Interspecific Relationships between White Clover, Kentucky Bluegrass, and Canada Thistle during Establishment

Forage establishment on fallow fields may be reduced by re-colonization and associated competition from hard to control perennial weeds that reproduce from residual root fragments. We assessed the competitive relationships between regenerating Canada thistle (Cirsium arvense L.) (CT) root fragments and seedlings of white clover (Trifolium repens L.) (WC), and Kentucky bluegrass (Poa pratensis L.) (KBG). Two greenhouse trials were conducted, each comparing 10 combinations of all three species grown in monoculture, or together (1: 1: 1 ratio), or in 1: 2 and 2: 1 binary mixtures in all possible combinations. All treatments were additionally done at two planting densities. After establishment, plants were grown for 70 d and assessed for biomass of all three species, as well as thistle shoot densities. Results from the two trials were similar. Canada thistle consistently produced the greatest biomass, and was more susceptible to intraspecific competition than competition from adjacent forage plants. Seedlings of neighboring WC were more effective competitors than KBG in reducing final thistle biomass and shoot density. White clover was also more likely to maintain shoot and root biomass under increasing weed presence. These results highlight the importance of reducing CT populations before forage seeding, as well as incorporating species such as WC that may be more tolerant of competition from the weed during the initial establishment period.