I've had it self-sow in various areas of my garden, just like dill does, and no problems.

If you're concerned that it may put out toxins that damage other plants, a la black walnut, you could either create its own separate bed, or plant in containers.

Last year I planted fennel by broccoli and acorn squash. No problems. In another spot where I had fennel last year, I have planted kale and dutch shallots. They are all thriving. I have read about the problems that fennel can have but I have no desire to plan an entire bed of it so I keep experimenting to find out what will work and what will not. I also have a bronze fennel planted in a perennial bed, again, no problems with the other plants. It does not seem to be inhibiting weeds or grass from growing in that bed either.

I have also read that fennel is supposed to be allelopathic. But I have never had any problems growing vegetables in a garden with clumps of bronze fennel growing in it. The fennel has been there years.

Great thanks. I have a spot about 4' from my baby peach tree but wanted to make sure before planting.

Same here. I have self-sown fennel mixed all through my perennial and veggie gardens. I've never had an issue with it.
Kim

Here is a link that might be useful: My Gardens