Lawn Care/Turfgrass Growth & Physiology
Turfgrasses used for lawns, athletic fields, and on golf courses can be divided into two main groups, warm season grasses and cool season grasses. As the names indicate, the cool season grasses grow more vigorously at cooler temperatures while the warm season grasses grow better at warmer temperatures. Examples of cool season grasses include Kentucky bluegrass, perennial ryegrass, tall fescue, fine fescue, and creeping bentgrass. The major warm season grasses are Bermudagrass, St. Augustinegrass, centipedegrass, zoysiagrass, and bahiagrass. Cool season grasses dominate throughout most of Indiana and Illinois where warm season grasses may be considered as weeds. Warm season grasses are better adapted to far southern Indiana and Illinois, and turf managers can maintain zoysiagrass and Bermuda grass satisfactorily in this region. Growth and development processes for turfgrasses are similar regardless if they a warm season or cool season grass. The differences in physiology between warm and cool season grasses will be discussed later.
Turfgrass plants are made up of two components, the shoot system and the root system. The shoot system is responsible for manufacturing the food supply that is used by the plant, while the root system is responsible for taking in water and nutrients. Grasses are often evaluated by how the shoot system looks but the root system is actually more important in maintaining healthy turf.
Seed germination is the first step in developing a new turfgrass plant. Before germination can begin, several conditions must be present. First of all, there must be a viable or live seed. When turfgrass seed is first harvested in the Pacific Northwest, it will not usually germinate, but must undergo a series of changes which are called after ripening. This has usually occurred by the time the seed has been cleaned, packaged, and shipped East. A guideline for storing seed for an extended period is to make sure it is not exposed to a relative humidity plus temperature combination in excess of 100. That is, if the storage room is 70°F, do not allow the relative humidity in the room to get above 30%. A more practical solution for most of us is to simply have the seed delivered when we plan to use it.
Assuming high quality seed is used, the conditions at planting must be right before seed germination can occur. Seed must be exposed to the correct temperature and have adequate moisture and oxygen to germinate (some turfgrass species also require light to germinate). The highest percent germination of cool season grasses occurs with warm days and cool nights. The temperature program used in laboratory tests of the germination of Kentucky bluegrass is one in which the seed is given eight hours of light at 86°F and 16 hours of dark at 59°F. Unless a soil is waterlogged, there will be enough oxygen available for germination to proceed. Moisture (water from the surrounding soil) is the single most important factor for germination to occur. If the seed starts to germinate and then dries out, germination will be reduced. The use of light frequent irrigation along with a mulch during turfgrass establishment helps insure that the seed does not dry out. Cool season turfgrasses are usually planted in the early fall because temperature and moisture conditions are optimal during this period and there is less pressure from annual grassy weeds such as crabgrass.
Placement of grass seed at the proper depth (1/8 to 1/4" deep) is of utmost importance since placing a greater depth may result in the food reserves in the seed being used up before the new shoot can reach the soil surface and start making its own food.
The newly emerged seedling to be made up of two parts, the root system and the shoot system. The root system shortly after germination is composed of the primary or seminal root system. This root system is active for only a few weeks and is composed of only a few roots. Approximately two weeks after germination, the seminal roots start to decay and the plant starts to develop an adventitious root system. The adventitious root system is so named because it does not arise from root tissue but rather from stem tissue. The adventitious roots, located at the base of the shoot, serve the plant's needs for the rest of the plant's life. This system is characterized by extensive branching and is very fibrous in nature. One researcher, who examined the root system of Kentucky bluegrass, found that in one cubic inch of soil there were 2,000 roots with 1,000,000 roots hairs. The combined length of all the roots was 4,000 feet and the surface area of the roots amounted to 65 square inches.
Turfgrass species have either an annual or perennial root system. This means that some species regenerate almost their entire root system every year while others, with a perennial root system, retain a portion of their roots on a year to year basis. All roots have a limited life span, the perennial vs annual designation refers to the time period over which the roots are regenerated. Perennial ryegrass has an annual root system while Kentucky bluegrass has a perennial root system.
