I have stitched the following together frrm snippets found on reliable websites. I see that I remembered the toxic substance name wrong. It's Juglone, not Juglase. 5-hydroxy-1,4-naphthalenedione. But you all knew that anyway.
Seems English Walnut is not so bad as Black Walnut. I think the latter only does well in UK in warmer areas. Walnus can struggle in Scotland, but bizarrely I found one doing well and fruiting on the very steep slopes below Stirling castle, on the most meagre of soils, almost bare rock. Judging by how hard it was to get to it to survey it, I think that brambles and nettles are very resistant to Juglone.
Many trees use allelopathy to protect their space by using their roots to pull more water from the soil so other plants cannot thrive. Some use their allelochemicals to inhibit germination or impede development of nearby plant life. Most allelopathic trees release these chemicals through their leaves, which are toxic once absorbed by other plants. Black walnut is a prime example of this. In addition to its leaves, black walnut trees store allelopathic properties within their buds, nut hulls, and roots. The chemical responsible for its toxicity, called Juglone, remains in the soil around the tree and is most potent at the drip line, though the roots can spread out well beyond this. The English walnut produces the chemical in lesser amounts.
Plants most susceptible to the black walnut’s toxicity include azaleas, pines, and birch trees.
Other trees that are known to exhibit allelopathic tendencies include maple, pine, and eucalyptus.
The tree of heaven, Ailanthus altissima, produces allelochemicals in its roots that inhibit the growth of many plants.
Ornamentals tolerant of walnut trees include forsythia, hawthorn, oaks, wild rose, daylilies, iris, phlox, Shasta daisy, and Virginia creeper.
Black walnut tolerant plants include the sugar maple, flowering dogwood and the boxelder to name a few. You can also plant crocuses, hyacinths and begonias. All of these plants are known to be black walnut tolerant plants. There are many more, and your local garden center can inform you of any intolerable plants so you don’t run into any problems.