Bermuda and St. Augustine grasses are warm-season turf types that are commonly grown in the southern parts of the United States. Both grasses provide distinct benefits for gardeners; for example, bermuda grass survives high levels of foot traffic, while St. Augustine grass performs well in shade. Lawns that have a mixture of both must use the right amount of nitrogen to encourage healthy growth.
Fertilizing
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Both Bermuda and St. Augustine grasses need nitrogen to grow green foliage and spread across the lawn. Excessively fertilizing either one of these grass types results in stressed-out root systems, which leaves the grass vulnerable to diseases and pest infestations. Contributing less than the recommended amount of nitrogen will not harm grass, but may not give gardeners the thick, lush-looking lawn that they desire. Fortunately, there are proper amounts of nitrogen that work for both grass types.
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Fertilizer
Bermuda grass needs between 4 to 6 pounds of nitrogen per 1,000 square feet each growing season, according to Texas A&M University. St. Augustine grass needs between 2 to 5 pounds of nitrogen per 1,000 square feet each growing season. By using 4 to 5 pounds of nitrogen on Bermuda and St. Augustine grass mixture, gardeners contribute enough nutrients to make both grass types healthy. Other factors determine how much nitrogen to use on St. Augustine and Bermuda lawns, such as shade.
Shade
Bermuda grass does not perform well in the shade, which is why gardeners plant their St. Augustine grass under trees and other shady areas. Grass grown in shade requires less fertilizer than grass grown in sunny areas. Gardeners must reduce their fertilizer amount by one-half to two-thirds when fertilizing grass in shade. A limited amount of fertilizer to St. Augustine grass grown in shade reduces incidences of disease. Shady environments are breeding grounds for fungal diseases. Fertilizer causes stress to grass' root systems, because it forces the grass to produce top growth. By limiting the amount of fertilizer in shady areas, gardeners reduce the stress on their St. Augustine grass.
When to Fertilize
One of the benefits of growing a mixture of Bermuda and St. Augustine grass is that they both can be fertilized at the same time in the late spring. However, gardeners must wait for their lawns to green up before applying fertilizer. Applying fertilizer to dormant grass encourages weed growth, because the grass is unable to absorb the nutrients. Waiting for the lawn to become at least 50 percent green reduces the weed population for the year.