It's probably not the greatest thing to do to bearded irises, but it doesn't seem to hurt them here and sometimes keeps them evergreen. I wouldn't ever do it except to protect adjacent plants, and you'd definitely want to remove the elm leaves in early spring before it warms up too much. Bulb-wise, I do purposely mulch xiphium hybrids to keep the winter-growing leaves from burning back, which also works on evergreen summer-growing moraeas. It will however cause junos and reticulatas to emerge too early for my climate. Sean Z Zone 6a SE Michigan