Treating Wood for Vegetable Gardens

Wood is a very versatile material for vegetable gardens. Whether it is used to create raised beds, the edges of paths or a frame for protective netting, wood is the natural choice for many gardeners. Apart from being a sustainable resource, it looks good and is great for creating a new structure for vegetable beds that will hold in compost or keep out pests. So when I set out to create a set of long-lasting raised beds for my front garden, wood was my material of choice. Yet, the question of how to keep the wood in good condition is not as simple as it might first appear...

The issue is that all of the common methods of preserving woods have their problems when it comes to growing edible produce. For many of us, one of the primary benefits of growing our own food is the knowledge that our vegetables have not been sprayed, treated or artificially enhanced. Organic principles can be applied very successfully to home gardens but treating the wood in contact with the soil and plants can cause many questionable chemicals to leach into the ground, contaminating the crops. Details of the extent of this problem are hard to come by but the following treatments are commonly questioned:

• Pressure treated (‘tanalised’) timber: this is where the wood is preserved before you buy it by subjecting it to chemical treatments under pressure, so that as much as possible is absorbed into the wood.  Most fences and posts are treated in this way, to prolong the life of the cheaper softwood used to make them.  The problem is that it is usually very hard to get information about the chemical concoction used in the process, particularly as the wood may have been pressure treated long before it reaches the place you purchase it from. There have been serious concerns about the use of arsenic compounds (CCAs) and heavy metals, particularly when these rub off on hands or get into the food chain.
• Creosote: The traditional wood treatment for many years, this has now been withdrawn from sale for domestic use within the European Union following advice from organisations such as the International Agency for Research on Cancer who believe it to be carcinogenic. Creosote continues to give off vapours for some time after application and can leech into the soil and groundwater, entering into the food chain.
• Oil-based preservatives: These penetrate wood very well, giving a deeper protection than many other paint-on preservatives and are the basis of many wood stains. Some are based on vegetable oils. However, they rarely list the ingredients and can contain a wide range of other compounds including fungicides, preservatives, UV blockers and pigments specific to the manufacturer. As a result, it is unclear whether they are safe to use around organic growing areas.

So what can be done to preserve the wood used on vegetable plots? You would imagine that there would be a number of products for the job but there are surprisingly few. The natural alternatives that do exist are often very expensive, not suitable for exposed outdoor areas in contact with soil, or prone to their own problems:

• Some paint-on wood treatments: which use acypetacs (such as ‘Cuprinol’) are believed to be safer than the above treatments. However, they don’t penetrate as deep into the wood and probably need re-treating more often. Of course, when you have earth permanently against a wood surface, you are not going to be able to re-treat it easily. Most of the products claim to work for years but this is for fences or woods which can dry out, not those exposed to damp soil.
• Other water-based preservatives: are based on boron salts which are widely considered to be safe to humans and are usually applied as a paint or gel. However, the water-soluble nature of these products means that they don’t chemically bond with the wood and can leach out. These are also harder to find in shops and take a long time to dry.
• Linseed oil: The classic wood treatment made from natural flax seed, linseed oil has excellent preservative properties and water resistance. However, it is very slow drying and in cold or damp weather it may not even be worth applying it because it can just remain sticky for weeks. As a result many available linseed oils are not pure raw linseed oil but a mixture with solvents such as mineral spirits, often called ‘boiled linseed oil’ to speed up the drying, which makes them much less natural. Even worse, other linseed oils contain many of the ‘nasties’ such as heavy metals used in pressure treated timber. So you have to be very sure you know what you are buying and remember that as a natural material, it doesn’t protect the wood from UV sunlight or mildew. [Also it is very flammable and rags used to apply it have been known to spontaneously combust - beware!]
• Plant based preservatives: Some vegetable-based natural products are available from specialist companies but they are largely for staining or interior use and tend to be expensive.

 An alternative approach is to choose hardwoods which, unlike the common softwoods  such as pine, will last much longer untreated (up to 20 years) but it is important to check that these are from forestry approved (FSC) sources and there is also the environmental impact to consider of how far they have been transported, coupled with the much higher cost. A better alternative is recycled plastic boards which are long lasting and available from companies such as Link-a-Bord (or Gardener’s Supply Company in the US). Plastic is not usually considered environmentally friendly but this company recycle the uPVC from windows and doors which mustn’t be incinerated or sent to landfill. Because the boards contain an air gap they are good insulators for the soil, helping it warm up more quickly in spring and keeping the plant roots at an even temperature. The cost is predictably higher than for wood but they are good value when you consider their length of life.

Having considered all of these options it seemed that the only wood treatments that are in keeping with my organic principles involved natural products that were going to almost double the cost of the wood required for my raised beds! So, I reached the rather surprising decision that it was best to use untreated wood, using thicker (2 inch) structural grade planks fixed together with galvanised decking screws. That way, although they will still rot, it will be a much slower process and I expect to get at least 5 years from them before I will need to start replacing them. My neighbour is unconvinced, predicting that they ‘won’t last 12 months’. He assured me that the only lasting way to treat wood is with a mixture of engine sump oil and creosote! However, my family is the one eating the vegetables from the beds and I would hate to have this concoction leaching into the fresh produce I feed to my children. If I had to choose an alternative option, it would be the recycled uPVC Link-a-Bords which many gardeners swear by. I was so impressed by the eco-credentials of the company producing these that I intend to use them for other parts of my garden. Ask me in five years time and I’ll tell you if I made the right decision!

EDIT: The beds lasted well for 3 years and then I had to move house and the new owner ripped them up! However, it was clear to me that they would easily survive another two seasons.