Raise Your Beds

Raised-bed gardening can be thought of as a hybrid between gardening in the ground and gardening in containers, and it has many of the same advantages as both of these methods.  Raised beds are essentially a small retaining wall of some sort - wood, brick, masonry blocks, even hay bales can work, and in some cases you might even see tires used for this purpose.  Sometimes the tires are painted white first - it all depends on the look you might be going for.  Regardless of your chosen materials, once your wall - or frame - is built, it is then filled with a good planting soil or compost mixture, and then your raised bed garden is ready for planting!

A raised bed garden is very appropriate when your underlying soil is either very hard, very rocky, very weedy, or infertile.  It gives you the opportunity to bring in the best garden soil or compost you can find (or you can afford) instead of relying on the dirt you happen to be blessed with.  Unlike container gardening however, a raised bed has a larger soil source available for root growth, for insulation against heat or cold, and for absorption and availability of water.  Four things are basic to building a raised bed garden: finished height of your garden, subsoil preparation, building the frame, and choosing your soil.

Finished Height of your Garden
Desired height of your raised bed should be decided upon before you begin, as it will determine how you accomplish all other steps.  The higher your garden, the less you have to work the subsoil.  If you are raising your bed a minimum of 24", your subsoil will only need a little scarifying.  As roots of vegetable plant thrive best when given at least 18" of friable soil, any height less than 18" would necessitate some digging or tilling of your subsoil.

The height of your garden will determine what type of materials you use for your frame - and vice versa - your preferred materials will influence the exact height of your garden.  For instance, if you are using simple wooden boards for your frame, they can be purchased in say 6", 8", 10", 12" or more widths.  Basic hollow concrete blocks are roughly 7 1/2" tall.  Two of them mortared together will provide a height of 15 1/2".  As you formulate in your mind the height you're going for, make sure the materials you might consider will easily accomplish your goal within budget.

The taller your garden frame, the more area you will have to fill with garden soil.  Simple mathematics can help you get an idea of how much soil you'll be needing.  Measure the ground area your garden is intended to cover.  If you're building a frame that is to be 4' by 8', you'll have 32 square feet of garden area.  If this frame is to be one foot tall, you'll need 32 cubic feet of soil.  If it is to be 2 feet tall, you'll need 64 cubic feet.  If purchasing bagged soils or compost, look on the bag for a determination of how many cubic feet of medium is included in each bag.  If purchasing garden soil or compost in bulk, you'll divide the number of cubic feet by 27 in order to communicate your needs in cubic yards.  If you're going to be shoveling your own compost from a local horse farm or the like, measure the interior of the truck bed you intend to use and compare the cubic feet with your garden needs to see how many truckloads you'll need to shovel.  Keep in mind that regardless of the soil volume you determine, your soil will settle throughout the growing season and you'll likely need to add soil or compost to your garden after each of the first few planting seasons. 

I strongly recommend that if you are planning out a raised bed garden, you carefully consider garden height with regards to your budget.  I know from experience that it's very easy to underestimate the amount of framing material or the amount of garden soil you'll need to accomplish your goal.  Again - if the work or costs seem to get too high quickly, start small.  Begin with your intended garden height, but go with a smaller square footage to start out with. 

Subsoil Preparation
Subsoil refers to the surface of the soil on which you plan to build your garden.  At the least, you'll want to remove any existing grass, weeds, or other vegetation from the soil surface and scarify, or scratch up the subsoil.  Removing any existing plants will prevent undue settling, and scratching up the soil's surface will improve drainage.  Imagine that if the surface of your subsoil were rock hard and you built a solid garden frame and filled it with soil, it would have the potential of holding in too much water similar to a container garden with no drain holes. 

Again, vegetable plant roots thrive when they have at least 18" of loose, friable soil, and so if your finished garden height is to be 12", you'll need to loosen the top 6" of your subsoil.  If your garden height is to be 6", you'll need to dig up or till 12" deep into your subsoil.  Keep in mind that this 18" would be measured from the top of the soil in your finished raised bed garden, which will likely be an inch or so lower than the finished edge of the frame.

If your decision to opt for a raised bed garden is based on an undesirable weed problem within your subsoil, you may wish to consider some sort of landscape fabric to be placed as a barrier between your subsoil and the fresh soil you are bringing in.  In this case you should be planning on providing a healthy 18" soil depth for your garden, and you should purchase some quality fabric.  A couple of years ago I went searching for some good fabric to use for a similar application and I was completely disgusted with what I found at Lowe's and at Home Depot.  The stuff tore easily when I pressed my fingernail into it, and I do not keep my fingernails long or sharp!  Instead I called a landscaper friend of mine and asked him if he had a spare length of silt fence - and that did the job I needed it to do!

Building Your Frame
Budget is often the guiding factor in choosing materials and construction methods for a good garden frame.  If your budget is very small, consider looking around online for a potential source of free concrete blocks, bricks, wood, or landscape timbers.  Pre-formed masonry blocks intended for use in building retaining walls are excellent choices and there's a good chance you could find enough left over from a contractor's job to meet your needs.  If your budget is substantial and you're looking for a permanent, professional-looking frame, mortared brick or concrete masonry finished in stucco is perfectly suitable; however, when constructing a solid masonry wall that is more than about a foot in height, you'll want to make sure that appropriate drainage holes are fashioned along the lower rows of bricks or blocks so that excess water does not undermine the construction over time.

When using wood there are the two options of natural wood or pressure-treated lumber.  Pressure-treated lumber has had chemicals applied to the wood under high temperature and pressure and these toxic chemicals (they used to include arsenic) have been proven to leach into garden soil.  It will have an obvious green color to it and will have labels or stamps indicating it has been treated.  Most if not all lumber sold as "landscape timbers" are highly impregnated with chemicals, and true railroad ties contain the additional evil of creosote.  Thus using natural lumber is a safer choice and the only choice if your desire is to garden organically, but you cannot count on natural lumber to give you a permanent garden structure.  If using natural lumber for your frame, you'll need to have in mind replacing a board here and there beginning in the 3rd or so year after starting your garden.  Nails work just fine in holding such a garden frame together; screws work even better.  Just make sure with either fastener that it is suitable for outdoor use so you'll get the longest life possible out of them.

Another great and easy way to put natural lumber together is this great little deal from Gardener's Supply Company:  

You simply slide your boards into the slots to form a corner.  They are also available in 10" and 12" heights, and in a version that pivots so that you're not restricted to right angles.  The finished garden can look something like this:

Choosing Your Soil
If purchasing bagged soils for filling your raised bed, I'll again recommend a 1/2 and 1/2 mixture of mushroom compost (or some other composted manure or organic matter) and soil conditioner.  If purchasing your soil in bulk where it will either be loaded into your own truck or delivered to your home in a dump truck, you'll want to be specific in what you ask for.  Simple "topsoil" is decent but is likely to have a high clay content and possibly a high content of weed seeds.  Often a mix of topsoil and compost will be available, and this is very suitable.  Compost alone will also likely do the trick.  Question the manager carefully about sources of topsoil, weed seed content, sources of compost, and whether the soil has been treated chemically in any way or if any chemical fertilizers have been added.   If either of those last two points are affirmative, it's not all bad, depending on your slant towards growing it all organic.