A planter box is a genuinely good first outdoor project: simple joinery, forgiving tolerances, and immediately useful. It also teaches every lesson that matters about building with wood outdoors, because a planter is permanently full of wet soil.
Material choice
Use naturally durable heartwood – cedar is the usual pick – or properly rated pressure-treated lumber. Note that the pale sapwood of a durable species is not durable; the decay resistance lives in the heartwood.
If you use pressure-treated material for anything that will hold soil for edibles, read the product’s own guidance on that use. Manufacturers publish it and it is worth two minutes.
Fasteners are not optional detail
Use stainless steel or fasteners specifically rated for exterior and for treated lumber. Ordinary steel screws will corrode, stain the wood black, and eventually fail. Cedar’s tannins accelerate this considerably. This is the single most common reason a well-built planter falls apart.
Design for water leaving
Three details do almost all the work:
- Generous drainage holes in the bottom, not a token few. Water sitting in the base is what rots a planter from the inside.
- Lift it off the ground. Feet, cleats or a stand – anything that keeps the base out of standing water and lets air move underneath. A planter sitting flat on a patio will rot at the bottom regardless of species.
- Do not line it airtight. A liner that traps moisture against the wood defeats the purpose. If you line it, perforate it.
Assembly
- Cut sides and ends, and cut corner posts that the sides fasten to. Corner posts make the box far more rigid than butt-joined sides.
- Pre-drill everything. Cedar splits readily near ends.
- Assemble the four sides to the posts, then add the bottom slats with gaps between them for drainage and airflow.
- Add feet or a base frame.
- Ease all the edges with sandpaper. Sharp edges shed finish first and are where weathering starts.
Finish, or do not
Cedar left bare will weather to silver-grey and last a long time. If you want to keep the color, use an exterior finish with UV protection and accept that it needs reapplication on a schedule. There is no permanent option. Choosing to let it grey is a perfectly legitimate decision and requires no maintenance at all.
