any of a number of plants, as the pigweed or Russian thistle, that break off near the ground in autumn and are blown about by the wind
See tumbleweed in American Heritage Dictionary 4
(tŭmˈbəl-wēdˌ)
noun
Any of various densely branched annual plants, such as amaranth and Russian thistle, that break off from the roots at the end of the growing season and are rolled about by the wind.