So, you’ve done some research and decided that raised garden beds are for you. Now you need to decide what lumber to use, and get some tips on just how to construct your beds.
Wood Characteristics You Should Look for
The wood that you use is going to be constantly wet and will have soil against it. You need something that will stand up to this abuse and that, at the same time, is safe for your produce. Pressure treated wood may seem like a great choice. It’s readily available and is meant to survive for years in the ground or in applications where it is constantly wet. However, you need to be aware that heavy metal chemicals are used to treat the wood to make it rot-, decay-, and bug-resistant. This is true even for the “new, safe” pressure treated wood that has been for sale since 2003. These toxins will leach into the soil around them and will be absorbed by plantings in that soil, i.e., your vegetables!
The wood you are looking for will be naturally rot- and bug- resistant. What does this mean? The resins in certain species help the trees defend themselves against the effects of nature, namely decay and pests. The lumber from these species will retain these desirable qualities.
You don’t need to have planed lumber – buying it in the rough will be more cost-effective, and the dimensions will be “true” (i.e., your two by six will actually measure two inches by six inches.) Just make sure you wear gloves if you want to avoid splinters.
Green or unseasoned lumber is fine for raised beds. Kiln-dried lumber has had most of the moisture extracted from it in order to keep it from shrinking and cracking in interior applications. Your lumber will be exposed to water, which has a 100% moisture content, and the summer air (think of the relative humidity number in the weather forecast). Kiln drying is one fewer process to pay for.
Three Great Lumber Species to Use
1. Native eastern hemlock
In New Hampshire, one of our most prolific indigenous species is eastern hemlock. We have found that, at a fraction of the cost of cedar or fir, it will last in the ground up to seven years. It is definitely the local favorite wood choice for raised bed gardening.
Eastern hemlock is also local and ecologically responsible in the following ways.
The logs are harvested and sawn here in New Hampshire
The sale of hemlock supports the local economy at every level of production
It avoids the cost and pollution of long-distance transport
When it decays or is discarded, an indigenous species is put back into the earth
2. Douglas fir
Douglas fir, or Doug fir, is another great wood to use. It is readily available locally, and although it is largely grown in the western states, the trees are often planted in New Hampshire.
There are several grades of Doug fir available. The most cost-effective is essentially a mill-run grade that allows for sound tight knots. This means that the knots will not fall out and create holes in your piece of lumber. Even this grade will cost approximately four times the cost of the eastern hemlock.
3. Western red cedar
Also grown in the western regions of the United States, western red cedar is readily available. If you are locating your raised beds more as planter boxes on your deck, you may want to consider using a knot-free grade, usually denoted by “A+” or “CVG” (clear vertical grain).
If you still like the idea of the red cedar, but not the price tag on the higher grade, take a look at the mill run or knotty grade. It’s still pricey, at a little more than six times the cost of the Eastern Hemlock but is a fraction of the cost of the clear grade.
Size of Your Raised Bed
A common width for raised beds is four feet. This makes the entire bed within your reach. But you need to be mindful of the length of your raised beds. The longer the bed, the greater the outward pressure from the soil. The maximum length of each bed should be six to eight feet. If you want it longer, you should add interior partitions. Construct the interior partition the same way you created your end wall.
When assembling your raised bed, screw-type fasteners are the way to go. They should be at least 3 ½ inches to four inches long and have these features:
Big heads or washers under the heads to resist the outward pressure of the soil.
Deep threads for better holding power. Self-cutting threads are the best.
Galvanizing or special coating for longevity.
Lumber supply stores have many types of fasteners. Ask for coated structural screws
Lydia West is the Chief Financial Officer at Goosebay Sawmill and Lumber located in Chichester, NH. Learn more at www.goosebaylumber.net.