Abutilon theophrasti is one of the worst agricultural weeds in North America, yet it has not reached that status in California in the 80 yr since it was first reported. The research reported here examined the distribution and modeled climatic requirements of A. theophrasti to determine whether it is likely to spread more widely in the state. Herbaria records and weed literature were surveyed to determine the historical occurrences of A. theophrasti in the state; current distribution was assessed through surveys sent to University of California personnel in each county. Combined results showed 42 counties out of 58 with A. theophrasti present historically or currently. A plot of the cumulative number of counties containing A. theophrasti by decade fit a logistic equation. The maximum rate of spread of this species occurred in 1962 and it is likely that its final distribution by county in California is leveling off and not likely to increase further. The climate-matching/mapping software CLIMEX® was used with observed and estimated parameters of environmental requirements of A. theophrasti to model its current distribution from India through China to Japan. The same model parameters were then used to map its potential distribution in California. Areas where A. theophrasti has been reported were predicted by CLIMEX to be poorly suited for its growth and development without added soil moisture in the form of irrigation. It appears that the Mediterranean climate is a deterrent to the integration of A. theophrasti into California. The climate-matching approach provided a biologically reasonable assessment of potential distribution of A. theophrasti in California. The approach also allowed assessment of the effects of common agricultural practices on potential distribution given the environmental requirements and limitations of A. theophrasti.

Nomenclature: Abutilon theophrasti Medicus, velvetleaf, ABUTH.