What's the Best Timber for a Wicking Bed?
- dom@urbanveg.com.au
- 5 hours ago
- 6 min read
A normal raised garden bed only has to hold soil. A wicking bed has to do much more. It holds a reservoir of water in its base, a deep body of permanently moist soil above that, and it sits out in the full Australian sun for years on end. That makes the choice of building material far more important than it first looks - the wrong timber can check, crack, warp or rot long before the garden inside it has hit its stride.
So before you build a bed, or pay someone to build one for you, it's worth knowing how each common material actually behaves. Here's an honest run through the options, and what we landed on for UrbanVeg beds after doing this for years.
CCA treated pine - cheap, common, and not for food
Most landscapers reach for treated pine sleepers because they're cheap and available everywhere. The trouble is what you're actually getting. CCA ("green") treated pine is preserved with chromated copper arsenate - it contains arsenic, which can leach into soil, so it was never intended for a vegetable garden. It's also usually sold wet, so as it dries out in the ground it warps, twists and cracks: screws work loose, boards bow, corners pull out of square, and the colour greys off within a season or two. It's the cheapest way to build a bed, and within a couple of years it usually looks it.
Cypress garden beds - natural, with a catch
Cypress is a cheap natural alternative, and cypress garden beds are marketed hard on a single idea: no chemicals. Cypress heartwood is genuinely durable and naturally termite resistant, and for a lot of buyers "untreated" feels reassuring around food.
But a wicking bed is exactly the setting that exposes the weak spots of cypress, and there are a few worth knowing before you spend the money:
It checks and cracks. Construction cypress is usually sold green (unseasoned). As it dries out in a hot, sunny bed it develops surface checking and splits - the official drying guidance even warns suppliers to slow the drying down to avoid it. Once it's in your garden, drying out in the sun is exactly what it does.
It's a knotty timber. Cypress is full of knots. Knots shrink back, weep sap and can work loose over time, leaving gaps and weak points along a board.
The durability is heartwood only. Those impressive "lasts for decades" claims apply to the dense heartwood. The pale sapwood that's often left in cheaper cypress is far less durable - so a cypress bed is only ever as good as the timber that was actually selected for it.
It splits at the fixings. Cypress is prone to splitting near board ends unless every hole is pre-drilled - which is exactly where a garden bed carries its load.
It still needs upkeep. To keep cypress protected and looking good, rather than greying off and going rough, it wants re-oiling every couple of years. "No chemicals" doesn't mean "no maintenance."
None of this makes cypress a bad timber - it's a perfectly good fence post or floorboard. But in a permanently moist, full-sun wicking bed, "natural" comes bundled with cracking, knots and ongoing maintenance that the marketing tends to leave out.
Steel and Colorbond - modern, but it cooks
Galvanised steel and Colorbond beds look sharp and won't rot. The trade-offs are heat and edges. Thin steel transfers the summer sun straight into the soil around the perimeter, drying out and cooking roots near the walls - the opposite of what a wicking bed is trying to achieve - and cut edges can be sharp and may rust over time. And in schools, they can really take a beating!
They can work, but in a Sydney summer the heat is a genuine consideration.
Hardwood sleepers - tough, heavy and pricey
New or recycled hardwood sleepers, like red gum, are very durable. The downsides are weight, cost and finish: they're heavy to handle, expensive, often still check and split as they age, and recycled sleepers can carry old oil, bitumen or unknown treatments you really don't want sitting next to food.
Untreated pine - skip it
Cheap untreated pine rots quickly in the constant moisture of a wicking bed. It's the false economy of the lot - you'll be rebuilding within a few years.
What we build UrbanVeg beds from - and why
After years of building beds across Sydney, we settled on an approach that fixes the trade-offs above rather than living with them.
Our timber is kiln-dried, so it's stable and stays put instead of checking and cracking the way green timber does. It's protected so it won't rot and termites won't touch it - and it never needs sealing or oiling to keep doing its job. It's food-safe, the iron-oxide colour won't fade, and there's no arsenic anywhere near your vegetables.
Then we over-build every bed. Each one stands on its own structural subfloor rather than bare ground. The heavy planks are bolted together with galvanised fixings - not just screwed or nailed - and the timber is locked in place from several directions so it can't bow or warp. The capping sheds water and the end grain is kept covered, so rain runs off instead of soaking in. It's more than a garden bed strictly needs, which is exactly the point.
Finally, every bed is fully lined with heavy-duty, food-safe pond liner - a second line of defence so soil and water never sit against the timber at all. Belt and braces.
So what's the best material for a wicking bed?
Honestly, there's no magic species. A bed lasts because of how it's built as much as what it's built from. The timber has to be properly dried, protected and assembled so it can't move, and the inside has to be lined so the structure never has to fight the soil. A "natural" timber thrown together green and unlined won't outlast a well-built, lined bed - and a cheap one will simply cost you again in a few years.
We'd rather you spent your weekends picking veggies than re-oiling timber or rebuilding a bed. Build it once, build it properly, and let it get on with the job.
Want a bed that's still going strong long after the novelty has worn off?
Talk to us on 0433 971 751 or email info@urbanveg.com.au. We design and install custom wicking beds across Sydney and regional NSW.
Frequently asked questions
Can you build a wicking bed out of cypress?
Yes, and plenty of people do. Just go in knowing that cypress is notorious for cracking and splitting as it dries in the sun. It's knotty, and it needs re-oiling with chemicals to keep its looks. And remember the durability claims apply to the heartwood, not the paler sapwood.
Is cypress good for raised garden beds?
Cypress heartwood is naturally durable and termite resistant, however most cypress sold in Australia is from the cheap sapwood. In a permanently moist, full-sun wicking bed, cracking, knots and ongoing maintenance are the things to weigh up.
What's the best timber for a wicking bed?
The best results come from timber that's kiln-dried so it stays stable, protected against rot and insects, and assembled so it can't warp - then lined inside so the timber never sits against wet soil. Species matters less than drying, protection and build quality. UrbanVeg only uses timber that does not lose it's colour in the sun, stands up to rot and termites, and we build in a way that locks in the timber so beds remaing looking good for years.
Is treated pine safe for a vegetable garden?
CCA ("green") treated pine contains arsenic and isn't recommended for food gardens. Look for food-safe, arsenic-free timber, and a fully lined bed for extra peace of mind.
Do you need to line a wicking bed?
Yes - a wicking bed needs a waterproof reservoir to work, and a full food-safe liner also protects the timber and keeps soil off the structure.
