For:
By: Daniel E. Mullins
Extension Horticulture Agent
Those Landscape “Sweet Spots” Can Give Gardeners Fits
Most landscapes have a sweet spot or two and unlike those found on baseball bats and golf clubs, they are generally not desirable. These are areas where the grass, flowers or shrubs turn yellow and decline even though adequate maintenance is being provided.
Such spots where plants struggle are usually found near the foundation of houses with masonry exteriors, near concrete driveways, sidewalks and where seashells have been used as mulch. These are areas where the soil is extremely alkaline or “sweet.” In horticultural terms, the pH is sometimes above the acceptable range for normal plant growth.
The alkaline condition can usually be traced to high calcium containing materials that were used during construction. Bits of mortar, when left on the soil around the house foundation, can constantly release calcium and keep the pH elevated indefinitely. Calcium also leaches from concrete and seashell drives and walks with the same result.
The effects of this soil condition are greatly magnified when acid loving plants such as azaleas, hollies, gardenias and centipedegrass are grown in such areas. These plants have great difficulty taking up iron and certain other trace minerals under alkaline soil conditions because they are chemically bound.
In general, it is extremely difficult to permanently reduce the alkalinity in these areas simply by applying a chemical treatment such as a sulfur compound. The constant release of calcium from the various sized bits of calcium containing debris rapidly raises the pH to its former level soon after each application.
Dealing with Sweet Spots
● Following construction, remove remaining mortar pieces, as well
as debris from the edges of driveways and sidewalks. Consider skimming away the upper 2 inches of
soil containing these fine particles and replacing it.
● Have the soil tested to determine if the pH is excessively high in the problem spots.
● If plants are already established and have begun to show nutrient deficiency symptoms, try applications of a chelated trace mineral product containing iron, sulfur, manganese and other minor elements. These can be obtained in dry or liquid form. Follow label directions carefully.
● Use coarse organic mulches such as bark or pine needles. A 2 to 3 inch layer on the soil surface helps to slightly acidify the soil and moderate stressful conditions.
● As a last resort, replace plants with those species that better
tolerate alkaline soil. Your
Question of the Week: There is a large yellow flowered shrub that I see growing in several
yards in my neighborhood. It is blooming
in September and has long formal looking spikes. What is this plant?
Answer: The plant that you have described is probably one of the Cassia shrubs. There are several species of these fall flowering plants. This is probably the species commonly known as candelabra plant. It can be found for sale at some nurseries and garden centers.