I confess to having used pesticides in the woods.
Preventing competition from weeds is crucial in the first few years of a tree’s life. Take a look at the picture I’ve taken from this pdf on weeding trees
Pretty compelling evidence that keeping weeds around the saplings under control is worth doing.
I have 1000 trees to keep clear, and right now the grass is 4” high in places. I’m not physically able to manually clear the ground (and keep it clear) around the trees. So I’ve resorted to spraying pesticides.
I’m clearing a spot around each tree with a diameter of around 1m. I’ll keep it clear of grasses and weeds for the first 3 or 4 years, until the canopy closes in.
The rest of the ground I’m leaving untouched, so the wildlife will still have some habitat. Though of course, with a young woodland coming the ecosystem is going to be quite a bit different in a few years time.