4 spacers to maintain consistent pathways. Install drip irrigation to all boxes simultaneously from a single timer-controlled valve — hand-watering multiple boxes daily in summer becomes a significant time commitment.

Drip Irrigation Integration

Drip irrigation in raised bed planters delivers water at the root zone, reduces foliar disease (water never hits leaves), and cuts water use by 30–50% compared to overhead irrigation or hand-watering. Components for a planter box drip system: a 1/2-inch main supply line from the hose faucet or irrigation valve, 1/4-inch distribution tubing to each box, and emitters (drip heads, micro-sprayers, or soaker tubing) inside each box.

Emitter spacing: one 0.5 GPH emitter per square foot of growing area is a starting point; adjust based on crop water needs and weather. Tomatoes and squash need more water than herbs and salad greens. Pair the drip system with a programmable timer — $25 to $50 timers set to water for 30–60 minutes daily in summer handle the routine completely automatically.