How to build a wooden planter box

Inspired by a popular UK garden build guide. This walk-through is based on the video "How to Make a Wooden Planter Box - Using Garden Decking", which shows how to make a solid, attractive planter from decking boards - a clever way to use the same material as a nearby deck, so everything ties together visually.

1. Choose the right timber

Softwood is fine for a planter, but it needs to be pressure-treated to survive outdoors. Decking boards work well - they are typically pre-treated, available in useful widths, and straight. Oak and larch are more expensive but will outlast most softwoods by years without any treatment at all.

Avoid using timber treated with older preservatives for food growing. If you are growing vegetables, line the inside of the planter with polythene to keep the compost away from the wood.

2. Decide on dimensions and cut a list

A planter between 300 and 600mm deep and 600 to 1000mm long is a practical size. Too deep and it takes a lot of compost to fill and gets very heavy. Too shallow and it dries out quickly in summer.

Draw a quick sketch with dimensions and write down all the pieces you need. A simple box is just two long side boards, two short end boards, four corner posts, and a base made from boards with gaps between them for drainage. Nothing complicated - just measure twice, cut once.

3. Cut all the boards to length

Use a circular saw or mitre saw for clean, square cuts. If you are using a handsaw, clamp the board firmly and use a try square to guide the first few strokes. An out-of-square end makes the assembly harder than it needs to be.

Cut all the pieces for one size before moving on to the next. Keep them in labelled piles so assembly is quicker and you do not mix up the similar-length pieces.

4. Assemble the corners

Fix a corner post inside each corner of the planter - the side boards screw into these posts from outside. Use 75mm or 80mm decking screws and pre-drill to avoid splitting near the ends. Galvanised or stainless steel screws only - standard steel screws will rust through in a couple of years and stain the wood.

Check each corner is square with a try square as you go. A planter that is twisted or racked looks messy and the base will not sit flat.

5. Fix the base and drill drainage holes

Screw the base boards across the bottom, leaving 10–15mm gaps between them for water to drain freely. A planter that sits in water will rot faster and kill most plants that are not aquatics.

If you are sitting the planter on a hard surface, add four short feet - just offcuts screwed to the base corners - to lift it clear of the ground slightly. This improves drainage and stops the base sitting in a puddle every time it rains.

6. Sand and treat the outside

A quick sand down with 80-grit paper removes any rough edges and mill glaze, which helps the treatment soak in better. Apply two coats of exterior wood stain or preservative on all outer faces and the top edges of the boards, letting the first coat dry fully before the second goes on.

There is no need to treat the inside - compost and plant roots will be there. Just keep the treatment away from the interior faces.

7. Line if needed, then fill and plant

For most flowering plants and shrubs, fill straight with good-quality multi-purpose or John Innes No. 3 compost. For vegetables, consider lining the inside with polythene and use peat-free compost. Punch a few holes in the lining at the base to match the drainage gaps.

Water the compost well before planting and again after. A freshly filled planter needs a good soak to settle the compost and remove air pockets before the roots go in.

When to call a handyman

If you want a larger planter, a built-in bench with planting either side, or a whole row of matching planters in a particular size, Richard can build them from scratch or from a design you have in mind. Custom planters to suit a specific spot are a regular kind of garden job.