Answer: Sometimes it is possible to divide these shrubs by carefully removing and transplanting some of the smaller, newer growth coming up from the roots in early spring. (Meanwhile leaving the original plant in place.)
According to Michael Dirr's "Manual of HArdy Landscape PLants" the following methods may also be tried.
It is possible to propagate nandina by seed (I have actually seen the occasional volunteer seedling, but only in gardens further south where the climate is much milder and the growing season much longer than your area.) Some accounts indicate a January sowing of freshly collected seed will result in seedlings the following spring while other reports indicate a double dormancy occurs, meaning seeds will require two chilling periods before they will germinate.
Tip cuttings taken from hardened wood can be rooted in mid-summer using sterile potting mix and probably also a rooting hormone (a special powder available in small bottles or packets sold at garden centers). The cuttings seem to do well in the warm soil and warmer, more humid summer weather. This last method is probably the easiest for the home gardener to handle.
Nandina is not all that easy to propagate so the commercial growers tend to use tissue culture. Unless you have a sentimental desire to replicate these particular plants, you might have faster results and larger plants sooner by purchasing them this spring.
Q&A Library Searching Tips