Base temperature (T_b) for leaf appearance and the thermal time interval between appearance of successive leaves (phyllochron) were determined for six common weed species from a series of growth chamber experiments. Mean leaf appearance T_b values for giant ragweed, velvetleaf, redroot pigweed, large crabgrass, woolly cupgrass, and wild-proso millet were 1.3, 8.0, 8.5, 4.5, 2.2, and 5.1 C, respectively, and mean phyllochron values were 37.2, 34.4, 17.3, 42.2, 65.2, and 34.2 growing degree-days per leaf, respectively. Phyllochron values increased slightly with mean temperature for each weed species. To our knowledge, these are the first reported leaf appearance T_b and phyllochron values for giant ragweed, woolly cupgrass, and wild-proso millet. The results confirm leaf appearance T_b and phyllochron values reported previously for velvetleaf and redroot pigweed. However, large crabgrass leaf appearance T_b and phyllochron values in our study varied somewhat from those reported previously. The results provide information needed for parameterization of plant growth models that predict leaf development based on thermal time.

Nomenclature: Giant ragweed, Ambrosia trifida L. AMBTR, large crabgrass, Digitaria sanguinalis (L.) Scop. DIGSA, redroot pigweed, Amaranthus retroflexus L. AMARE, velvetleaf, Abutilon theophrasti Medicus ABUTH, wild-proso millet, Panicum milliaceum L. PANMI, woolly cupgrass, Eriochloa villosa (Thunb.) Kunth ERBVI