Cedar Planter Box — Plans, Cut List, and Step-by-Step Build Guide

Cedar Planter Box at a Glance

A cedar planter box is a strong first woodworking project. Simple joinery, forgiving material, and a finished product you'll use for years. Western Red Cedar resists rot naturally without chemical treatment, so it's safe for growing food. Build one in an afternoon for under $70.

In this guide:

• Why cedar outlasts every other planter wood
• Tools, materials, and cut list
• Step-by-step assembly with squaring tips
• Drainage, liners, and finishing for food-safe and ornamental planters

Why Cedar Outlasts Every Other Planter Wood

Cedar heartwood contains thujaplicins, natural fungicides that stop decay fungi and insects from breaking down the wood. Heartwood is the darker center of the board. Sapwood is the lighter outer wood. The darker the board, the more heartwood, and the longer it lasts. After those fungicides deplete over the first few years, other protective compounds in the heartwood keep working. Lakeland Mills has a good breakdown of the chemistry.

The numbers back this up. US Timber rates Western Red Cedar heartwood as Class 2 durability above ground, with a 15-40 year expected life. Put it in direct ground contact and that drops to 5-15 years (Class 3). That's why you want feet on your planter, not flat on the ground.

Cedar also moves less than most softwoods. The Wood Database lists shrinkage at 2.4% radial and 5.0% tangential, with a volumetric shrinkage of just 6.8%. Your planter will cycle between rain-soaked and sun-baked dozens of times per season. Low movement means fewer cracks and tighter joints year after year.

And cedar is safe for growing food. University of Maryland Extension confirms untreated cedar for vegetable beds with no caveats. Tannins leach slowly but at harmless concentrations. No liner required for food safety, though one can extend wood life.

Buying Cedar at the Store

At Home Depot or Lowe's, you'll find Western Red Cedar 1x12 boards (actual size: 3/4" x 11-1/4") in 6-foot and 8-foot lengths. Price runs $18-28 per 6-foot board depending on your region.

Pick boards with more dark-colored heartwood and less light sapwood. Sapwood rots faster because it lacks the protective extractives. If more than a third of the board face is pale sapwood, grab a different one.

Check for twist by sighting down the board like a rifle. Set it on edge on a flat surface and look for rock. Cupped boards can be ripped on a table saw, but twisted boards are trouble. Budget 30-60 minutes for board selection. Cedar stock at big-box stores varies wildly in quality, and sorting through the pile is part of the project.

A lumber yard sells better grades for less per board foot. Ask for "tight knot" or "select" grade cedar. If you're building several planters, the trip is worth it.

Tools, Materials, and Cut List

Minimum Tools

You probably own most of what you need:

• Drill/driver with #2 Phillips bit and 1/2" drill bit (for drainage holes) and 1/8" bit (for pre-drilling)
• Circular saw or miter saw for crosscuts
• Measuring tape and speed square
• Sandpaper (120 grit)

Nice additions: bar clamps for holding panels during assembly, a block plane for rounding edges, a pocket hole jig if you prefer hidden fasteners.

Cut List

All pieces cut from two 1x12 x 6' Western Red Cedar boards (actual thickness: 3/4").

Hardware

• 1-5/8" exterior-rated deck screws (stainless steel or ceramic-coated), about 24
• Optional: exterior wood glue (Titebond III) for cleats

Estimated Cost

Sizing Your Planter for What You're Growing

Interior dimensions determine how deep your soil goes. Subtract 1-1/2" from the exterior width and length to get the inside measurement (3/4" wall on each side).

For tomatoes or deep-root plants, build the box 18-24" tall using two stacked 1x12 boards per side. Add a mid-height support cleat to prevent the sides from bowing under soil pressure. Oregon State Extension warns that any bed taller than 18 inches or longer than 6 feet needs structural reinforcement.

Step-by-Step Assembly

Step 1: Cut All Pieces

Measure and cut every piece from the cut list above. Mark each one with pencil (LS = long side, SE = short end, BT = bottom). Check that matching pieces are identical length by stacking them together.

Step 2: Drill Drainage Holes

While the bottom panel is still flat on your bench, drill 1/2" drainage holes spaced every 4-5 inches. Seven to nine holes across the panel works well. Drilling now is far easier than after assembly.

Step 3: Attach Corner Cleats to End Panels

Position a corner cleat flush with the edge of each short end panel, inset 3/4" from the outside face. This creates a pocket that the long side panel will screw into.

Pre-drill with a 1/8" bit, then drive two 1-5/8" screws through the cleat into the end panel. Repeat for all four cleats (two per end panel).

Step 4: Attach the First Side

Stand one end panel with its cleats facing inward. Butt the long side panel against it so the side covers the end of the cleat. Pre-drill through the side panel into the cleat, then drive three evenly spaced screws.

Check for square with your speed square. Adjust before the screws are fully tight.

Step 5: Attach the Second Side

Repeat Step 4 on the opposite side of the same end panel. You now have a U-shape.

Step 6: Install the Bottom

Drop the bottom panel into the U-shape, resting it on the lower edges of the side and end panels. Screw up through the bottom into the sides using 1-5/8" screws every 6 inches. The bottom sits inside the box, supported by the walls.

Step 7: Close the Box

Attach the second end panel by screwing through the side panels into its cleats. This is the tightest step because you're working in a partially enclosed space. A right-angle drill attachment helps, or just angle your driver.

Step 8: Check Square

Measure both diagonals of the box. If they're equal (within 1/8"), the box is square. If not, push the longer diagonal corner inward and add one more screw to lock it.

Step 9: Add Feet and Sand

Screw 1-1/2" blocks to the bottom corners. These lift the planter off the ground so air circulates underneath. This alone can double the life of the bottom panel.

Sand all exposed edges to 120 grit. Round over sharp corners with a sanding block. Cedar is soft and sands fast.

Drainage, Liners, and Finishing

Drainage and Feet

The 1/2" holes from Step 2 handle water flow. The feet from Step 9 keep the bottom out of puddles. Together, they prevent the two things that kill planters: waterlogged roots and trapped moisture.

Skip the gravel layer. It's a persistent myth. Research from horticulturists shows gravel creates a "perched water table" that actually traps moisture above it. Drill enough holes and use well-draining potting mix instead.

When to Use a Liner

A liner between the soil and wood slows moisture transfer and extends the planter's life.

Use one when: extending lifespan is the priority, or you want to grow food and eliminate any tannin contact (though cedar tannins are harmless to plants).

Skip one when: maximum drainage matters, you're growing ornamentals, or the planter is small (herbs, flowers).

For liner material, heavy-duty landscape fabric works best. It breathes, drains, and costs a few dollars. Cut it to fit the interior, fold the top edge over the rim, and staple it in place. Punch holes at the bottom to align with your drainage holes.

Finishing Options

For food-growing planters: leave the interior raw, or apply pure tung oil (100%, no additives) or food-grade mineral oil. Avoid boiled linseed oil on soil-contact surfaces. It contains metallic driers (cobalt, manganese salts) that leach into soil. Apply other exterior finishes only to the outside faces.

For ornamental planters: a penetrating exterior oil preserves cedar's warm color and adds UV protection. Apply two coats to all six faces, including the bottom. Let each coat soak in for 15-20 minutes before wiping excess.

Raw cedar turns silver-gray within 6-12 months of outdoor exposure. This is cosmetic, not structural. Many builders prefer the weathered look and skip finishing entirely.

Five Variations on the Basic Box

Window box (36" x 8" x 8"): Narrower and lighter. Mount below a window on L-brackets rated for the weight of wet soil (about 30 lbs for this size). Use the same joinery but reduce the end panel width to 6-1/2".

Tall planter for tomatoes (12" x 12" x 24"): Stack two 1x12 boards for each side. Add a horizontal cleat at the 12" mark (mid-height) to stop the sides from bowing under soil pressure. Use the same corner cleat joinery.

Tiered planter: Build three boxes at different heights (8", 12", 18") and stack them staircase-style. Screw through the touching walls to lock them together. Great for herbs at the top and flowers at the bottom.

Raised-leg planter (table height): Add 30-32" cedar 4x4 legs. Attach with carriage bolts through the corner cleats. Add cross-bracing between the legs for stability. Good for balconies and accessible gardening.

Self-watering planter: Build the standard box, then add a false bottom 2" above the real bottom using a perforated panel. The reservoir below holds water that wicks up through the soil. Add a fill tube (1" PVC pipe) through one corner and an overflow hole in the side at the false-bottom level.

Common Mistakes and How to Avoid Them

Buying sapwood-heavy boards. Sapwood (the pale outer wood) rots faster than heartwood. Pick boards with mostly dark-colored wood. A board that's half sapwood will fail at the sapwood portions first.

Skipping drainage holes. Waterlogged soil kills roots and accelerates wood rot. Drill more holes than you think you need. Seven to nine 1/2" holes in the bottom panel is a good baseline.

Setting the planter flat on the ground. Moisture gets trapped underneath and the bottom rots from below. Add feet. Even 1" of clearance makes a difference.

Using interior screws. Standard zinc-plated screws rust in outdoor conditions. The rust stains the wood and weakens the joint. Use stainless steel or ceramic-coated exterior screws.

Skipping the pre-drill. Cedar splits near edges and end grain. Pre-drill every screw hole with a 1/8" bit. Stay at least 1" from any edge. The 30 seconds per hole saves you from cracked boards.

Not accounting for soil weight. Wet soil weighs about 80-100 lbs per cubic foot. This 24" x 12" x 12" planter holds roughly 1.5 cubic feet, so expect 50-70 lbs when watered. Plan where you'll place it before filling, because you won't want to move it.

Over-tightening screws. Cedar is soft (Janka 350 lbf). Screws drive fast and strip easily. Stop when the screw head is flush. Use a clutch setting on your drill if it has one.

Where This Fits — Your Next Build

This planter teaches layout, cutting to dimension, assembly sequence, and squaring. Those skills scale to every project you'll build.

A simple shelf adds wall mounting and level/plumb to your skill set. A cutting board introduces edge-joining and finishing for daily-use objects.

For a joinery upgrade on your next planter, try pocket holes. Hidden fasteners and a cleaner look for minimal extra effort.

Sources

This guide draws on wood science data from species databases, extension service research on garden bed safety, and field-tested planter construction methods.

• Wood Database — Western Red Cedar — species properties, shrinkage coefficients, hardness
• US Timber — Western Red Cedar — durability class ratings, physical properties
• Real Cedar — Characteristics & Properties — density, thermal properties, dimensional stability
• University of Maryland Extension — food safety of untreated wood for raised beds
• Lakeland Mills — How Cedar Resists Rot — thujaplicin chemistry, heartwood vs sapwood
• Gardenary — Best Wood for Raised Beds — cedar vs redwood vs pine comparison
• Homestead and Chill — Make Garden Beds Last — sealing options, liner recommendations
• Cedar Planters — Tannins Myth or Fact — tannin safety for plants
• My Chemical-Free House — Non-Toxic Sealers — food-safe liner and sealer options
• FarmTilling — Best Wood for Planter Box — wood species comparison
• Wood Database — Northern White Cedar — Eastern White Cedar properties for comparison
• Bear Creek Lumber — WRC Physical Properties — detailed dimensional stability data