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July 16, 2007

Raised bed materials

As one who changed from a field-type vegetable garden to raised beds, I strongly recommend others do the same.

Raised beds have a number of advantages:

1.The plants are higher and easier to reach.
2.Likewise the undesirable plants (weeds) are easier to reach.
3.Raised beds warm up faster in the spring (soil temperature is more important than air temperature to many plants).
4. It is easier to amend soil in raised beds. Likewise for watering.
5. You can make raised beds any length you want. The width is usually no more than four feet so you can reach the middle from either side. The height can be anything you wish. I have seen raised beds of wheelchair height.
6. Having raised beds means you do not have to walk on the planting soil, thus compacting it.
7.You do not need heavy-duty tilling tools. In fact there is more and more evidence that heavy tilling, by hand or machine, destroys the soil structure and harms the tiny critters that amend the soil naturally.

The most frequently asked question is what do you use for edging a raised bed?

Pressure treated lumber will last but many gardeners prefer not to use it even in its new, more benign formulation (arsenic-based materials are no longer used in PT wood that is commonly available to homeowners)

Ordinary dimension lumber (2 by 10s 2 by 12s etc. made of hem/fir, pine etc) can be used but it will rot out in 10 years or less. If you don't mind replacing it, this is the most economical way.

At the Master Gardener Veggie Demonstration Garden on the main URI campus, we have three types of raised beds that will last a long time.

Several of them are made of Trex (a composite material that is designed for decks. It is made of wood fibers combined with recycled plastic). Trex (there are other brand names sush as Choicedeck) lasts but it has very little lateral strength and thus you have to build in vertical supports every four or five feet.

There is one bed there made of cedar. Liberty Cedar of West Kingston donated these boards which seem to be holding up well. Cedar is more expensive than PT or regular lumber and can be more expensive than Trex

Finally there is a bed made of PlasticLumber donated by PlasticLumberRI.This is a solid plastic material (all recycled stuff) which has multiple uses such as decks, bleachers, docks. Since it is solid plastic it does not get mildew (as some composite materials will) and seems to have superior lateral strength. It is also the most expensive but will last indefinitely.

This garden is located off East Alumni Avenue which is off Upper College Road. The garden is enclosed in a picket fence adjacent to the College of the Environment and Life Sciences Outreach Center (formerly the URI Cooperative Extension and Education Center).

A final note: Don't overlook materials that Mother Nature provides. If you have a lot of rocks (who in RI doesn't) use rocks to make the walls of a raised garden. They heat up during the day and give off heat at night. They look the most natural and of course they will never rot and the price is right!

Posted by Rudolph A. Hempe at 1:56 PM to How to... | Permalink

Comments

Rudi, my first post here (Art & science project: A raised bed) concerned our raised bed, made of salvaged oak floor beams from a factory being torn down. A commenter asked how we preserved it, and I offered this nontoxic wood preservative recipe from the USDA. It may be useful to your readers to repeat it here:

Non-Toxic Wood Preservative Recipe The U.S. Department of Agriculture's Forest Products Laboratory has developed this recipe for preserving lumber used aboveground (such as for fences and picnic tables). The treatment is also safe for wood to be used in the ground -- that is, it won't leach toxic chemicals into your garden soil -- and the wood will last longer than if left untreated.

Here's the recipe (and please be sure to follow it carefully):

# Melt 1 ounce of paraffin wax in a double boiler (DO NOT heat over a direct flame).

# Off to the side, carefully place slightly less than a gallon of solvent (mineral spirits, paint thinner or turpentine at room temperature) in a bucket, then slowly pour in the melted parrafin, stirring vigorously.

# Add 3 cups exterior varnish or 1.5 cups boiled linseed oil to the mix, stirring until the ingredients are blended. When it cools, you can dip your lumber into this mixture or brush it onto the wood.

Among these choices, we used turpentine and boiled linseed oil.

And, as I mentioned, we lined it with heavy plastic so the soil wouldn't come in contact with it.

Sheila Lennon | July 16, 2007 1:55 PM link

Your mention of lining a raised bed with plastic reminds me of one thing I forgot to mention--make sure a raised bed has good drainage. Some soils in RI do not percolate well and if that is the case in your yard, a layer of crushed rock or gravel beneath the planting soil is recommended.
Actually raised beds can be built on any surface--even an asphalt driveway--provided you give the bottom good drainage.
The depth of the soil in a raised bed depends on what you intend to grow. Things like lettuce don't need much depth but root crops such as turnips and carrots obviously do. Many crops need more room for their roots than it appears. The deeper the better.
One year I experimented with a double-dig bed at the East Farm Demonstration Vegetable Garden.(I'll explain this method some other time). The result was parsnips and carrots that were very long. Some carrots of a certain variety grew to 20 inches long.

Rudi Hempe | July 17, 2007 8:05 AM link

I should have mentioned we slashed the bottom of the plastic, so there'd be drainage.

Sheila | July 17, 2007 10:10 AM link

July 16, 2007

Raised bed materials

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