Clonal structure of invasive hoary cress (Lepidium draba) infestations

Hoary cress is a perennial herbaceous weed that has invaded agricultural and natural areas of western North America. Invasions are often composed of dense patches, and it is unclear whether clonal growth via lateral rhizomes or seedling recruitment is the dominant method of patch expansion. To study the clonal structure of this invasive, six patches from three USA populations (194 ramets) were analyzed with the use of Amplified Fragment Length Polymorphisms (AFLPs). Known siblings and clones were also included to ensure sufficient variation for discrimination between clonal and nonclonal ramets. Patches had low genet/ramet ratios (mean G/N = 0.25) and low diversity levels (mean D = 0.49) compared to similar clonal studies. Single genets represented 55-85% of the ramets sampled in a patch, and the largest genet was 38 in across. Hoary cress exhibits a strong bias toward patch-size increase from clonal reproduction rather than from seedling recruitment. Results indicate that biological control methods that focus on reducing or eliminating seed production would do little to stop expansion of a patch. Despite the domination of a patch by one or a few large genets, other smaller genets are able to persist or are occasionally recruited into dense areas of a patch.