1. Choose the site before the timber
This design sits directly on suitable garden ground and has no timber floor. It is not a freestanding balcony planter, a raised table or a retaining structure. Wet growing medium is heavy; do not adapt it for a roof or balcony without checking structural capacity, waterproofing and drainage with the appropriate professional.
Choose a reasonably level location with light conditions that suit your plants. Leave access for watering and keep the box away from air bricks, damp-proof courses and building drainage. Remove persistent weeds and check that the underlying soil drains rather than remaining waterlogged after rain.
2. Compare timber options
Untreated durable timber avoids added preservative but can cost more and still needs sensible detailing. Ordinary untreated softwood is easier to source in many places, although ground contact can shorten its useful life. Treated timber should be explicitly suitable for the intended exposure and, where relevant, use around edible crops. Ask the supplier for the treatment documentation.
Avoid reclaimed timber with unknown coatings or treatments. Do not assume that an old sleeper or pallet is appropriate for food growing. Use exterior-rated screws compatible with the chosen timber and treatment; ordinary indoor screws may corrode. Wear suitable protection and collect cutting dust.
3. Use an actual-size cutting plan
Here is a worked example using boards that actually measure 20 mm thick and 140 mm wide. Two stacked courses create a box approximately 280 mm high, with outside dimensions of 800 × 400 mm. Nominal timber sizes may differ from delivered dimensions, so measure your stock before cutting.
- Four long-side boards: 800 × 140 × 20 mm.
- Four end boards: 360 × 140 × 20 mm.
- Four internal corner posts: 40 × 40 × 260 mm.
The end boards fit between the long sides: 400 minus two 20 mm thicknesses gives 360 mm. If the thickness changes, recalculate that length. Allow for saw kerfs and unusable board ends when buying stock. Posts remain slightly below the rim.
4. Cut safely and assemble square
Mark each cut with a square. Clamp the timber on a stable work surface and support the offcut so it cannot pinch the blade. Follow the saw’s instructions, keep hands away from the cutting line and use eye and hearing protection as appropriate. Smooth splintered edges before assembly.
Make two long panels by fastening two stacked boards to a post at each end. Position posts so the end boards can fit between the long sides and fasten into their adjacent faces. Dry-fit the complete box first. Measure both diagonals; matching measurements indicate a square rectangle.
5. Fasten for outdoor conditions
Pre-drill near board ends to reduce splitting and use the screw maker’s guidance for pilot size, edge distance and engagement. For this example, a compatible exterior screw around 50 mm long may suit a 20 mm board entering a 40 mm post, but confirm the actual assembly and fixing requirements. Stagger screws on adjacent faces so they cannot collide.
Keep screw heads flush without crushing the timber. If using treated stock, protect cut ends only with the treatment system recommended by its supplier. Apply any compatible finish before filling and observe its drying requirements. Coating the outside does not make the box permanently rot-proof.
6. Set, fill and look after it
Place the assembled box on level, prepared soil. Do not line the bottom with impermeable plastic. An optional side liner must be suitable for the intended growing use and should leave the bottom open; it does not eliminate decay. A gravel layer is not a substitute for draining soil.
Fill with growing medium appropriate to the plants, leaving watering space below the rim. Account for settlement and water gently. The nominal internal volume is about 77 litres before posts and freeboard, so use that only as a purchasing estimate. Inspect annually for loose fixings, splits and soft timber; empty the box before moving or repairing it.