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 Building a Raised Bed

When you’re going to make the investment of building a raised bed for your home garden, you’ll want to consider the different types of materials you can use to construct that raised bed.
A raised bed can be constructed from almost any material as long as there is no concern about any toxins in that material.  It used to be said that we did not recommend treated lumber because it was treated with an arsenic compound.  Now, most treated lumber is done with a copper compound.  Most plants need copper, so it’s not a concern as far as being a toxin. 
The cheapest material you can use for a raised bed would be untreated lumber.  The down side of that untreated lumber is that it’s usually going to start decaying and may need to be replaced in about 3 – 5 years.
The next material you can consider is treated lumber or treated landscape timbers.  Both of those are relatively inexpensive as well.  The treated lumber is going to last longer than untreated lumber.  It can last anywhere from 5 – 15 years depending on the conditions in your garden.
From there, if you’re looking for a little more decorative or upscale material, red cedar would be a good option for a raised bed.  It’s also going to decay a little bit slower.  After that, you’ll get into materials that are much more durable and also more expensive.  Landscape pavers or concrete blocks are very functional in raised bed edging materials.  They obviously are very durable if they’re constructed well.  But, they have a little higher initial expense. 
A newer material that we’ve been using in raised beds is plastic lumber – or decking materials.  The plastic lumber is very durable, but it’s also quite expensive – especially if you’re using a thicker, higher quality plastic lumber.  The other challenge that we’ve seen with the plastic lumber is that if you get a less expensive variety, and it’s a little bit thinner, it tends to warp quite a bit when you start making it into a raised bed.  So, it can be a good option for a raised bed, but you need to be aware that it’s going to need more bracing on the interior to hold it in it’s shape.
Generally, we don’t recommend any specific liner in a raise bed.  The exception would be if you were building on a site that has known soil contamination.  Perhaps it may have been an old industrial site that is known to have heavy metals in the soil.  That would be a time that a weed barrier fabric would be an appropriate thing to use as a liner.  In most cases, raised beds are intended to improve the existing garden soil.  So, you don’t want a barrier between your existing soil and your raised bed.  You want to add to your existing soil with improvements like compost and additional topsoil to fill your raised bed, rather than creating a barrier and creating a large container.
When you’re getting ready to build your raised bed, you’ll want it to be in a location with as much sun as possible.  You’ll always want a full sun location.  As far as the dimensions go, we generally recommend a bed that’s not more than 4 to 5 feet maximum in width.  This is because you want to be able to reach to the middle of the bed without having to walk in the bed.  Then, you have to consider length.  The length is very flexible according to the space you have in your yard.  You should consider that if you’re not walking in your bed, that means you have to walk around it if you need to get to the other side.  So, it may be easier to make several shorter beds with walkways in between, rather than making one really long bed.
This feature story prepared with Rebecca McMahon, Kansas State University Research and Extension Horticulture Agent, Sedgwick County. For more information, visit your local county extension office or visit our website at KansasGreenYards.org.