it depends on the biological activity in the soil, and also where the leftover roots are. near a root is much worse than away. Juglone lasts hours in the presence of some common bacteria, but it lasts year in heavy, barren soils. If you want to do your best, till the area, tilling in good amounts of undigested OM (leaves, but manure or grass clippings or hay or straw would do too). That will create a bacteria rich soil, nicely prepare the bed for the asp, and you will be safer next spring.

That area is about 10 to 15 from the root zone. I have several Holly Osmanthus shrubs and oak seedlings there. They grow well. I move all the plants and found some black walnut roots under them. It appears they are tolerant of the roots and the toxin.

The weather is still warm. I can dig up the roots, or deep the soil to 1' or more. It is easy to dig. I've already covered the area with leaves and some yard debris. I just do not know if this enough....

till them so the bacteria are injected, and BW roots get chopped. They are very distinctive, black outside, yellow wood. IME it is only near big roots that juglone lasts.

I dug the soil closest to the stump, about 2' deep. Then I dug the trenches to plant the crowns. I'll leave the trench open for the winter and plant the crowns next spring.

I only found a couple of large roots, about 1/2" diameter. They grow at the top 1' of the soil.....