Well I would imagine that your laws allow you to cut off any foliage that is in your yard i.e. the over hanging branches. If you wanna dig them up then I'd wait until they went dormant.

Sometimes you can pull up little trees after a rain, but when I have a seedling tree that I can't pull out, in my case it's likely to be hackberry with roots that go deeply very fast, I cut it off near the ground and put a drop of full strength broad leaf weed killer on the stump. That kills it. Be careful when applying it; don't breath it and don't get it on your skin.