Elm getting back on its feet

[leatherback]

a successful air- or ground-layer depends on a lot of healthy roots to feed the layer while it forms its own roots. Sound about right?

Yeah. From a fysiology point it also makes sense. THe roots can rebuild based on being fed from the leaves. During a layer you stop energy flowing from the leaves to the roots. So.. trimming the roots means taking away the reserves. And then we expect them to grow trunk-roots and recover the regular roots. Bad idea.