Raised beds

Raised beds

  • By David Garth
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  • April 2017 - Vol 3. No.4
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  • 1 Comment

You just cannot manage a big vegetable garden next summer —  either because of sheer laziness or the need to slow down —  but neither can you do without fresh produce you grew yourself.  Or, maybe you’ve dreamed of raising your own veggies, but you’re inexperienced. One answer to these dilemmas is a raised bed garden, big enough to grow a few tomatoes, a little lettuce, maybe carrots or beets, but not overwhelming you with work.  Note the following advantages of a 4X8 raised bed:

  • Less space to plant and weed with higher yields
  • You don’t stoop and bend down so far
  • Soil warms faster and works easier, starting the growing season sooner
  • Less water and exactly where you need it, while having better drainage
  • Raised beds can be handicapped accessible
  • Solves the problem of rocky or rugged terrain (see photo of rock walls)
  • You concentrate on best gardening practices

That last benefit depends, of course, on diligence; but sometimes it helps to practice on a small project before biting off too much.  The first step is choosing a place where your bed will get at least 6 hours of sunlight, preferably more, and frequent, accessible watering.  The soil will dry out faster because the bed gets moisture exclusively from rain or from your efforts.  A couple of two-gallon watering cans applied daily usually will take care of a single bed even in dry spells; and you don’t want to carry water too far.  Water and sun are essential to a good site.

Materials: Constructing a raised bed involves some choices if you go with wood.  There’s lumber treated with chemicals that prevent rot due to contact with the soil, and this lumber lasts longer, but those chemicals leach into the soil.  The good news is that wood treated with arsenic-based chemicals, such as chromated copper arsenate (CCA) are not available for residential use, but there’s less consensus about how much harm the newer chemicals might do to our health.  To be officially organic in Washington State for example, no pressure-treated wood can have contact with the soil or a growing medium. “The National Organic Program Part 205.206f clearly states, ‘The producer must not use lumber treated with arsenate or other prohibited materials for new installations or replacement purposes in contact with soil or livestock.’”  (see the Alabama Extension and the Fine Gardening articles listed below).

Perhaps the most toxic options are old creosote-treated crossties or utility poles still oozing tar, used by gardeners for years; but the risk of contaminating the soil is greatly reduced after they weather extensively.  It’s small comfort that most plants taking up harmful chemicals would die before producing anything for people to eat.  The bottom line is that all treated lumber leaches, although the effect on food seems minimal.  That leaves several possibilities for building materials, each necessitating a compromise in durability, expense, appearance or environmentalism:

  1. Untreated lumber, preferably heartwood from red cedar, redwood or cypress.  Some recommend balsam fir and juniper.
  2. Treated lumber wrapped with heavy plastic on the sides touching the ground.
  3. Concrete blocks; 16 standard blocks make a 4X8 bed.
  4. Brick or stone, which necessitates thicker walls.
  5. Mounding dirt on four sides of the bed, similar to a “hill” for potatoes or cucumbers.
  6. Plastic wood or wood-plastic composite materials, sometimes available as scrap.

A simple wood frame might have sides made from 1 x 12 or 1 x 8 inch stock held together at the corners with 2 x 4 posts.  More durability can be achieved with 2-inch thick lumber for the sides.  The 8 or 12 inch height is a choice that depends not only on how much bending the gardener wants to do, but also on the amount of soil available to be placed inside.  A 4×8 foot bed uses standard length lumber, but the size really depends on your needs for gardening and landscaping.

 

Try not to skimp on the quality of soil placed in the bed.  Although the bottom of a raised bed is usually open to the ground, most of the nutrition and moisture for your plants will depend on what soil and water you add inside the frame.  Working the ground where a raised bed will be sited assures        a) better transfer of ground moisture into the bed, and b) space for deep-rooted plants to grow.  A good mixture of composted manure and bagged soil, all weed free, will help insure good results.  Local dirt may be used if you don’t mind a few weeds; but in any case, the addition of a little vermiculite or perlite to our red clay helps keep the earth more friable for the plants and makes your work easier.

Ideally, leave 3 feet for walking or kneeling on all four sides.  The great thing about raised bed gardening is the ease with which plants can be reached for weeding, cultivating, watering, and harvesting. Paths outside the bed can be made softer underfoot with mulch.  Wood chips may be free for the asking when a neighbor is having a tree pruned.  Grass paths are another way to go.

Options:  There’s no magic size for a raised bed.  It certainly can be smaller and more like container gardening using concrete or other heavy planters.  Higher beds can provide gardening for those in wheelchairs or on walkers.  They do require more soil.  Albemarle High School has accessible beds constructed from heavy timbers that are three feet high.  Whatever size or arrangement you choose, keep in mind how accessible the garden will be for the intended gardener.  Some people make their whole garden an array of these beds.  For comfort, you can rest a flat board across the frame, providing a temporary seat.

What to grow  A 4X8 bed can easily support three or four tomato plants, especially when staked or caged.  Most salad greens do well and afford much satisfaction.  Carrots can be fun to grow in the amended soil since mine typically struggle in our local clay.  Note that the soil in a raised bed will dry out more quickly in summer heat, thus requiring more frequent water; therefore mulching around plants will help.  If leaching of treated wood is a concern, be aware that root crops generally absorb the chemicals, although peeling the vegetables removes most of the residual contamination. The main limitation on crops is that taller plants such as corn or okra need more root structure, limiting the size of the crop; but the addition of posts for pole beans or hoops to protect late veggies from frost give added flexibility.  

Since a raised bed offers more intensive gardening, interplanting species that mature at different rates becomes possible.  For example radishes planted next to beans will be ready to eat before neighboring beans attain full height for harvest.  What could be more appealing than a well-tended raised bed overflowing with fresh produce all summer?

Not only vegetables, but also flowers, herbs and other ornamentals can be grown in a raised bed, adding beauty and creativity to any layout.  Beds can be large containers, contrived from salvaged materials or built in interesting shapes and sizes.  Vines and other greenery can fill in the spaces between blossoming flowers that may be changed to keep the display attractive over the season.  Especially for those of us who plan to age in place in our gardens, it’s nice to take a break from weeding while we watch the bees pollinate our lovely flowers.  

 

References

“Treated Wood in the Garden,”  Alabama Coop. Ext. (2013)

“Are Pressure Treated Woods Safe in Garden Beds?” www.finegardening.com/pressure-treated-woods

“Raised bed lumber, pressure treated safe?”extension.oregonstate.edu/question-of-the-week/raised-bed-lumber-pressure-treated-safe

“Environmental Soil Issues: Garden Use of Treated Lumber,” extension.psu.edu/plants/crops/esi/treated-lumber

“Raised-Bed Gardening,” extension.missouri.edu/p/G6985

“Gardening in Raised Beds,” Univ. of Fla. Ext.edu (2016)

“Using Treated Lumber in the Garden,” http://web.extension.illinois.edu (2015)

“Gardening in Raised Beds and Containers for Older Gardeners and Individuals with Physical Disabilities,”  www.hort.vt.edu (1995)

1 Comments

  1. Kirk Barley

    There are a number of products that may give us an alternative to treated wood for our raised beds. I have not found any certified by the US or Canada, but I’ve also failed to find that they use any prohibited chemicals. They all use similar titles, here is one example: “ECO-SAFE Wood Treatment – Stain & Preservative by Tall Earth … Non-Toxic/ VOC Free/ Natural Source.” All appears to be made in Canada and the various products are available through Amazon, Home Depot or direct.

    The product comes as a ~1/4th cup of powder, to be mixed with a gallon of water. The mixture, now in solution is applied to the wood using a brush or sprayer. Wood to be in ground contact should be soaked. Once dry, they are ready for your raised beds.

    This seems an acceptable compromise to exotic woods or plastic wrap, but it may not be. I’ve reached out to Organic.Org and made other inquiries to discover if there are any undisclosed risks from using this somewhat expensive ($20/gal) product in a vegetable garden. Let me know if you discover any concerns.

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