For very heavy ivy cladding, I use Woodland Scenics 'Foliage Clusters F58'. This is an altogether different product, a dense, rubbery sponge of coloured foam, which may be sliced very thinly with a blade taken from a Stanley snap-off knife. Simply glued in place with PVA it can be roughened as you wish with a sharp point or left flat. It can be wrapped around curved surfaces if sliced thinly enough.
Doug