The materials you use in the garden influence your soil content, the growth of your plants, and what you consume from home-grown food. Lately, the topic of pressure-treated wood and which is best for the garden is one of the things people wonder about.
Knowing what you’ve included in your raised bed design or the content of the grow bags you’re using is as important as knowing what your plants need to thrive. Understanding the discussion and concerns around pressure-treated wood is among the many things to consider before you add a wooden bed to your growing space.
To give the topic a full examination, we have decided to tackle the questions about pressure-treated wood. What is it?
What Is Pressure Treated Wood?
Pressure treatment preserves wood by forcing chemicals into it, making it more durable against outdoor conditions.
Well, there are alternatives! Before we get into those, let’s define “pressure-treated wood” and discuss the overarching concepts behind this commonly used building material.
You may have heard the term “green-treated.” This is just another name for pressure-treated wood, so we’ll use the terms interchangeably here. Both refer to wood with chemicals forcibly pressed through the grain for preservation purposes. It takes on a green tint in the process – hence, green-treated wood.
The goal of pressure-treating is to fortify it against the elements. Pressure treatments make wood less porous, limiting the ability of bacteria and fungi to colonize the grain and break it down. These treatments also increase density, making it stand up to outdoor conditions for much longer than raw wood.
Types of Treatments
There are two main types of treatments. One is more prevalent now than in the past. The other is less desirable due to scientific studies that showed the potential for the chemicals used in the treatment to leach away from the wood.
Organic or Inorganic Treatments
Wood can be treated with either organic or inorganic compounds.
There are two ways to pressure treat: with organic compounds or inorganic compounds. Organic compounds are generally options like an oil used to treat the wood, whereas inorganic compounds are infused into water.
Organic treatments are also present in commercial retail but are less common. When people worry about chemical compounds from treated wood leaching into soil, they focus on inorganic compounds rather than organic ones.
In that light, let’s discuss the most common forms of inorganic treatment typically sold in stores. Note that two of these treatments have been approved by the EPA for use in homes, gardens, construction, and more. We’ll detail which applications are officially approved in the two sections below.
We aren’t going to detail absolutely every type of chemical used in treatments, but these should give you a good idea of what you’re working with most of the time.
Creosote and Pentachlorophenol
Since the early 90s, chemicals like PCP and creosote are no longer used in treatments due to health concerns.
Previously, with certain treatments, chemicals like pentachlorophenol (PCP) and creosote or coal tar were predominant. However, since the early 90s, these chemicals are no longer used among the public due to concerns about toxic chemical inhalation.
As a result, these treatments are less available than a couple of decades ago. This is, on the whole, a really good thing for consumers, as they don’t risk ailments associated with these chemicals.
It is still possible to find creosote-treated wood, most commonly as railroad ties or previously used power poles.
Alkaline Copper Quaternary
ACQ is approved for various applications, including ground contact use for garden beds.
Also called ACQ, this water-based treatment preserves wood’s integrity when exposed to fungi, insects, and general elemental decay. The compound is composed of copper oxide and quaternary ammonium compounds suspended in water. These have been approved for use in the following settings:
- Lumber
- Timbers
- Landscape ties
- Fence posts
- Building and utility poles
- Freshwater and marine pilings
- Sea walls
- Decking
- Shingles
- Other wood structures
ACQ is approved for ground contact use – a standard that we’ll discuss further in just a bit. This means it’s approved for use in garden beds placed on the ground.
Chromated Copper Arsenate
CCA contains chromium, copper, and arsenic, with concerns arising from potential arsenic contact over time.
CCA contains chromium, copper, and arsenic. When people express concerns about pressure-treated wood in the garden space, this is likely the kind they are worried about. Their concerns stem from the fact that trace amounts of arsenic can come off when contact is made with hand or soil to the wood.
While experiencing immediate effects of arsenic poisoning is not a result of touching this kind of green-treated wood, residual effects can develop with repeated exposure over a long period of time.
Since the 1940s, the EPA has approved using these woods for the same applications we’ve outlined for ACQ treatments. However, since 2004, the recommendation has been to avoid CCA treatments. They do not suggest this means that all structures with CCA wood should be removed, however. Instead, people are advised to limit exposure and avoid building new structures with CCA.
Today, this kind of wood is still used and recommended for certain commercial, industrial, and agricultural purposes. Home gardening is not a good setting for CCA-treated wood. It is better used in the following settings:
- Shingles
- Shakes
- Foundations
Note that if you do use CCA for roof shingles, it’s best if you do not harvest rainwater that has run off those shingles for edible garden use due to the potential for leaching into the water.
Pressure Treated Ratings
There are two types of ratings associated with pressure treatments. Here, we’ll discuss the basic tenets of each rating and how to know which situation is suited for specific grades. These grades refer to where the wood can be used in relation to the ground’s surface.
Above Ground
Green-treated wood approved for above-ground use is suitable for structures at least 6 inches above the ground.
Green-treated wood that is approved for above-ground use is great for structures that exist completely above the earth’s surface by at least 6 inches. These are structures that can easily be maintained or replaced as needed. Good drainage and ventilation are essential to remain at this standard.
Common examples of above-ground graded wood include decks and similar platform structures built above ground.
Ground Contact
Ground contact wood is used similarly to above-ground wood but for applications where it directly touches the ground.
Wood approved for ground contact can be used the same way you would use above-ground wood. It is also good in applications where the wood touches the ground itself. This is generally the choice for projects where direct contact with the ground cannot be avoided. In these situations, the wood is often not in a structure that can be easily amended, nor is it in a situation where it can be replaced.
Common applications for ground contact wood include structural posts, wood foundations, garden boxes, and landscaped walls.
Pressure Treated Wood In Gardens
Green-treated wood, while approved for garden use, does leach some chemicals into the environment.
While the various grades of green-treated wood do have approved use in the garden, the truth of the matter is they do leach some chemicals into the environment. There haven’t been enough studies to suggest whether or not this causes long-term problems, outside of discontinuing approval for CCA-treated wood due to known effects.
This leads a lot of gardeners to question whether or not pressure-treated wood is appropriate for growing plants, namely, growing food. We can say for sure copper and arsenic are present in soils regardless of their contact with pressure treatments that contain those chemicals. In some cases, trace minerals can improve the chemical structure of the soil.
Instead of waiting for the next round of studies to develop conclusive results, gardeners concerned about the long-term effects of approved chemical treatments can opt for alternatives.
Alternatives to Pressure-Treated Wood
Alternatively, consider metal raised beds, straw bales, or grow bags for those avoiding wood.
The simplest way to avoid all green-treated wood is to go for an untreated, raw wood that can withstand the elements due to its grain. Hard cedars and redwood will last the longest. Add a layer of protection with a natural oil applied to the outside of the structure.
Linseed oil is a great choice for prolonging the life of untreated wood, especially since it comes straight from flax seeds. Avoiding the treatment with natural oils is fine, too, but be aware that the wood will not hold up to the elements as long.
Other alternatives include not using wood at all and opting for a metal raised bed, straw bales, or even grow bags. All of these are viable options for those who don’t want to mess with treated wood.
Final Thoughts
Now that you know which pressure treatments are EPA-approved and which ones have been phased out, you can make more informed decisions about using green-treated wood in your garden space.
Whether you choose to use pressure-treated wood or go with untreated wood, both have benefits. You know what’s best for you and your garden!
