Jamie Buhlman, University of Florida | IFAS
One of the most popular turfgrasses for lawns and landscapes is St. Augustinegrass. It is easily differentiated from other warm-season turfgrasses by its larger stolons and coarse-textured leaves. Visually differentiating between cultivars of St. Augustinegrass is challenging, but there are a few tips that can help.
A St. Augustinegrass cultivar is a vegetatively propagated clone, meaning that each plant or propagule of a given cultivar is genetically identical. Despite being genetically identical, the same cultivar may vary in appearance based on its environment. Environmental variation can be caused by abiotic factors such as soil type, soil moisture, nutrition, day-length, or shade or by biotic factors, such as the presence of pathogens or pests. Some examples of environmental variation are yellowing and stunting of a grass due to low nitrogen levels, and elongation and thinning in shade. All of these factors can make it difficult to visually identify a St. Augustinegrass cultivar, as the grass may not be exhibiting characteristic features due to its environment.
In addition to environmental variation, there is also genetic variation between cultivars. These are differences that can be seen between two cultivars when they are exposed to the same environment and are the result of differences in genes. Genetic variation may only be notable under certain environmental conditions, such as cold hardiness after a frost or disease resistance in the presence of a pathogen. Genetic variation may also be relatively consistent across a wide range of environments. Both types of genetic variation can be used to help identify and differentiate between St. Augustinegrass cultivars. However, the former is only useful when the correct environmental conditions are present.
Morphological features are often used to differentiate between St. Augustinegrass cultivars. These can be broadly grouped as either vegetative or reproductive. Vegetative morphology includes stolons, leaves, and roots. Vegetative stolons, often called ‘runners’, are stems that grow horizontally above ground and send out roots and new shoots at nodes along the stem. The portion of stem in between the nodes is called the internode. Stolons often vary in overall length, the amount of branching that occurs at the nodes, and internode length, thickness, and color (Fig. 1). Leaves can vary in color, as well as in length and width. The width of leaves is referred to as the texture of the grass, with fine-textured grasses having narrower leaves and course-textured grasses having wider leaves.
Morphological features that have genetic variation also frequently display environmental variation, which can lead to some confusion. Excessive shade can cause elongation of stolons and leaves, drastically changing the appearance of vegetative morphology of a cultivar compared to growth in full sun. Likewise, differences in nutrient availability and disease can significantly impact the overall growth and look of many St. Augustinegrasses. When trying to compare vegetative morphology of two or more cultivars, it is essential to look at these features in a nearly identical environment. The most ideal situation is to compare the morphological features of cultivars growing side-by-side.
Reproductive morphology refers to floral structures. A grass flower is referred to as an inflorescence or seed head. The type of inflorescence produce by St. Augustinegrass flower is termed a spike (other grass species may produce a raceme or panicle time seed head) and is produced at the terminal end of a stolon. Reproductive stolons grow vertically before producing an inflorescence and are typically thinner than vegetative stolons for the same cultivar. The inflorescence emerges from a terminal leaf sheath, called the flag leaf. The main body of the inflorescence is referred to as the spike, and within this are multiple partially embedded spikelets. Each spikelet bears two or more florets, which look like small seeds. From each floret emerges the stigmas and anthers of the flower. The stigmas are the female part of the flower and look like small feathers. The anthers are the male part of the flower and look like two small slippers on stalks. (Fig. 2).
St. Augustinegrass inflorescence morphology shows some environmental variation, especially in regard to the overall size and height of the inflorescence. However, they also show considerable genetic variation which is relatively stable across a wide range of environments and has been shown to be very useful in differentiating between cultivars.
One of the most easily distinguishable inflorescence characteristics is stigma and anther color. St. Augustinegrass stigmas vary from white, to white and purple, to completely purple. Anthers range from pale yellow to dark golden, and yellow mixed with purple (Fig. 3 and Fig 4). Stigmas and anthers that have emerged in the morning have vivid colors which fade throughout the day. However, the general classification of the stigma color does not change. White stigmas do not turn purple with age; they only turn brown and shrivel. Completely purple stigmas emerge a dark purple, again shriveling and turning brown with age. The white and purple stigma category shows the most variation. Purple coloring may localize at the tip or base of the stigma or be dispersed throughout. The overall amount of purple color in a white and purple stigma may also vary based on the cultivar.
Many of the cultivars available in Florida’s market are easily distinguishable from one another based on key vegetative and reproductive traits. Below are descriptions of six cultivars and how to distinguish them. Although these guidelines are useful, there are limitations. There are several ways in which unusual genotypes may be introduced to established turf-grass, especially if St. Augustinegrass has been present at the site for many years. In a home lawn, it is always possible that multiple types of St. Augustinegrass have been planted at the site. St. Augustinegrass is native to the southeastern U.S.; mature, viable seed from wild genotypes can be introduced and established in new areas via flooding during storms. Likewise, mature seed from fertile cultivars can fall to the ground, germinate, and establish, resulting in a grass that is not true-to-type. Finally (and least likely), mutations may build up in vegetatively propagated turf, resulting in new traits. Off-types may be easily distinguished from a known cultivar or appear extremely similar, especially if the two are closely related.