Stephen Henning wrote in
news
The maples are a big part of the problem. Not much of anything
including grass grows under maples. They have shallow roots and
take all the water besides having dense shade. I would dig a
trench down about a foot deep around the walnut out several feet
to cut any roots that are coming into the area of the walnut. I
would also open up the shade by pruning the maple tree(s).

that depends on the type of maple. if they are Norway maples (an
invasive weed tree), then you are correct. those do have shallow
roots & dense shade. red, silver, & sugar maples are not nearly as
dense. i have black walnuts growing in with sugar maples, & there
are no growth issues due to excessive shade or lack of water. i
also have all types of grasses, ferns & understory shrubs growing
in my sugar bush...

We have black walnuts growing in our yard that squirrels plant.
They seem to be 3 foot tall before we see them to pull them out.
If growing in a suitable location, they should grow fairly
quickly. I cut one down yesterday that was over 40' tall and it
was about 30 years old.

did you sell it for lumber?

I have seen green water bags called treegators that people place
around the base of new trees to keep them watered during the
summer. They come in different sizes. That would be a good
idea.

i agree, for a couple years until it gets established.
lee