20 January 2010
In The Garden With the Fluvanna Master Gardeners
By Irene C. Burke - Fluvanna Master Gardener
Fluvanna County Extension Office: 434.591.1950
For plants that require 8 12 vertical inches of well-drained soil, the raised bed meets the challenge of Virginias piedmont clay. But the frame of raised beds proves to be equally as important as the contents.
Site, shape and dimensions
A four-foot wide bed makes it easy for you to reach plants from all sides. The beds length depends on your garden design and location: Sun, slope, drainage and prevailing winds.
Consider the novel keyhole-shaped garden with an eight-foot diameter, pioneered by the charity Send-a-Cow, for the people of Lesotho. This gardens compost pile fills the round section of the keyhole with walk-about room at its edges. Entry to the keyhole is from the long portion of the hole.
Though raised beds can be as simple as an earthen mound, the framed bed provides a more durable solution to maintaining and refreshing the dirt. There is a lot of discussion about the best materials to use for your raised beds frame synthetic or natural.
Synthetic materials include creosote or pentachlorophenol-treated wood, which you should avoid, for they damage plants as the chemicals leach into the soil.
In May 2009 the revised Virginia Cooperative Extension (VCE) fact sheet on raised beds said that, pressure-treated lumber is toxic in vegetable gardens since it contains copper arsenate compounds. But it adds that Research at Texas A&M found that arsenate compound movement in soils was insignificant. However, in 2003, the lumber industry voluntarily adopted a resolution to use an arsenic-free preservative.
VCE recommends alkaline copper quarternary (ACQ) treated lumber in lieu of chromated copper arsenate (CCA), or ammoniacal copper arsenate (ACA). The Environmental Health Department of Thurston County, Washington State, recommends copper borate azole (CA) treated lumber.
The University of Missouri Extension says that Old, discarded (railroad) ties do not injure plants. However, injury may occur if ties are still oozing black, sticky creosote or smell intensely.
The U.S. EPA does not have any warnings for plastic lumber. For example: Heavy-duty vinyl from recycled sources, recycled lumber made from HDPE (recycled milk jugs) with added colorants and UV inhibitors; UV protected polyethylene; recycled plastic from proprietary blends of plastics, which include post-industrial, post-consumer waste, wood flour and virgin plastic; curved composite wood grain timbers, made from wood fiber and recycled polyethylene, with joints made from recycled ABS plastic.
These materials are often touted as green because they are made from recycled products. Though risks are not apparent at this time, I havent seen anyone questioning the manufacturing process or primary sources of these plastics. At this writing Ive seen no published studies testing recyclables for soil contamination.
The EPA is silent on the use of poly vinyl chloride resin (PVC), fiberglass or corrosion resistant aluminum for garden bed frames.
To side-step the uncertainties of green synthetics, use rot-resistant wood like cedar, or bricks, rocks, or cement blocks.
Cement blocks will leach lime into the soil making it alkaline, which may inhibit the growth of plants, that prefer acid soil, but alternatively it will favor plants that need neutral or base soil.
Whatever you choose, like a wise consumer weigh risks against benefits.
Tip of the Week
2010 is the International Year of Biodiversity. To keep insect pests and disease to a minimum, to reduce your gardening costs, plan for variety. When losses are confined to one or two plants, devastation is limited.