Answer: If your trees are 40 feet tall, I'd call an arborist to consult about topping them. It can be done, but poplars are short lived trees and they probably are getting towards the end of their lives and ultimate height they will grow. Topping may envigorate them for awhile. Also, since they grow so fast, the topping will have to be done every few years to keep the growth in check. I'm not familiar with the cultivar of poplar you're growing, but all that I know of form the white, cottony seed masses in spring. I'm sorry we didn't answer your black walnut question. Below are some plants that can be successfully grown under black walnuts and perhaps meet your requirements: red cedar; crabapple; hawthorn; black raspberry; 'Exbury' azaleas and honeysuckle.
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