I was wanting to grow heartnut trees on a hugel, but even though our soil is alkaline, fixing a hugel will cause the tree to be in acidic soil. Will the roots grow through the hugel fast enough to allow the tree to flourish?
What is going to cause such a drastic change in pH? Do you know the current pH of the soil where the hugel will be installed?
If you have not got one, do get a pH tester or get pH strips so you will know for sure both at the beginning and when construction is completed.
Butternut and Blackwalnut grow pretty fast in the right conditions, once they get about seven years old, they start to slow down, a little more each year until they reach maturity (around 100 years) when growth is as slow as it gets (around a foot per year branch wise).
The first seven years both trees will grow about 3 or 4 feet per year, root growth matches or is faster than branch/trunk growth on all trees.
(roots first then branches, that's why transplanted trees need that first year without fruit, so the roots can do all the growing for better support of the tree from then on)
If you know the pH is going to change, then plan for that by adding the correct buffer. (Lime for adjusting acid and sulfur for adjusting alkalinity)
The trees you want to plant like a pH of 6.3 to 6.7 (perfect pH range) they will do good in a soil that is a few % of either end of that range.