Would a tumbleweed sucked into a jet engine on takeoff cause a serious problem?
Luckily tumbleweed rolls and doesn't fly [like some geese].

I tried to check how much an average one weighs, I couldn't find out. But it will cause some degree of damage. The engine can fail, but if it's just one tumbleweed then it's just one engine, it'll be serious but not dangerous. Airliners can fly with one engine inoperative, they are designed for it.

If it happens before the airplane reaches what's called a V1 speed, the flight crew can just abort the takeoff.

I didn't find any occurrence/report of it happening before. And it hasn't been tested before (they test using chicken guns).

Airport tower controllers and ground staff are vigilant in spotting such things, as this story shows:




  Escorted back for questioning!