Be careful with what you use to kill trees, because it will likely leech into your first harvest if not more beyond that. If you are looking for long term answer, kill the trees and plant some bag seed to leech up the toxins. 1001 ways to use Hemp/Cannabis- It can be used to pull up toxins from the ground, even with heavy metals etc. To that first crop will be unsmokeable due to the toxins used to kill the trees, so chop it down and then you should have healthy and safe ground to grow your product in next harvest. Then use raised beds to hold for pests etc. Bury chicken wire under the ground to stop any large root eaters from getting you from the bottom-up. From there, beneficial pests and helpful fungi are natures gold IMO. I love praying mantises, lady bugs, wasps, nematodes, and mycorrhizal fungi as a good mix. Stops aphids, thirps, large leaf eaters, not to mention those that burrow into the grown to lay eggs. Make a circle in the soil around your plants to give a nice deep watering, then cover it in gravel to stop pests from having easy access to the plants stem and root mass from the top down. The mychorrizal fungus is awesome. It stops most negative root fungi from ever having a chance to grow onto the roots and significantly increases the surface area of the roots and its ability to take in water and nutrients. Good luck man, cheers.