Crown bud, tiller, and rhizome development was studied in 10 quackgrass biotypes at two stages (three-to four- and seven- to eight-leaf stages) of development to determine if differential development of these structures influenced control with sethoxydim and haloxyfop. There was a differential response among the 10 biotypes to both sethoxydim and haloxyfop at both stages of development. The biotypes differed in the number of crown buds, tillers, and rhizomes that formed at both stages of development. In the three- to four-leaf stage, the biotypes with the greatest number of crown buds were the most difficult to control. These results suggest that in the susceptible stage (three-to four-leaf) the quackgrass biotypes with the least number of crown buds will be the easiest to control; however, by the time all the biotypes reach the seven- to eight-leaf stage even the biotypes with the lowest number of crown buds have enough crown buds that they become difficult to control with either sethoxydim or haloxyfop.

Field experiments were conducted to determine whether aerial and subterranean stem sections taken from Canada thistle [Cirsium arvense (L.) Scop. # CIRAR] plants in four stages of development (spring-vegetative, bud, postbloom, and fall-vegetative) could survive and produce infestations the following year. Partially buried aerial stem sections from all stages of development had greater survival 28 days after planting than completely buried aerial stem sections. Few completely buried aerial stem sections survived. Partially or completely buried subterranean stem sections from Canada thistle at the postbloom stage had the highest survival rate. Few partially or completely buried subterranean stem sections from the other three growth stages survived. Surviving stem sections from spring-vegetative, bud, and postbloom stages of Canada thistle produced adventitious roots that overwintered and produced new infestations the following spring. Surviving stem sections from Canada thistle at the fall-vegetative stage did not develop an adequate root system for winter survival.