Question
We live in SE MN and have lots of black walnuts growing on our farm.  Thanks to some busy squirrels and lack of time to weed for a few years, we have been inundated with trees growing too close to buildings and in the middle of garden plots.  We have tried to pull them out, but they are very slippery and have extensive root systems, even the first year.  We have tried cutting them back to the ground, but they sprout again.  Is there a chemical or other mechanical way to kill them?  I realize that the roots will remain toxic in the ground for years if we don't remove them, too, but I'm willing to live with that.

Answer
The best way is to either spray or wipe a herbicide called Roundup on the foliage of the plants wanted to be killed. The Roundup will be taken up by the leaves and translocated to the roots killing the plant. Beware that Roundup will kill any green plant it get on so do not spray in the wind or on plants not wanting to be killed. you can wear rubber gloves and put the Roundup on  rag and wipe the foliage with the Roundup this will keep the spray form getting on other plants especially in your garden plot.