Is Ulmus parvifolia allelopathic - ie. does it exude chemicals toxic to other plants? I am having great difficulty getting azaleas to grow under the canopy of a mature specimen.
Yes they are allelopathic, Peter. As an alternative to Azealea, Virginia bluebell (Mertensia virginica), a perennial, spring bloomer with blue flowers will thrive under elm trees. Hardy in climate zones 3 through 9, bluebells resist the toxins secreted by trees. Narcissus also go well.