A very large (or it feels that way, since it kind of leans toward the house) black walnut anchors the very back edge of our yard. Another smaller walnut is in the corner just past our property line. Walnuts can wreak havoc on your planting plans. Have you noticed?
People who are gifted with a black walnut tree have both an asset and a curse. The asset is a protein source that both you and your squirrels (and other animals) can enjoy. The curse is two-fold: like any nut or fruit tree, they can be messy. But their messiness is far outweighed by their ability to inhibit or kill other plants around them.
All other trees in the genus Juglans have the same ability, but to a lesser extent. Essentially, these trees give off a compound called juglone through their leaves and roots that has an impact on other plants. Anything in the drip line can be hit, and anything within reach of its roots can be hit. Cutting down the tree will get rid of the drip line impact, but the roots have to completely rot before their impact will be gone. So you may as well keep the tree and choose plants that can take it! That's my reasoning, anyway.
There are, however, Things You Can Do.
Here I quote from a publication from Virginia Tech: [t]he accumulation and depletion of toxins in the soil is affected by factors such as soil type, drainage, aeration, temperature and microbial action. Soil microorganisms ingest allelochemicals as energy sources, and metabolic decomposition can render the chemicals non-toxic to plants. When soils are well drained and aerated, a healthy population of aerobic microorganisms can accelerate this process."
In short, the most important thing you can do to encourage healthy plants around a black walnut tree is create healthy soil. So once again, you will do best to get rid of any grass and create mulched beds that will support the insects and microorganisms that eat walnut "juice."
Some plants will never do well around a walnut since they are just too sensitive to the impact. Other plants are strongly armed against it. The rest fall into that gray area that is simply undocumented. By researching garden forums for your area you may find a consensus of opinion on certain plants that will do well. The key to some of these is simply making sure the plant is having all its other needs met (sun, water, soil preference)--reading some of these forums leads me to believe that it was owner error rather than the walnut that "did in" a particular plant.
To start you off, I am adding a new page to this site (see above just below the page header) of some plants known for black walnut tolerance. [photo: monarch on (likely) tolerant milkweed]
Thanks for reading!