If decision-aid software models of weed emergence and growth are ever to help producers better time weed management, these models must be able to predict perennial weed shoot emergence from vegetative propagules. In this research, Cirsium arvense shoot emergence from adventitious root buds in spring was modeled using degree-day heat sums. Fractional C. arvense shoot emergence was best modeled as a logistic dose–response function of degree-day heat sum as follows: Y = 1.108/(1 [X/488.344]^−5.161) where Y = fractional C. arvense shoot emergence (0 to 1) and X = heat sum in degree-days above 0 C after day 91 of the year (April 1) with an upper limit of 800 degree (C) days (r² = 0.83). This empirical model was validated by graphing observed vs. model-predicted C. arvense shoot emergence using two independently gathered data sets, one of C. arvense emergence in autumn chisel-plowed Triticum aestivum (r² = 0.82) and the other in no-till fallow (r² = 0.63). The model slightly overestimated emergence at low fractional emergence (< ∼7% at 0.1 fractional emergence) and underestimated emergence at high fractional emergence (10 to 20% at 0.8 to 1.0 fractional emergence). Below an emergence fraction of 0.8, the model adequately estimated observed emergence to within about 10% of the predicted regression line. Using the model, about 1% and 80% of C. arvense shoots should emerge from adventitious root buds after a heat sum accumulates of about 197 and 587 C d, respectively, starting from day 91 of the year. Consequently, farmers should begin monitoring C. arvense patches for emergence and height growth after about 197 C d accumulate and expect to control C. arvense before about 587 C d accumulate, which is when about 80% of shoots have emerged.

Nomenclature: Cirsium arvense (L.) Scop. CIRAR, Canada thistle; Triticum aestivum L., wheat.