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Raised Bed Gardening and Construction

Raised bed gardening is a great way to making gardening more efficient and enjoyable.  Building raised beds involves basic materials and construction;and if that doesn’t sound like something you are interested in doing, raised beds can be purchased from gardening supply and home improvement centers.

Benefits:

  • Easier to reach – less bending and stooping
  • More production per square foot since you don’t need the 24” – 36” space between rows
  • Better soil by making your own soil mix
  • Improved drainage, eliminating erosion
  • Less weeding.  After the plants grow, there is little room for weeds.
  • Less water usage – you use less water because there is less space.
  • Easy pest control – trellises and fences can be installed to keep out pests
  • Harvesting is easier and less messy

Construction:

Materials you can use:

  • Pressure treated lumber (use products made after 2006 to avoid arsenic and other poisons)
  • Cedar, Cypress, Redwood and other outdoor woods (can be expensive)
  • Plastic, stone and composite materials are now available, but can be expensive
  • Cinder block, brick and stone

You can also construct raised beds with no material at all.  Just make mounds 12” to 24” high with 45 degree slopes; rake the top so it is flat.

If you use wood, make sure it is a minimum of 1 ½” wide (i.e., 2 by 6, 2 by 8 or 2 by 10 dimensional lumber).

Do NOT Use:

  • Freshly treated railroad ties because they contain creosote and other toxic materials
  • Pressure treated lumber made before 2006 because it may contain arsenic and heavy metals
  • Concrete blocks because they will raise the PH of the soil (calcium carbonate will leach from the block, making the soil more alkaline)

You can also purchase ready-made raised beds.

Raised Bed Design:

Width is the most important dimension and will depend upon your reach.  The general rule is no wider than 4 feet and a minimum of 12 inches deep.  Soil depth needs to be a minimum of 10 inches and you will be filling the bed to within 2 inches from the top of the bed.  Length is not a factor, but you will need to add side braces for beds 8 feet or longer.

Special considerations:  For individuals in wheelchairs, the maximum width is 3 feet.

Construction:

If you are using wood, you can use nails or deck screws.  If you are using nails, a power nailer is a handy tool to have available.  If you use deck screws, use the screws with square drivers because they are easier to install and remove.  Both nails and screws should be galvanized or stainless.  If you are using nails, use the Spiral type.

constructing a raised bed

If you are stacking 2 by 6s or 2 by 8s, staple plastic on the inside to prevent soil from escaping.  Also, if the soil under the bed is poor, it is a good idea to line the bottom of the raised bed with heavy fabric–do NOT use plastic because it will interfere with drainage.

Corner braces should be a minimum of 2″ wide, preferable 4 x 4’s. Sharpen one end to make it easier to pound the braces into the soil.  Trellises can be used, either as a separate narrow bed, or attached to the side of another bed.  If a trellis is used, the bed can only be reached from one side, therefore adjust the width.  Special Anchor Joints and Stacking Joints can be purchased instead of braces at a cost of about $10 each.

If you have a problem with burrowing animals, install chicken wire on the bottom of the bed and extend about 3 inches up the side of the bed.

Coat the wood with a non-toxic sealer.

Location:

Raised beds should be located in an area of full sun for a minimum of (6) hours when growing vegetables.  Some vegetables the leafy ones (lettuce, Spinach, Cabbage, Broccoli etc.) can get by in partial shade followed by root vegetables, (potatoes, carrots, beets).  All get love the sun.   Flowers can get by with afternoon shade (after 2 pm.)  Do not locate you bed near the drip line of trees, and at least 100 feet from Walnut Trees.

The area should be fairly level, adjust beds if necessary.   Orient the raised bed in a North/South direction if possible.   As a practical matter, locate your beds near a water source.

Site Preparation:

Kill turf if it exists by placing moistened layers of newsprint (not colored) down to smother grass before installing beds or install a heavy fabric cloth.  Level the site, or you may have to add additional lumber to the raised bed to make it level.   Add sawdust, straw, wood chips, or mulch to the space between beds to prevent weeds and grass from growing.

Soil:

Soil warms up faster in a raised bed so the growing season begins earlier, of course this means it cools down faster in the fall.  Get the soil tested, but be aware some testing facilities cannot give a proper test if organic materials such as mushroom compost are used.   Soil for most vegetables should have a PH of approximately 6.0 to 6.5 (very slightly acidic).  If no soil test is taken, apply 10-10-10 fertilizer per 100 square feet of garden area.

To increase the soil PH, use lime.  To decrease PH, use sulfur or aluminum sulfate.  Follow the instructions on package.  Additions of sphagnum peat moss (rather expensive), well rotted manure, leaf mulch, mushroom compost also help the soil fertility.  Work in 2 to 3 inch layer.  Fill the bed to within 2 to 3 inches from the top.

A mini tiller for mixing the soil in a raised bed works great or if you need the exercise shovels and hoes work great.

Drainage:

As soil is above ground, heavy rains drain off faster, standing water is basically eliminated.  Use straw or pine needles mulch to help retain moisture.

Watering:

Be aware that vegetables in raised beds require more watering than those planted in the field.   If you have the time, watering by hand is best, and you can also weed at the same time.  Watering at the roots is thought to be best because wetting the leaves increases diseases if done late in the day.  If you water the leaves, do at a time that they will dry out before nightfall.

It is difficult to water the base only of small plants.  Besides what do you do if it rains?  If I were a plant I would enjoy being watered in the heat of the day.  Soaker hoses and drip irrigation are additional options.

Pest and Insect Control:

Pest control is easier as you have a smaller area to view and control.  Some flowers and herbs control pest, including marigolds, garlic.  See the list of companion plants (insert link)

Planting/Harvesting:

Some plants do not work well in a raised bed primarily because of their size.  These plants include okra, corn, sweet potatoes and squash.  Otherwise, you can plant much closer together in a raised bed.  The traditional gardening spacing the rows are 24 to 36 inches apart.  This spacing is not required in raised beds.  The table below shows which plants are compatible in the confined space of raised beds:

plant compatibility in rasied beds

Planting storage onions 4 inches apart in a raised bed that is 4 feet by 8 feet will give you 230 storage onions.  In a standard garden with only 24 inches between rows will give you 46 storage onions.  The average increase in quantity is approximately 2 to 5 times as many vegetables depending upon their size.

Watch for future workshops on constructing raised beds.  Raised bed workshops are generally held in the spring of each year before planting season begins.

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