Fall flowers and vegetables only live a season, but they can benefit greatly from amended soil. Gardeners who take the time to amend their soil will be rewarded with a bountiful fall vegetable harvest and more blooms on their pansies and other fall annual flowers.
Most annual flowers and vegetables prefer moist, well-drained soils. Annual plants have a very limited root system and require consistent soil moisture to survive.
The limited root system of fall annuals means that gardeners must water these flowers and vegetables more often than perennial plants require. Because they require more water, they are more susceptible to root rot and soil-borne diseases.
Roots need oxygen to survive and grow. If a soil becomes too compacted or too wet, plant roots will die from a lack of oxygen. The best amendments for clay soils are pine bark humus (less than half an inch in diameter) and composted leaf mold.
Ideally, amendments for clay soil should help loosen compacted soil, improve soil drainage and increase soil porosity, or the amount of pore spaces in soil available for air and water.
Pine bark humus is preferred because it is cheap, readily available and easy to work with. It’s also called “bark fines,” “milled pine bark” or “finely ground pine bark mulch.”
Soil conditioners, or soil amendments, are sold under a variety of brand names at local garden centers.
These soil amendments are not the same as potting soils, which are often blended with materials like peat moss and vermiculite that help retain moisture. Potting soils are fine for use in potted plants, but not in clay garden soil. Clay already holds plenty of moisture by itself. Clay soil actually has the highest water-holding capacity of any soil in the world.
To add amendments, till the soil 6 to 8 inches deep in the area you wan to place annual plants. If the site is excessively wet, plant on a raised bed 6 to 12 inches high. The raised bed can either be enclosed using treated lumber or it can be a mound of soil called a “berm.”
Soil amendments need to be added at a minimum of 25 percent by volume and a maximum of 50 percent. This is roughly equivalent to the addition of 3 to 4 inches of soil amendments incorporated into clay soil 8 to 12 inches deep.
This provides an ideal root environment for annual flowers and vegetables. One cubic yard (nine 3-cubic-feet bags) of soil amendment will cover about 100 square feet. Mix amendments thoroughly with the underlying clay soil.
Add a starter fertilizer to annual plants immediately after planting. If needed, lime or sulfur can be added to adjust soil pH based on the results of a soil test. (Garden soil can be tested through your local University of Georgia Extension office. Call 1-800-ASK-UGA1 or visit extension.uga.edu to find the Extension office in your community.)
When planting most trees, shrubs and other perennial plants, research shows there is practically no benefit to amending the soil. The only exception to this rule is plants that must have extremely well-drained soil, like azaleas, rhododendrons, mountain laurel and blueberries.
In Georgia, most organic materials or compost added to soil will break down and decompose within a year or two. This is primarily due to rainfall and high temperatures. Most soils in Georgia contain, on average, less than 1 percent organic matter.
Therefore, those plants that live longer than a year will eventually outgrow their planting hole. The sooner trees, shrubs, and other perennial plants’ roots get acclimated to Georgia’s red clay, the better off they will be in the long run.
The greatest benefit you can provide is to dig a wide hole, at least two or three times larger than the original plant’s root system, and loosen up the clay soil with a pickax, shovel or garden tiller. No soil amendments are necessary for perennial plants.