Wood is the construction material of choice by the home gardener. Wood products used in gardening projects include timbers for separated or raised beds, posts for decorative or protective fences, latticework, boards for decks, containers, and walkways. Lumber used in an outdoor garden or landscape must either be naturally resistant to insect and fungal attack or it must be treated to provide an extended service life. The safe application of treated lumber in the home garden or landscape makes economic and environmental sense which results in the efficient use of harvested timber.
Causes Of Decay
Wood products used in gardening and landscape environments are susceptible to deterioration from insects and fungal attack. Decay organisms grow most rapidly at temperatures between 70 deg to 90 deg F, wood moisture content above 20 percent, and in the presence of oxygen. The protection requirements of wood products used in the garden or landscape depends to a large extent on end use. For example, wood used above ground is not exposed to the same level of decay organisms as wood used in-ground. It is most appropriate to use a level of treatment commensurate with the end use. Wood is most vulnerable to decay at the ground-line. Below ground-line there is insufficient oxygen to support fungal growth. Above the ground-line with proper care, wood moisture content will be too low to support fungal growth.
Outdoor Wood Products
The home gardener has several choices regarding wood products suitable for outdoor use:
- Lumber with natural preservatives
- Lumber treated by the home gardener
- Pressure treated lumber
- Treated landscape timbers
- Recycled products such as rail ties
The home gardener must also feel comfortable with issues associated with the safety of products chosen for use in the garden and landscape.
Natural Decay Resistance
Lumber or timbers containing natural preservatives can be used effectively in gardening and landscape applications. Certain lumber species naturally resist attack by decay organisms. It is important when using naturally resistant species that the lumber contains a high percentage of heartwood, the dark-colored, center portion of the tree. The heartwood contains extractives that provide the decay resistance. The sapwood or light-colored, outside portion of the tree does not contain these extractives and therefore is not decay resistant. Examples of commercially available species with natural decay resistance include redwood, cedars (western red, eastern red, northern white), walnut, Osage orange, white oak, and locust. At a minimum, these species can have a service life in excess of 10 years. However, some woods when green, like oak and walnut, have extractives that are harmful to plants.
The lumber or wood can be treated by the homeowner with a variety of available products. Since the wood preservatives used commercially are classified by the Environmental Protection Agency as pesticides, these products are not available to the homeowner. There are a number of water repellent finishes from which the homeowner can choose for preserving wood above ground. Typically, exterior finishes contain a resin or drying oil, paraffin wax, solvents and possibly a preservative that may include materials toxic to decay organisms. Exterior finishes are a product in which quality is closely aligned with price (higher price usually indicates higher quality).
Pressure treated lumber that has the characteristic green or light-brown color is the result of a treating process in which the preservative is forced into the wood and bound in the cell wall. This wood is then resistant to attack by fungi and insects. The three primary chemical preservatives are creosote, pentachlorophenol, and chromated copper arsenate salts (CCA). Most wood products marketed through retail lumber yards and home centers as pressure-treated lumber contain the preservative chromated copper arsenic (CCA). This product usually includes a 30 to 40 year guarantee. All CCA pressure-treated products will have a quality stamp that indicates the level of preservative treatment. The quality stamp specifies whether the material can be used in-ground or above ground. The absence of a quality stamp on the lumber means the product is not pressure treated. The retailer will also provide a Consumer Information Sheet (CIS) that includes guidelines regarding the handling and use of the products treated with preservatives.
A product developed specifically for gardening is the treated timber that may be advertised as “landscape timbers.” This product is the unused center of a log resulting from plywood manufacturing. The product is machined flat on two sides and rounded on two sides. These products are usually treated, however they are not pressure treated and therefore do not conform to the standards required for extended in-ground use. This product will not contain a quality stamp, however it will be treated (soaked or dipped) with preservatives. Use and handling precautions are similar to pressure-treated products. The expected service life of this product is difficult to predict. Due to the high percentage of heartwood, an estimated in-ground service life would be five to seven years. Tests of untreated southern pine posts resulted in a service life of slightly more than three years.
Treated rail ties and utility poles are examples of products taken out of service and often marketed to the homeowner for use in landscaping. These products have been pressure treated with one of the three majors chemical preservatives pentacholorophenol, creosote, or chromated copper arsenic (CCA). Most often, the preservative used is either creosote or pentacholorophenol and therefore these recycled products are not recommended for use in gardening applications.
Chemical products forced into lumber as preservatives will leach out into the soil and water. The important question is whether chemicals leached from treated lumber used in a raised bed represent concentrations sufficiently high to cause damage to plants or humans. The use of CCA treated lumber has been reported as safe to humans and plants (Hickson Corp., 1993) in raised bed applications. Human health problems attributed to treated lumber have occurred in treating plants and manufacturing facilities. Concerns regarding the use of CCA treated lumber in garden applications have focused on potential hazards from the toxic chemicals rather than demonstrated results. Specific guidelines must be followed when using treated products. These guidelines, developed by the Environmental Protection Agency, are available as Consumer Information Sheets from the building products supplier.