Root growth dominates the growth of the cotton plant during germination and seedling establishment. In fact, the taproot may be as deep as 10 inches by the time the cotyledons emerge. This is a critical time for the development of the root system. Cold soils, seedling disease, low soil pH, water stress, hard pans and herbicide injury all inhibit root growth and development, but careful crop management can minimize most of these stresses. The roots absorb water and nutrients that are vital to the development of the plant, and any hindrance of root development in these early stages of cotton growth may cause a disappointing production season.

Cotton emerges the quickest from warm, moist soil. Low temperatures (below 60 degrees F) or less than adequate soil moisture may hinder germination by slowing metabolic processes (see the discussion on heat units). Physical impedance, such as crusting, does not slow germination, but it can prevent the hypocotyl from emerging. This often causes thickening of the hypocotyl and a condition referred to as “big shank” or “thick-legged” cotton, resulting in reduced seedling vigor. Generally, the longer it takes for emergence to occur, the greater the risk of plant death and yield loss. A rule of thumb for planting cotton in most regions of the U.S. Cotton Belt is that the soil temperature at 4 inches deep should be at least 65 degrees F for 3 consecutive days, with warm temperatures in the forecast.

Figure 1. Thick shank in a cotton seedling. Soil crusting, compaction or other mechanical factors can cause thick shank.

Root Development

As the cotton plant grows, the radicle that originally emerged from the seed becomes a taproot, from which lateral roots begin to form and grow. Lateral roots and the taproot collectively make up the basal root system. Other “higher order” roots then develop from this basal root system. These higher order roots have a functional life of about 3 weeks. They form when environmental conditions are good, and then die when nutrients and water are depleted in the area in which they developed.

As the plant matures, the roots continue to spread and probe deeper in the soil profile for water and nutrients. Therefore, the distribution of roots tends to match the most fertile soil zones. Figure 2 shows an example of the root distribution of an unstressed cotton community. Most of the roots in this case can be found between 1 and 3 feet deep in the soil, but large quantities of roots can still be found more than 4 feet deep in the soil. The amount of roots generally peaks during the cotton flowering phase then declines as the plant partitions more carbohydrates to the developing bolls.

Figure 2. (a) Comparison of root quantity with soil depth for a mature cotton plant. New roots are constantly produced in areas of the soil profile that have water and nutrients. (b) Comparison of root quantity with cotton growth stage. Roots begin to decline after flowering as the cotton plant shifts its energy from root to boll development.