What's inside
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A wooden seedling tray is a useful first-season project because it involves straight cuts, simple joinery, and no critical structural loads. This version holds twelve seedlings in a 3-by-4 layout, with dividers that lift out when plants outgrow their cells. The tray is sized for a sunny windowsill, greenhouse bench, or sheltered outdoor table.
Contents
Project overview
The finished tray measures approximately 16 inches long, 11 inches wide, and 3 inches high. The removable divider grid creates twelve spaces about 4 by 4 inches each. Those cells are large enough for tomatoes, peppers, herbs, and flowers before transplanting.
| Material | Advantages | Drawbacks | Best use |
|---|---|---|---|
| Cedar | Rot-resistant, lightweight, attractive | Costs more; can split near screw holes | Repeated use or damp greenhouse conditions |
| Pine | Cheap, easy to find, easy to cut | Swells and rots faster if left wet | Indoor use or a one- to two-season project |
| Exterior plywood base | Stable and inexpensive | Edges must be sealed; delamination is possible | Budget tray base |
For most beginners, cedar is worth choosing if the tray will sit in a greenhouse or be watered heavily. Cheap pine is fine if the tray will live indoors and dry between waterings. Avoid treated lumber around edible seedlings, and do not use reclaimed wood unless you know it has not been painted or chemically treated.
Materials and tools
Buy one 1-by-4 board, 8 feet long, for the tray sides and dividers. The actual thickness of standard 1-by lumber is about 3/4 inch, and the actual width is about 3 1/2 inches. You will also need a 16-by-11-inch piece of 1/2-inch exterior plywood for the base, plus exterior-rated wood glue and 1 1/4-inch galvanized or stainless screws. A small box of exterior wood screws is preferable to drywall screws, which rust and snap more easily outdoors.
Required tools are a tape measure, pencil, square, handsaw or circular saw, drill, 1/8-inch drill bit, screwdriver bit, and sandpaper. A pair of small woodworking clamps makes assembly easier but is not essential. Add a 1/4-inch drill bit if you want drainage holes in the base.
Cut list
- Two long sides: 16 inches long
- Two short sides: 9 1/2 inches long
- Three long dividers: 14 1/2 inches long
- Two short dividers: 9 1/2 inches long
- One plywood base: 16 by 11 inches
The divider lengths assume the long sides overlap the ends of the short sides. Measure your actual board width before cutting. If your 1-by-4 measures 3 1/2 inches wide, the dividers will stand slightly below the top edge of the tray, which is useful because they are easier to remove. If your board is wider or narrower, adjust divider lengths so the grid fits inside the side walls without force.
Cut and prepare the parts
Mark each cut with a square rather than relying on the factory board ends. Cut the four walls first, then test them on a flat surface. Sand the cut ends with 120-grit paper and lightly round every exposed corner with 150-grit paper. Sharp corners catch clothing, cut fingers, and tend to splinter after repeated watering.
Lay the plywood base on the assembled wall pieces and check that it is flush or slightly inset on all sides. If the base overhangs, trim it now. Drill four to six 1/4-inch drainage holes through the plywood, keeping them at least 1 inch from the edges. Without drainage, excess water collects under the root zone and encourages damping-off and root rot.
Assemble the tray
Place the short sides between the long sides, forming a rectangle. Apply a thin bead of exterior wood glue to each mating end. Drill two pilot holes through each long side and into each short-side end, then drive the screws. Do not skip the pilot holes, especially in cedar; the ends can split even with small screws.
Check the frame for square by measuring both diagonals. If the two measurements differ by more than 1/8 inch, push the frame into alignment before the glue sets. Position the base underneath, drill pilot holes every 4 to 5 inches, and screw it to the walls. Use enough screws to prevent gaps, but stop when the heads are flush. Overdriving them crushes the plywood and creates leak paths.
Make the removable dividers
Mark the three long dividers at 4 3/4 and 9 1/2 inches. On two of them, cut a centered half-depth notch from the bottom edge at each mark. Make each notch about 3/4 inch wide and 1 3/4 inches deep. On the two short dividers, mark the center and cut a matching notch from the top edge. A handsaw and chisel work well; make several shallow saw cuts in the waste area, then remove the chips.
Slide the notched pieces together to form a grid. The notches should fit snugly but not require hammering. If the grid binds, widen the notches with a file or sandpaper. A tight fit becomes worse when the wood absorbs water. The dividers should lift out as one assembly, or you can remove individual pieces as seedlings need more room.
Test-fit the grid inside the tray. If it is too tight, trim the ends by 1/16 inch at a time. Do not glue the dividers in place; removability is the feature that makes this tray more useful than a fixed-cell planter.
Finish and use
Sand the tray to 150 or 180 grit. For indoor use, you can leave unfinished wood, but it will stain and absorb water. A coat of food-safe wood finish on the outside and base edges slows moisture absorption. Let it cure fully according to the label before adding soil. Do not coat the inside with a product that is not intended for garden or food-contact use.
Set the tray on a waterproof saucer or boot tray. Fill each cell with damp seed-starting mix, not garden soil, which is heavy and may contain weed seeds. Water gently so the mix does not wash through the drainage holes. If you need a very cheap liner, use a plastic nursery tray or saucer beneath the wood rather than stapling plastic inside; trapped water against the wood shortens its life.
Common mistakes to avoid
The most common failure is using undrained plywood or placing the tray directly on a wet bench. The second is making the divider grid exact to the inside dimensions with no clearance. Wood expands across its width, so leave roughly 1/16 inch of side-to-side play. Finally, do not carry the tray by a single divider. Lift it by the outer walls, especially when the cells are full and wet.
At the end of the season, empty the tray, rinse off soil, and let it dry completely. Replace any divider that has swollen badly or split. With cedar and reasonable drainage, the tray should last several growing seasons; pine may need replacement sooner, but it remains a sound low-cost way to learn basic measuring, drilling, and joinery.