Heavy metals and other chemicals can accumulate in the soil near raised beds, but they can also spread farther than your garden's confines. Toxic pollutants are most mobile in clay, sandy or moist soils, where they end up in the water table.
The first rule of gardening really should be: do no harm. Here are the worst raised bed materials that can have a dire impact on the environment:
pressure treated wood
Before 2004, copper chromate arsenate (CCA) was the most widely used wood preservative. Discontinued due to concerns about arsenic exposure, alkaline copper quaternary (ACQ) is the standard wood treatment these days.
While it's far less toxic than its predecessor, ACQ contains large amounts of copper, which can leach into the surrounding soil.
Copper is highly toxic to fish and aquatic life, and the use of ACQ pressure-treated wood to house moist soil increases the likelihood of copper seeping into watersheds.
MB tray
Wooden pallets can be an inexpensive and waste-free way to build your bed -- but beware of those that say "MB".
Methyl bromide is a broad-spectrum insecticide that is extremely harmful to human health. Treated wood is not recommended for use in any way.
It easily kills fungi, insects, roundworms, and even rodents. MB trays emit exhaust gas into the atmosphere, directly destroying the ozone layer.
In any DIY pallet project, indoor or outdoor, only use pallets that are printed with "HT" or heat treated. This means the tray is sterilized at 132°F and above for at least 30 minutes. The HT tray is completely safe to upgrade to loft beds and beyond.
railway relations
Wooden railway sleepers are treated with creosote, another harsh insecticide that should never be used around humans and plants.
Creosote is a soot that repels termites, fungi and other pests. It is made from tar from coal, petroleum and other fossil fuels.
Prolonged and frequent exposure to creosote railroad sleepers is not only harmful to human health, but can also penetrate into the soil and cause damage to plants, insects and small animals.
cinder block
Cinder blocks made from fly ash or coal particles contain arsenic, lead, mercury and other heavy metals. While cinder blocks haven't been mass-produced for about 50 years, if you're making a raised bed from recycled materials, you may want to avoid them entirely.
Modern concrete blocks look the same as old cinder blocks, but are made from Portland cement and other aggregates. Concrete is considered non-toxic and safe to use in the garden. However, the concrete industry has a huge carbon footprint and is one of the major producers of CO2 globally.
used tires
The effort to upgrade junk into something useful is truly admirable, but some items — like used tires — are generally best avoided in the garden.
Tires contain cadmium, lead and other harmful substances that can theoretically seep into the soil. Some believe that used tires release most of the toxins in their first year on the road, and they take decades to degrade.
But the jury is still out on the issue. To date, no scientific studies have been conducted to determine whether used tires contaminate garden soil. But why take the risk? Especially when using raised beds to grow food, it's better to be safe than sorry.
Once your raised beds are built and ready, the next thing you'll want to do is fill them with rich and healthy soil.
