Submitted to: Weed Technology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/14/2007
Publication Date: 1/1/2008
Citation: Tipping, P.W. 2008. Mowing-Induced changes in above and below ground populations of plumeless thistle and musk thistle.. Weed Technology. 22:49-55. Interpretive Summary: Musk and plumeless thistles are exotic invasive plants which invade pastures, rights-of-way, and fields. Their ability to invade and dominate an area depends on many factors including abundant seed production, a soil seed bank, and competition from other plants. Most seeds are located in the top few centimeters of soil but seed is buried deeper in more disturbed areas like grazed pastures. Eliminating seed rain in plumeless thistle by mowing the plant quickly reduced the below ground seed bank which demonstrates its short-lived nature. The most effective treatment for musk thistle was mowing the plants in the fall after they had died. Normally, seedlings are found in close proximity to the parent plant which may be providing physical or allelochemic protection. Once the parental litter is removed however, the thistle seedlings are outcompeted by cool season grasses. The long term trend in Maryland has shown a sharp decline in musk thistle populations to a low level while plumeless thistle populations have remained stable.
Technical Abstract: Plumeless and musk thistle plants were clipped using a rotary mower at various growth stages at two separate sites during a 6-year period to elucidate relationships between soil seed banks and seedling density. One site was an old field with minimal disturbance and the other a heavily grazed pasture. The seeds in the soil were vertically distributed with over 96% in the top 7.6 cm at both sites. Clipping plumeless thistle when the majority of flower heads were at the full bud or post bloom stage had no effect on seedling density or the seed bank while clipping at full bloom significantly and quickly reduced seedling and seed densities. Musk thistle populations responded differently to clipping. Only the post-bloom treatment done when the entire plant was typically dead caused a decline in seedlings but not the seed bank. This indicates that other factors that limit the recruitment and maintenance of musk thistle populations. This may include allelochemical production by mother plants as well as interspecific plant competition. Long term trends of thistle densities at one site showed a steady decline in musk thistle over an 11-year period and the maintenance of plumeless thistle densities. The seed-destroying activities of Rhinocyllus conicus on musk thistle may explain this disproportionate landscape level decline in Maryland.