Have you changed the soil? May just need to add some extra nutrients. I know when I change soil or add nutrients to my old soil my plants seem to have a growth spurt.

I just amended it with a bunch of composted cow manure and some new vermiculite this spring.

Stunting is a symptom of black walnut poisoning. First off, is this a black walnut? If it is an english walnut, it can not be the culprit. The leaves are very different, english having the same leaf as a hickory, large and smooth-edged, black having smaller, serrated leaves.

Assuming it is, what happened is that the tree sensed the good compost in those beds and eventually sent a root there. Every tree does that. It was not there in years past, but it is now. Dig a trench on the bed side facing the tree, find the root, and cut it. The roots are also black on the outside, and they should be fairly superficial. In fact, the farther back from the bed you can cut the root, the fewer these interventions over the years.

It will take some time for the juglone to dissipate. Unfinished compost, buried in the bed, will speed things up (not the sterile finished, the smelly, fermenting one, all crawling with worms). You can plant carrots, beets or chard, they will make it, and give you a crop in the Fall. Even they will not grow as much as they could, juglone is a nasty compound.

Thank you for the advice. It is a black walnut. How deep is "superficial" for root depth? Are we talking less than 6 inches?