Weekly Express-News Article
Saturday, December 31, 2005
By Calvin R. Finch, PhD, SAWS Water Resources Director, and Horticulturist

“Building a Raised Bed”

            If you have rocky or heavy clay soil like most San Antonians and you want to raise flowers or vegetables you may want to build a raised bed garden.  A raised bed garden allows you to have a planting bed filled with good soil.  You can raise more flowers and vegetables in a small raised bed that you can in a large garden of very poor soil.

 

            The raised bed can be elaborate, but it does not have to be.  My favorite raised bed construction material is used railroad ties.  They are heavy enough to hold the soil in place without anchors.  I place them in a row on relatively level ground in full sun.  Level the ground with a shovel to eliminate the highest peaks and backfill to fill in low spots on the ground where the ties will be laid end to end.  You can use a string or eyeball the ties to get a relatively straight bed. 

 

Fill the resultant bed with landscape soil from Keller's, GardenVille, Fertile Garden Center or another source.  Be generous with the soil because it will settle as much as two or three inches.  Fill the beds to the brim on the edge with a slope to a high point in the center of the bed.  For every eight inches of bed, one railroad tie high, 1.5 cubic yards of soil is required. 

 

            Landscape soil is approximately one-third sand, one-third soil and one-third compost.  The soil needs to be heavily fertilized because the commercial compost is usually only partially composted and may be dominated by sawdust, which produces a nitrogen deficit for about two years.  Add one cup of slow release lawn fertilizer per eight feet of raised bed garden every eight weeks for two years. 

 

            Railroad ties are very heavy so this building option may not be the best for you.  Treated landscape timbers (three pieces high) also work.  Put stakes at the four corners of the proposed bed and tie a string to the stakes to match the borders of the timbers.  Each timber is 3.5 inches tall so the string should be at that level on all the stakes.  The idea is not necessarily to make the timbers and raised bed absolutely level, but it is relatively important for aesthetics and timber fit that the bed be rectangular with 90º angles.  Use a square to check the 90º on the string if you do not trust your eye. Like with the railroad ties it is easiest to do dimensions in multiples of eight feet, but landscape timbers are easy to saw with a chain saw or hand saw, so a four feet width is easy to construct.  Remember to add 3.5 inches to the outside dimensions when setting a string to allow for the thickness of a timber and to allow overlap. The advantage of a four feet width is that you can reach in to work the garden with out having to be on the soil.

 

            Unlike railroad ties, landscape timbers are not heavy enough to stay in place without fasteners.  The easiest fasteners are five inch spikes pounded into the top timber and through to the one under it.  Lay the first layer on the ground using the string for a guide.  Rough leveling can be done by shoveling off high spots and back filling low spots.

 

            Place the second layer on the first layer so that the pattern results in a Lincoln log like overlapping arrangements.  That means that there is overlap at the corners so that the spike interlocks the whole row and not just the timber below.

 

            The end pieces whether they are 4 feet or 8 feet need to be staggered to allow for an overlap with the second row.  Do this by having the end pieces extend to the outside boundary of the bed on one side and butt up against the side timber on the other side.  If you do the same for the other end of the bed except at the opposite side, you create the opportunity for overlap in the second row.  Do the same for the third row.  Fill with soil and you are ready to plant. 

 

            Other raised bed materials that work relatively well are cement blocks, treated lumber, 6-8 inch rocks and plastic timbers manufactured for the job.  Research completed in San Antonio documented that the treated railroad ties; timbers and woods were safe for vegetable gardens.