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Wood is an excellent material for constructing homes, furniture, and sheds, but there are several reasons why it is ill-suited for creating raised beds.
Microorganisms play a pivotal role in transforming dirt into soil. A soil’s carbon content is essential as it supports soil biodiversity. Microorganisms consume carbon, reducing complex compounds into a bioavailable format, and decay-resistant wood destroys them.
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Why You Should Reconsider Using Wood in Your Garden Beds
There’s no denying it; wood is fabulous. Antique furniture pieces are a testament to wood’s versatility, beauty, and durability. For millennia humans have used wood to construct, craft, and build – from ships to shelters.
But some reconsideration is needed when using wood directly with the soil we want to grow plants. This piece explores seven reasons why wood is inadvisable to shape raised beds.
1. The Beneficial Microorganisms in Your Soil Destroy Wood
Nature is highly effective at recovering waste and reusing it to benefit life. In the life cycle, everything that lives eventually dies. When it does, there are mechanisms to ensure that any remaining energy and minerals stored in the organic matter are made available for use by living organisms.
All energy is sourced from the Sun. Even fossil fuel is a storehouse of the Sun’s energy, able to be released by combustion. Plants are the primary absorbers of the Sun’s energy on the Earth’s surface, storing that energy as carbon.
Animals (including humans) get their energy by consuming plants, plant-consuming animals (herbivores), or animals that feed off plant-consuming animals (omnivores and carnivores). All the predator/prey cycles transfer the Sun’s stored energy to sustain life.
Once a plant dies or digests, plant matter is excreted, and saprophytic organisms decompose the carbon (stored sun energy), producing CO2 and releasing water (reversing photosynthesis).
Any additional mineral compound (including nitrogen) in the dead matter is eventually released into the soil, making it available to plants (bioavailable) and avoiding their eventual depletion.
So, when you add wood to your garden bed, nature’s response is to break it down into CO2 and water and extract all the minerals that may be stored in that piece of wood; it’s nature’s way.
Nitrogen is a bit like Steven Spielberg’s E.T.; it wants to go home. Home is the atmosphere where our air is 78% nitrogen. If we do not have a diverse and abundant population of microorganisms in our soil, nothing will use that nitrogen for cell growth, and it will fit into the atmosphere.
But because these microorganisms use nitrogen for cell growth and carbon as food, the carbon is broken down, and the nitrogen is captured in their bodies. Diversity ensures a balance between predators and prey. As the billions of microorganisms go through their cycles of eating and being eaten, the nitrogen in their bodies is released to become available to plants.
They also produce glue that helps the soil aggregate, preventing erosion and enhancing water and air retention capacities. The carbon also improves the cation exchange capacity of the soil, the ability to hold some cations in suspense closer to the roots, increasing their bioavailability.
Microorganisms are also responsible for actively forming partnerships with plants, trading sugars for nitrogen, and even creating a communication network. Research shows that these networks are responsible for transporting nutrients from one plant to another but also trigger defenses against pest and disease attaches.
In short, microorganisms are essential to soil and plant health. Wood is a natural food source for microorganisms and will decay unless treated to kill them. The speed of decay depends on water and air availability, both of which should be abundantly available in healthy soil.
Allowing the killing of microorganisms is short-sighted and will negatively impact your plants’ health, productivity, and resilience, not to mention the impact on soil health.
2. Wood is Treated to Destroy Your Soil’s Biota
Scientists have long sought to extend the durability of wood, mainly to protect it from water, microorganism, and insects. If these natural processes of decay are interrupted, wood can last for centuries.
To understand some of the tricks used to protect wood, we need to explore the building industry, the primary consumer of treated wood.
Wood Preservative Pesticides
The main types of wood preservative pesticides are fumigants, oil-based, or water-based. The efficacy of each substance in the different categories depends on the type of exposure and their respective levels of toxicity.
Wood exposed to the ground below the surface has a chance of decaying and needs heavy-duty preservatives. The wood is classified as having a high decay hazard and usually applies to wooden utility poles or other buried or partially buried wood.
The wood used above the ground (low decay hazard) does not usually require pressure treatment but will have surface treatments of different strengths, depending on the environment. Railroad ties, called sleepers, are hardwoods (usually teak) saturated in creosotes.
Saline or marine exposure has high decay hazards and needs heavy-duty preservative or dual treatment. Some active microorganism-killing ingredients can be used in both oil-based and water-based preservatives.
Treatment efficacy is affected by numerous factors, including formulation choice, application technique, sapwood-to-heartwood ratio, wood moisture level, amount of preservative retained, chemical penetration depth, and dispersion throughout the wood.
Most wood products used in construction and other applications likely to be attacked by insects, fungi, and marine borers require pressure treatment.
Oil-based Preservatives
- They are highly toxic to fungi and insects.
- They are vicious and need to be dissolved in petroleum or oil.
- Have a strong odor that may irritate plants, animals, and humans.
- Tend to bleed or leak toxins into the soil.
Structures such as fence posts, railroad ties, and other similar items are often treated with CCA (chromated copper arsenate). Coal tar distillate, or creosote, is a thick, oily substance that increases the wood’s resistance to the elements, protects it from insects and fungi, and makes it more water-resistant.
It is used for fence posts, poles, pilings, and timbers for building railroads. It contains arsenic and should not be used near vegetable beds.
Crystalline pentachlorophenol (Penta) is a soluble chemical in light petroleum oil. Lumber, timber, utility poles, cross-arms, and fence posts can be treated with Penta.
Naphthenate of a copper solution protects poles, pole tops, beams, and timber from rot, termites, and mildew. COP-R-NAP (77 percent copper naphthenate) is diluted with a hydrocarbon solution (diesel fuel, mineral spirits, etc.) and applied to the wood’s surface.
Water-based Preservatives
- The principal active ingredients are copper, chromium, arsenic, and fluoride combinations.
- Water-borne preservatives are used for plywood, lumber, fence posts, timber, pilings, and poles.
- Are odorless.
- It may consist of acid copper chromate (ACC), chromated copper arsenate (CCA), ammoniacal copper zinc arsenate (ACZA), or toxic elements.
- Should not be used in vegetable gardens (for human health) or gardens in general (for soil health)
Internal Treatments
Borate wood preservatives are made from all-natural minerals and guard against wood-destroying fungi, termites, and more.
Several compounds combine copper and boron to eliminate fungi.
3. Treated Wood can Affect Your Health
As noted above, some wood preservatives contain actual arsenic and other ingredients that may be toxic to human health.
The guideline for organic certification warns explicitly against the use of treated wood. Still, most gardeners are unaware of how dangerous treated wood can be to their health and the health of their loved ones.
EPA Rulings
The Environmental Protection Agency (EPA) specifies different chemicals that may be used as an alternative wood preservatives. Still, the EPA has not opted to remove any wood in circulation that may harm human health.
This is because there are limited risks to using wood treated using dated methods AS LONG AS IT IS NOT USED FOR GROWING FOOD IN.
4. Railroad Ties (Sleepers) are Toxic
I love furniture made from teak reclaimed from disused railroad ties. It is dark, complex, and has the most beautiful flames. It’s not easy to work with, and carpenters are strongly advised to wear masks.
The sawdust is toxic and causes cancer, one of the reasons producers of chromated copper arsenate voluntarily stopped its use as a lumber treatment process. Still, it is beautiful.
Pesticides containing chromium, copper, and arsenic, such as chromated copper arsenate (CCA), are used to prevent termites, fungi, and other wood-destroying organisms from damaging or destroying wooden structures.
It is unsuitable for a garden frame to produce a raised garden bed. I know it’s heavy and doesn’t need spikes to hold it in place, but it destroys the soil biota and poses a real risk of harm to vegetable growers.
Levels of Danger Warnings
Pesticides are categorized as needing caution, providing a warning, or outright dangerous:
- CAUTION represents the lower toxicity products.
- WARNING indicates medium toxicity products.
- DANGER stands for the highest toxicity products.
EPA Warnings in Using and Disposing of CCA-treated Wood
- Even though chromated arsenical pesticide products are not sold to the public, chromated arsenical-treated wood could still be in circulation.
- Generally, homeowners can dispose of chromated arsenical-treated wood through regular trash collection (i.e., as municipal solid waste). However, state and local governments may have specific guidance or instructions for disposing of treated wood, so check with your state or local waste management program.
- Mulch and other products made from recycled wood should not contain wood treated with chromated arsenicals.
- As the smoke and ash from CCA or other preservative-treated wood may contain harmful compounds, it should not be burned in an urban setting.
- If you must see wood treated with chromated arsenicals, use protective eyewear and a dust mask, and wash your hands afterward.
Consider railroad ties as DANGEROUS.
5. Water and Oxygen Speed Wood Decay
Almost every issue that might arise with a wooden structure can be traced back to excess moisture. Water causes several problems for wood, including paint peeling, rust, warping, cracking, and general shrinkage.
Moisture Challenges
It is accepted knowledge that when wood absorbs water, it swells, and when the wood dries, it shrinks. However, wood will only contract and expand below its fiber saturation point, around 28% moisture content. Wood expands and contracts to depend on the amount of moisture.
Lumber surfaces at 15% moisture content or lower can be stamped MC 15 or KD 15. However, these labels reveal the timber’s original moisture percentage. The equilibrium circumstances the wood will experience in service must coincide with its moisture content.
Installed wood moisture levels for interior use, including completed trim, cabinets, and flooring, should be about 8%. However, the fate of wood exposed to the elements might be quite different.
The relative humidity of wood used for raised vegetable beds will vary continuously between 60 and the regional average. Eventually, the wood will split and rot.
Factors that Cause Rot
Four conditions enable rot to grow. They are moisture, oxygen, higher temperatures, and food sources.
For decay to take hold, the wood’s moisture content must be between 20 and 30 percent. Since wood is porous, it will take in moisture. The fungi will die if the wood absorbs too much water, making it impossible for them to get enough oxygen.
A fungus is a living entity that needs oxygen to breathe. The air must contain at least 6% oxygen for it to live. Fungi thrive at warmer temperatures. Most fungi go dormant at temperatures below 40°F/4°C.
6. Cedar is Not All It’s Made Out to Be.
Regarding decay and pest resistance in wood trim, cedar is frequently considered the material of choice. If you’re looking for a softwood that will outperform pressure-treated wood, go no further than cedar.
While cedar does an excellent job of fending off fungi and bacteria, it isn’t the greatest choice for every house. Cedar still has other flaws, which may result in callbacks and increased expenses.
If you watched the 18-minute TED-talk video shared above, you might have noticed that the experiment showed that Cedars live in their universe, not communicating with other trees (Fir and Birch in this case).
Cedar, for instance, should never be used as a mulch as it kills soil organisms and will cause an imbalance between beneficial and pathogenic microbial populations.
Eastern Red Cedar and West Coast Cedar
Juniperus virginiana (Eastern red cedar) has a high resistance to decay and can last for decades, even in contact with soil. This timber is quite solid and might be challenging to work with.
Because the Eastern red cedar is not commercially cultivated and is notoriously difficult to come by, it can be costly.
Although West Coast cedar (Thuja plicata) is widely available and straightforward, it can split if wood screws are used without pre-drilled. While sustainable production is essential, it does not come without its share of environmental challenges.
Transporting it may take a lot of gas because it is grown on the west coast. Compared to Southern yellow pine, cedar can be four to five times more expensive, and its durability in the ground is a matter of debate.
7. Even Untreated Hardwood Decays
Hardwood and softwood are not accurate descriptors. Needles-bearing trees like pine, fir, and hemlock are commonly considered softwoods. Leaves from trees like maple, ash, and cherry make their hardwoods.
On the other hand, density is the single most reliable indicator of hardness and strength. Dense woods are more challenging for cutting, milling, and fastening. Thus, this is important.
Specific gravity is the standard measure of density and is a comparison to water which is standardized as 1.0. Items less dense than water float (wood), and those denser than water (rocks) sink.
Below is a list of different woods and their respective specific gravities (densities):
- Black Ash: 0.48
- Red Maple: 0.49
- Douglas Fir: 0.50
- Longleaf Pine: 0.60
In Closing
Wood is beautiful, versatile, and great for making furniture, sheds, fences, and many other items and structures. It is, however, unsuitable for use in raised beds, especially when there are so many alternatives to choose from.
