Plant Answers  >  Successful Raised Bed Gardening


Return to Gardening Columns Main Index

Successful Raised Bed Gardening

There are some soils in Texas that are so bad they shouldn't be talked about in public. Sending a sample of such soils for analysis would result in a conveyance of condolences and the recommendation for improvement would be to add everything but sand and rocks. If things are really bad, a "For Sale" sign is enclosed.

There are gardeners who do not have the luxury of being able to collect enough loose soil in their planting area to have it tested. These poor unfortunates have to call in explosives experts to till their gardens! They don't "dig up the garden" since rock doesn't dig. Then there are those growers who have to specialize in aquatic plant production because of the swamp-like conditions that exist in their garden. Some folks do not appreciate the opportunity of snorkeling for garden fresh produce!

How can problems such as rocky soil and poor drainage be overcome for optimum production? The answer is simple – you have to elevate the situation ? move up in life. You must rise above your problems. Leave the disastrous rock and water below. You must build a raised bed garden!

The advantages of raised or elevated planting beds are obvious. For example, there are no rocky encounters, it’s high?and?dry and the soil type has not been predetermined for you. Yet, there are some cautions.

First—consider the location. As with all gardens or planting beds, optimum production will only be realized in a full-sun situation. This means 8 to 10 hours of sun during the day. Shade during the evening hours will enhance summer production. Since a raised bed does involve considerable construction expenses, don't put your money in the wrong place—in the shade.

Once you have the ideal location you will want to choose the best possible filler. Remember, most gardeners build raised beds to get away from less?than?desirable soil situations. It would be foolish to not carefully choose the best possible soil. Many times, heavy soils will not drain properly and subsequently prevent optimum growth and production of plants when used in a raised bed situation. It is a good rule of thumb to consider raised bed gardens as large containers. Just as you should not use heavy, clay soils for potted plants, you should not use this type of soil for raised garden beds. A well?draining, organic rich soil that is free of weeds, weed?seed, roots and pathogens should be used.

This may be easier said than done. If you cannot locate such a soil you may have to mix your own. Strive for one part washed sand, one part soil and one part organic material (compost, bark, peat moss). Another caution: “buyer beware!” Do not telephone your local soil dealer and ask for a loose, well?drained garden soil ? they all have what you need if you have the money. Many times the "sandy loam" that you buy can be used for either the garden or to make adobe bricks. Go out and examine the "garden soil" before it is dumped on your lawn. If it is not as organic or as sandy as it should be, the seller may be convinced to custom mix you a batch. Even if you have to pay a little extra, it will save you years of misery and disappointment. Also, you will find that front?end loaders available where soil mixes are sold are much faster and easier than a strong hand shovel and weak back!

Now that you’ve chosen the ideal site and found an appropriate soil mix, it is time to prepare the site. Remove from the area all rhizome?type grass (Johnson grass, bermuda, St. Augustine carpet grass, nut grass, etc.). Do not think for a minute that you can "cover it up" and kill it. Some of these grasses grow through concrete and will certainly penetrate and proliferate in your loose, fertile garden mix. Remove these grasses by digging them out or using chemical treatments. Chemical treatments used must be non?residual so that plant phytotoxicity will not occur after planting. The product which is safe for this procedure is glyphosate (Roundup, Kleanup, Doomsday, Weed and Grass Killer). Glyphosate is effective only against actively growing weeds and grasses. Glyphosate requires several weeks, depending on stage of growth and time of year, before the kill is complete.

Place a retaining wall around the area using railroad ties, treated lumber, tile, rocks or other suitable materials. Railroad ties have been used for many years to construct raised planting beds. Their popularity is due to a variety of reasons, but primarily for their low cost and desired "old and weathered look."

The genuine railroad tie also has been treated through and through with preservatives, so they seem to last forever. In addition, the ties are easily transported, easily handled by two workers and can be cut to various lengths. Ties have therefore been used as curbing, walls, steps or planters.

Because of their popularity and the need for top quality timbers, coupled with the growing scarcity of usable old ties, new ones have become more available. These new timbers vary in dimensions much as the older ones do, but usually are sold as 6 inches by 8 inches by 8 feet. Landscape timbers that are even smaller in dimension have become available at garden centers and lumber yards. The new landscape timbers and railroad ties also consist of various kinds of wood, both hardwood and softwood.

The best, naturally resistant woods are red cedar, redwood, cypress and black locust. The least resistant ones are primarily softwood species such as pine, spruce and hemlock.

However, no tree species wood will last for many years if not treated with a preservative. Even the best species, if available, will eventually decay, especially if the wood is in direct contact with moist soil.

Once the retaining wall has been erected, seal all cracks to keep out creeping rhizome?type grasses. Begin several inches below ground level (where the wall rests on the surface) and seal up to the desired height of 6 to 8 inches. The depth of the raised bed is up to the individual. Plants perform best with a root growth zone of 10 to 12 inches. Also, timbers will tie together stronger when stacked 2 or more high. Fill to within an inch or 2 from the top of the retaining wall.

How much soil will be needed to fill a raised bed growing area? This, of course, will depend upon the square footage (length times width) of the area, plus depth. The following table is a quick and easy guide to help you determine how many yards of soil to buy.

MATERIAL COVERAGE GUIDE

(approximate cubic yards required)

Depth Required, in inches *

Depth Required, in inches *
Area
Square Feet
1 inch 2 inches 3 inches 4 inches 6 inches 8 inches
50 4 cu. ft. 8 cu. ft. ½ yd. ¾ yd. 1 yd. 1-¼ yds.
100 8 cu. ft. ¾ yd. 1 yd. 1-¼ yds 2 yds. 2-½ yds.
150 ½ yd. 1 yd. 1-½ yds. 2 yds. 2-¾ yds. 3-¾ yds.
200 ¾ yd. 1-¼ yds. 2 yds. 2-½ yds. 3-¾ yds. 5 yds.
300 1 yd. 2 yds. 3 yds. 3-¾ yds. 5-½ yds. 7-½ yds.
400 1-¼ yds. 2-½ yds. 3-¾ yds. 5 yds. 7-½ yds. 10 yds
600 2 yds. 3-¾ yds. 5-¾ yds. 7-½ yds. 11-½ yds. 15 yds.
800 2-½ yds. 5 yds. 7-½ yds. 10 yds. 15 yds. 20 yds.
1000 3-¼ yds. 6-½ yds. 9-½ yds. 13 yds. 19 yds. 25 yds.
1500 4-½ yds. 9-½ yds. 14 yds. 19 yds. 30 yds. 40 yds.
2000 6-½ yds. 13 yds. 19 yds. 25 yds. 38 yds. 50 yds.

 

The above chart is based on the formula: V = LXWXD divided by 324 when V is Volume in cubic yards, L is length of garden in feet, W is width of garden in feet and D is depth of soil required in inches.

For example, if you want to build a 200 square foot (10 feet wide and 20 feet long) planting bed which is 2 crossties or 12 inches deep, you need to order 7? ½ yards of soil (6 inches for 200 square feet is 3? ¾ yards; for 12 inches you need twice 3? ¾ yards or 7? ½ yards).

The width of the area should be given careful consideration. Planting beds are usually spaced 2 to 3 feet apart so increments of that measurement should be considered. Also, remember that crossties are 8 feet long so increments of 8 feet widths will be convenient unless crossties are to be cut. If you’re making a small raised bed area, consider a 4 foot width. All areas of a raised bed garden that is only 4 feet wide can be conveniently reached from one side or the other.

Just because you have a Cadillac of a garden doesn't mean that you won't have a wreck. Raised bed gardens do offer the advantage of height, which means that you can sit on the wall while gardening or harvesting. However, these gardens are not high enough to prevent insects from flying in and depositing eggs, or the wind blowing in fungus. Some people claim that fungus problems seem to be lessened because of better air circulation. However, air circulation depends mostly upon the planting system (space, height arrangement, foliage density, etc.). Also, plants that are reaching optimum growth have a certain amount of natural pest resistance.

Remember, too, that high performance vehicles such as a good garden soil mix must be maintained. Follow the standard cultural practices of adding more organic material, seasonal and side?dress fertilization and sanitation (removal of dead or pest?ridden plants). Raised bed gardens can make your plant-growing life much easier and more productive.

 

 


Listen to the Garden Show live!
Saturday & Sunday from Noon-2PM
Call (210) 308-8867 or (866) 308-8867
and have your gardening questions answered
- during show hours ONLY -
Milberger's Gardening South Texas
on 930 AM THE ANSWER
Hosts: Dr. Calvin Finch, Dr. Jerry Parsons, and
Milton Glueck, radio personality and host
Last weekend's shows ON PODCAST
Podcast Logo
Milberger's Specials
On Sale This Week | Newsletter Signup
Local Gardening Events
Open 9 to 6 Monday-Saturday & 10 to 5 Sunday
3920 N. Loop 1604 E.  San Antonio, TX 78247
Phone: (210) 497-3760
Three exits east of 281, inside of 1604.
Next to the Valero station.
Email Us | Map & Directions
Copyright © 2017 PLANTanswers.com - All Rights Reserved. PLANTanswers and PLANTanswers.com are trademarks of Jerry Parsons.