I have a Nandina (heavenly plum blossom) that is getting really top heavy and I need to find out how to divide it or cut it back and root the cuttings. I’ve been reading up on it and there is very little information about propagating them.
The book American Horticultural Society Plant Propagation by Alan Toogood (1999) recommends taking nodal greenwood (similar to softwood) cuttings in the summer. The shrub
can also be propagated by division but this is recommended in the early spring and not in summer due to the increased risk of wilting and scorching.
Rainy Side Gardeners in the Pacific Northwest notes that fresh seed (as soon as it is ripe) can be germinated in six to eight weeks. Old seed may take up to two years to germinate. Semi-ripe cuttings can be rooted in the summer.
As for pruning the Nandina, the American Horticultural Society Pruning and Training: A Fully Illustrated Plant-by-Plant Manual by Christopher Brickell and David Joyce (1996) suggests that plants can usually be renovated by cutting back old canes to ground level in the early spring when the older leaves have turned from red to green. Rainy Side Gardeners suggest cutting the oldest canes down to the ground, discouraging the shrub from getting top heavy and falling over. The pruning will keep it growing a denser growth lower down on the shrub.
A Practical Guide to Pruning: How and When to Prune for Better Shrubs, Trees, Fruits and Climbers by Peter McHoy (1993) suggests cutting one out of every three canes to the ground. His recommendation is not to do this each year.
Paghat’s Garden (a website maintained by a local gardener) had this to say:
“Nandina thrives in considerable shade, but has a tendency to become leafless underneath unless it can get sunlight around the lower part of the plant. Before I transplanted this one, it was in a lot of shade, & needed to be staked because it became top-heavy. This did not necessarily harm its looks, because the species’ tendency to lose leaves at the bottom gives it the appearance of a miniature tree with long trunk, & I used the “empty” space around its base for small ferns. But when transplanted to a sunnier garden, it became more broadly bushy & the trunk became stronger, no longer needing to be staked.”