When it comes to building your garden beds, choosing the right materials is important for many reasons. Pressure-treated lumber offers durability, resists decay, and can be a great solution if you need a long-lasting garden fence.
Prior to the early 2000s, pressure-treated wood was treated with chromated copper arsenate (CCA), which contained arsenic, chromium, and copper. The chemicals in CCA-treated wood were found to be toxic and could leach into the garden soil over time. CCA-treated lumber is no longer sold in stores, so if you're buying new lumber, there's no concern that the chemicals could potentially contaminate nearby groundwater or affect edible plant growth and wildlife.
Modern pressure-treated wood treaters use alternative solutions, primarily Micronized Copper Azole (MCA), and prior to 2020, Alkaline copper quaternary (ACQ). "Today, Copper Azole is used for virtually all pressure-treated wood sold into residential markets," shared Timm Locke, Director of Marketing for Western Wood Preservers Institute. "Treaters in the South use Micronized Copper Azole because that form works well with Southern Yellow Pine species. In the west, the non-micronized version is used because the micronized version doesn’t work well with western species due to their cell structure."
New Research-Backed Data Shows That Pressure Treated Wood is Safe For Gardens
While modern pressure-treated wood treatments are considered safe for most outdoor applications, including garden fences, some environmentalists continued to voice reservations about its use, wary about potential risks in organic gardening or environmentally sensitive areas. WWPI reported on a recent study from Oregon State University that works to debunk that. The researchers tested their theory using two raised planter gardens, one made of untreated Douglas-Fir lumber, and a second made of Douglas-Fir treated with Copper Azole (CA-C):
- Soil and plants within 3-4" of the wood frame were tested throughout the study, including plant tissue and root crops. It concluded that there was no measurable difference in soil between the vegetable garden beds at this distance from the wood frame.
- Soil within 1" of the treated border did experience a small increase in copper (57 PPM), which the study confirms is within the range of natural soil.
Furthermore, the untreated wood garden boxes experienced decay after one year, while treated boxes showed no deterioration.
Can you use non-pressure-treated fence posts around the garden?
Yes, you can sink untreated lumber into the ground for certain applications, but keep in mind that if they're untreated or lack a protective barrier, they won't be immune to deterioration over time.
To prolong the lifespan of non-pressure-treated wood, it's important to use gravel or crushed stone at the base of the hole to improve drainage and reduce ground contact. Allowing direct contact with soil or organic matter will accelerate rot. Additionally, applying a sealant to the portions of the wood that will be buried can help slow down decay. A few common sealants include:
- Oil-Based Preservatives: These penetrate natural wood for lasting defense against decay, insects, and moisture. They often contain linseed, paraffin, or tung oil.
- Water-Based Preservatives: Water-based wood preservatives are similar to oil-based ones but use water as a carrier instead of oil. They're a popular choice because of fewer toxic chemicals, and an "environmentally friendly" label. They're also easier to clean up but may not penetrate as deeply into the wood.
- Boron-Based Preservatives: Boron-based wood preservatives are suitable for areas with low moisture levels. They are less toxic and inhibit fungi.
FAQ
How quickly will untreated wood rot underground?
The durability hinges on several factors. First, the types of wood used play an important role; hardwoods like cedar and redwood offer greater resistance to decay than softwoods like pine or spruce. Additionally, environmental conditions, including moisture levels, sunlight exposure, temperature fluctuations, and soil composition will significantly influence the wood's degradation rate. Constant exposure to moisture, such as from rain or irrigation around your vegetable beds, accelerates deterioration. When used as fence posts, untreated wood may last for 10 years in one environment, but decay and weaken in fewer than two years in another.
Are cedar fence posts a better option than treated pine?
While the natural oils and resins in cedar wood act as a protective barrier against moisture, fungi, and insects, they're not immune to rot over their lifespan. While they will last longer than non-pressure-treated pine, you'll want to consider if cedar is an economical option. Factor the cost and benefits of each board into your decision.
How long does pressure-treated wood last in a garden fence?
Pressure-treated wood can last 15-20 years or more, depending on the environmental conditions and maintenance. Regular sealing and staining can extend its lifespan by providing additional protection against moisture and UV damage.
Can you burn pressure-treated wood?
Disposal of pressure-treated wood should be done carefully, as it contains chemicals that can be harmful if not handled properly. Do not burn it; instead, check with local waste management facilities for guidelines on proper disposal or recycling options.
