Growth and Development of Arizona Cottontop (Trichachne Californica [Benth.] Chase)

Arizona cottontop is a productive, palatable, dominant perennial grass on many southwestern semidesert ranges. Although cottontop is a warm-season grass and grows mostly during the summer, new basal shoots normally start their development in the spring growing period, go dormant during the May-June drought, resume growth with the onset of summer rains, and produce an inflorescence. Most culms produce branch shoots from axillary buds, primarily on the upper part of the culm. These axillary shoots may mature during the same summer or go dormant over winter and mature the following spring or summer. Cottontop culms live more than 1 year, some 3 or more. Numbers of internodes on mature reproductive culms vary from three to 21. Generally, the shortest internodes are at the base of the culm, internode lengths increase gradually toward the top; the top two, and especially the topmost (panicle internode), are greatly elongated. An axillary bud generally is present on each internode of the culm except the panicle internode. Axillary buds at the base of the culm sprout primarily in the spring, producing the year's crop of basal culms. Axillary buds above ground sprout throughout both the spring and summer growing periods, depending on the availability of soil moisture. The first structure to elongate in a newly developing shoot is a leaf blade. The youngest six leaves of a vegetative shoot are in the process of elongating at any one time, of which only the oldest two or three exceed 1 mm in length and are elongating rapidly. In the field, new leaves are initiated at the rate of one about every 4 days. The lengths of leaf sheaths and leaf blades vary with position on the culm, but in different ways. The top, or flag-leaf sheath, is always the longest (to 202 mm), twice as long as the second sheath and three times as long as the third. The longest blade (to 173 mm long), however, is generally on one of the leaves between the third and the sixth from the top, inclusive, and blade length decreases toward both the top and bottom of the culm. Most florets that reach anthesis flower during the first 3 hr after sunrise on the part of the panicle that has emerged from the sheath during the previous 24 hr. Inflorescences begin emerging from the boot 2-3 weeks after growth starts in the summer. The first panicles to emerge are always on culms that started their development the previous fall or spring. New panicles continue to be produced as late in the fall as soil moisture is available. Individual basal culms, during their 3-4-year life, produce up to nine panicles. Perennial culms, long-lived roots, and the ability to produce new shoots quickly and to flower seasonlong make Arizona cottontop an exceptionally valuable forage grass for the semidesert.