NZ TIMBER EDITORIAL GUIDE
Raised Garden Bed Timber
A raised bed keeps timber close to damp soil, irrigation and compost. The useful question is not simply 'treated or untreated?' It is how long the owner expects the bed to last, what will be grown, which treatment is documented, and whether the sides can drain, dry and eventually be replaced.
Choose your risk and replacement strategy
| Priority | Material direction to investigate | Trade-off |
|---|---|---|
| avoid preservative-treated timber near food soil | verified untreated naturally durable timber | variable durability, higher cost or more frequent replacement |
| longer ground-contact durability | documented treated product for the exposure | confirm preservative and follow EPA/producer advice |
| low initial cost | common pine product with clear specification | short life if untreated/inadequate class; treatment choice matters |
| easy future repair | modular boards fixed from accessible side | more connectors and deliberate detailing |
EPA New Zealand provides public advice on treated timber and gardening. Use the current page and product safety information rather than folklore based only on timber colour or an old preservative name.
Keep edible soil and structural soil conceptually separate
A planter box holding its own contained growing medium is different from a wall retaining natural sloping ground. Once the structure supports surcharge or significant retained ground, obtain retaining-wall advice. Labelling the space a garden does not reduce soil and water loads.
A quantity example for a 2.4 x 1.2 m bed
A one-board-high rectangle with 2.4 m boards on the long sides and 1.2 m boards on the ends needs 7.2 linear metres before posts and waste. If the available stock is 2.4 m, three boards can yield two long sides and two 1.2 m ends with no cutting waste in theory. In practice, add corner posts, internal ties where needed, rejected ends and the exact stacking detail.
| Line | Planning quantity |
|---|---|
| two 2.4 m sides | 4.8 LM |
| two 1.2 m ends | 2.4 LM |
| net perimeter | 7.2 LM |
| corner/support members | design separately |
| additional courses | multiply perimeter by course count |
Details worth deciding before the timber arrives
- Finished internal dimensions and working reach; very wide beds are hard to tend from the edges.
- Board thickness and support spacing to limit bowing.
- Food-growing policy for treatment and any liner or barrier.
- Drainage outlet that will not stain paving or saturate framing.
- Fixings compatible with wet timber, soil environment and preservative.
- A replaceable assembly that does not trap every board behind landscaping.
Liners, membranes and drainage layers
A liner can separate soil from timber or slow moisture movement, but it can also trap water against the board. Specify material suitable for the intended food-growing context, leave a drainage route and avoid puncturing it randomly with fixings. A waterproof membrane is different from a permeable root or separation fabric.
Do not line the bottom unless the garden design calls for it; connection to underlying soil can support drainage and soil biology, subject to site contamination and pest considerations. On paving or a roof, obtain a complete drainage and waterproofing design.
| Detail | Purpose | Failure to avoid |
|---|---|---|
| top cap | shed water and protect end grain | flat ledge holding irrigation water |
| corner post | resist board bowing | post too small or fixings close to board ends |
| internal tie | limit spread on long beds | corroding or inaccessible connector |
| base drainage | release excess irrigation | blocked outlet saturating timber |
Plan access before height
A tall bed reduces bending but increases soil pressure, fill volume and working reach. Calculate the volume: internal length x width x soil depth. A 2.2 x 1.0 m internal bed filled 0.45 m deep needs about 0.99 m3 before settlement. Confirm the subgrade can carry the mass and that deliveries will not damage paths.
- Keep the bed narrow enough to reach the centre without standing in soil.
- Allow space for a barrow and future board replacement.
- Place irrigation so leaks are visible.
- Separate treated-timber cutting from edible soil and clean the site.
Sources and technical guidance
- EPA New Zealand - treated timber (official)
- NZ Farm Forestry Association - macrocarpa species information (technical)
- Timber Dealers - landscaping timber (commercial)