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Originally published April 18, 2007 at 12:00 AM | Page modified April 18, 2007 at 2:01 AM

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6 good reasons for raised beds

Pop quiz: What's the Holy Grail for Seattle-area vegetable gardeners? If you said heat, you win. Short of moving to Arizona, how can we...

Special to The Seattle Times

Pop quiz: What's the Holy Grail for Seattle-area vegetable gardeners? If you said heat, you win.

Short of moving to Arizona, how can we triumph over our cool maritime climate and snag that golden chalice? If you answered raised beds, you're right again.

Here are six reasons why a raised bed is a great choice for food gardening in our region:

1. Warmer soil. A raised bed will shake off the soil's winter chill faster in the spring because of sun exposure, air circulation and better drainage. Solar heating is enhanced if the walls of the raised bed are made with stone or concrete.

2. Quicker drying. Your raised bed will be ready to plant earlier in the spring because the soil will dry sooner. Less compaction allows better drainage and evaporation out of the planting zone. However, this also means your bed will dry out sooner after watering, so keep a close eye on your plants' needs and use soaker hoses to water well.

3. Better drainage. When you create a raised bed and use the proper mix of soil components that are not too heavy with clay or loose with sand, you create a loamy layer of earth through which water can move.

4. Less compaction. "A good soil contains 50 percent air space," says vegetable gardening guru John Jeavons in "How to Grow More Vegetables." This is key to good root development. Since you don't generally walk on raised beds, its soil is not damaged by compaction.

5. Solves problem areas. Do you have a heavy clay soil, or one filled with rocks? Instead of breaking your back and taking years to build the soil into what you need, simply place a raised bed on top of it and fill the bed with the type of soil you want. Caveat: The bed must be deep enough to accommodate the root zone of your plants. Many vegetables have a 6- to 8-inch root zone, but some perennials need 12 to 18 inches.

6. Pest and weed control. A raised bed is easier to weed because the soil is generally looser, so weeds come up easier. Also, since you're not planting in rows and walking between them, you can plant closer together, leaving less space for weeds to sprout. Finally, if you have a raised bed made of wood, you can staple a continuous copper strip to the wood edge, which organic gardeners have found effective as a slug barrier.

How to create your own

There are two basic methods for making a raised bed: mounding the soil and building a structure. Mounding the soil is the easier method and requires no building skill or materials, while a structure with walls can be higher and warmer.

Mound the soil

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To create a mounded raised bed, measure and stake the area first. Raised beds should be between 3 and 6 feet wide. Make them wide enough so that you can reach to the center from either side and not step into the bed for planting, weeding or harvesting.

Next, prepare the ground. If you are adding new soil on top, use a garden fork to loosen the ground before piling on the new mixture.

Consult with a garden center or provider of topsoils and garden mixes to choose the right blend of soil, compost, manure and other amendments for the type of garden you're creating. It will take a surprising amount of material to significantly raise the soil level, so provide your measurements to that company to ensure you order enough.

If you want to blend the existing soil when adding amendments, consider "double-digging" the plot. Use a square-ended garden spade to dig a trench, piling the soil along it. Loosen the soil at the bottom and mix in organic matter and soil amendments. Dig a second trench next to the first one, depositing soil into the first trench and mixing in more soil amendments. Complete the process across the entire bed. In the last trench, mix in the soil that was dug up first.

When the mounded bed is complete, it should have a flat top and gently sloping sides.

Build a box

Building a box for your raised bed can be a source of artistic expression. It can also cause frustration for people with limited construction skills. Raised beds can be built from wood, engineered lumber such as Trex, bricks, sheet metal, steel, cement blocks or salvaged material, like concrete chunks of an old sidewalk broken into uniformly sized pieces.

If using wood, start with a 2-by-10 or 2-by-12 board on its edge, setting the bottom in a shallow trench a bit below grade. Attach it to sturdy corner posts, and you will have a bed 10-12 inches high, great for most vegetables. Be wary of treated lumber that may leach chemicals into your food crops, and realize that your wood frame may last only five years before rotting. Some garden centers and catalogs sell pre-formed corner pieces and even raised-bed kits.

Build block walls with stackable cement blocks found at large home centers and stone yards. Many of these have interlocking features and are quite easy to use. Consult with the manufacturer or seller about preparing the ground for a level, sturdy wall.

Once the structure is in place, prepare the ground as outlined for the mounded beds, or fill it with the amended soil mixture.

A well-formed raised bed will provide many benefits to the vegetable gardener. It may not cause the sun to shine, but it can solve problems related to our climate and soil.

Bill Thorness is a freelance garden writer in Seattle: bill@thorness.com

Resources

"How to Grow More Vegetables: (and fruits, nuts, berries, grains, and other crops) than you ever thought possible on less land than you can imagine," by John Jeavons.

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