Can it not drive around the tumbleweed? Or, if it's completely walled in by them, make an assessment of their density and likely harm caused, and decide to just run over them?

If we're crediting the car with being able to handle merging into a crowded multi-lane highway and dealing with idiot human drivers who don't indicate or hold lane position very well, it seems a bit odd to arbitrarily presume that it won't be able to identify non-hazards like said tumbleweed.