Should You Use Pressure-Treated Wood for Garden Fence Posts?

Originally posted on Men's Journal | By Emily Fazio | Last updated 3/17/24

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. While the upfront costs are higher, its longevity often justifies the investment; however, be mindful of environmental impacts and learn more about the product options. Historically, pressure-treated wood was treated with chromated copper arsenate (CCA), which contained arsenic, chromium, and copper. The chemicals in CCA-treated wood are toxic and can leach into the garden soil over time, potentially contaminating nearby groundwater and affecting edible plant growth and wildlife.

When choosing a pressure-treated wood product, consider alternative treatments. Alkaline copper quaternary (ACQ) and micronized copper azole (MCA) are noteworthy alternatives:

Alkaline Copper Quaternary (ACQ): ACQ replaces toxic elements with copper and quaternary ammonium compounds, reducing environmental concerns. It provides excellent protection against decay and insects, making it a preferred choice for eco-conscious gardeners.
Micronized Copper Azole (MCA): Similarly, MCA utilizes copper combined with an organic azole compound. This treatment is less toxic and environmentally friendly compared to traditional methods. It offers robust resistance to decay and insects, ensuring the longevity of your garden fence without compromising environmental sustainability.

While modern pressure-treated wood treatments are generally considered safe for most outdoor applications, including garden fences, some environmentalists and individuals may still have reservations about its use, particularly in organic gardening or environmentally sensitive areas.

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 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 are typically less toxic and labeled "environmentally friendly." 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.

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?

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 and are less dangerous than CCA-treated lumber, you'll want to consider if cedar is an economical option. Factor the cost and benefits of each board into your decision.

This article first appeared on Men's Journal and was syndicated with permission.

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