With small ones, what RD said is so, they tend to attract them selves in a huge mass, They are brittle but only the small ones should be run over with a bike, If it is bigger than your front wheel avoid it. If it is bigger than your bike, It can drag you down into the ditch, Tumble weeds have built in stickers, Not a fun thing to deal with. They can be crossing the road any time of year. When passing through my part of Texas, watch out for tumble weeds. They can ruin your day.