With its bluish green hue, St. Augustine grass (Stenotaphrum secundatum) prefers hot summers for the most vigorous growth. This warm-season grass acclimates well to U.S. Department of Agriculture plant hardiness zone 8 through 10, but can create problems when paired with nearby trees. Competition for natural resources, from sunlight to moisture, creates a garden battle that is quite often detrimental to both plant species.
St. Augustine prefers a full sunlight location, but it can tolerate some shade. Cultivars "Seville" and "Floraverde" tolerate shady conditions the best, according to the University of Florida. If you regularly prune your trees, sufficient light should penetrate down onto the grass below. However, dense tree shade causes widespread grass dieback -- turf cannot photosynthesize adequately for vigorous growth. To combat this problem, maintain your trees as airy canopies. Also, allow your grass to grow between 3 and 4 inches tall. The extra grass blade height provides more surface area for photosynthesis and successful growth in shady conditions.
Competition for soil resources is one of the major issues between St. Augustine grass and trees. Because tree roots remain in the top 24 inches of soil, they naturally spread amongst the grass roots. As a result, moisture, oxygen and nutrients are constantly fought over between the root systems. To compound the soil problem, St. Augustine is known for its thick thatch layer. Composed of decaying stems and foliage, thatch forms a barrier to incoming rainfall and soil air circulation. If your grass is not properly maintained, both the trees and turf suffer from nutrient, oxygen and moisture deficiencies.
As breezes pass across an open turf area, water evaporates from soil and grass blades. This air movement prevents bacterial and fungal infections from settling into the turf. However, if several trees are planted near your St. Augustine grass, this can prevent normal air circulation. Tree leaves and limbs create a barrier to the wind as it redirects the air flow. As a result, the grass cannot evaporate as quickly and may harbor pathogens and pests that feast on stressed turf. Pruning your trees and spacing them correctly in the landscape allows the turf to gain access to air flow and healthy evaporation.
As St. Augustine grass grows, it spreads stolons, or above-ground stems, across the topsoil. If you do not maintain the turf's perimeter, the stolons grow up to the trees' trunks. Because the grass remains directly under the canopy, it may turn brown from lack of water -- the leaves and limbs stop direct rainfall to the grass below. For the best separation between turf and trees, spread organic mulch on the ground beneath the tree until it reaches the drip line. With a 2- to 4-inch mulch layer, the stolons cannot grow under the tree and compete directly with feeder roots.