So-called dwarf arctic willow (Salix purpurea ‘Nana’). Photo: Susan Hagen, pinterest.com
Question: A few years ago, we were sold arctic willows as a hedge. The nursery told us this willow was easy to grow and needed little care. We planted them at the base of an embankment to isolate us from our neighbors. Now the branches almost cover the slope and prevent any garden plants (iris, ajuga, etc.) from blooming. Also, in winter, snow flattens its branches. It’s become quite a mess and certainly not what we expected.
Could we prune our Arctic willows enough to get our slope back and to keep it reasonably hedgelike? If yes, when and how?
Answer: The salesperson was partly right. This shrub is easy to grow. However, I’d scarcely say it needed little care, as it generally needs to be coppiced (cut to the ground) annually.
The so-called “arctic willow” (Salix purpurea ‘Nana’ or S. p. ‘Gracilis’) is indeed a willow, but is not at all of arctic origin. Its true common name is purple osier willow or basket willow and it’s native to central Europe. No one knows why it’s sold under the name arctic willow in North America, especially since it is not particularly hardy, being often damaged by the cold even in USDA hardiness zone 3—cold, yes, but hardly arctic conditions!
The true arctic willow (S. arctica) is a small creeping shrub of circumpolar distribution and extremely cold tolerant (zone 1). In fact, it is the most northerly shrub in the world, growing well beyond the tree line as far north as the north coast of Greenland.
The highly flexible branches of S. purpurea are widely used in basketry, the art of braiding plant fibers into objects (baskets, hats, mats, furniture, etc.). That natural flexibility explains why it is so often bent flat by the snow. A shrub truly native to the Far North would have long ago adapted to snowy conditions.
The purple osier willow tolerates pruning very well, even coppicing, and regrows quickly. You could prune it at almost any time of the year, except August or early September. Avoid pruning at that season so as not to stimulate late regrowth that would be too frail to survive the cold of the coming winter.
The most logical time to prune a purple osier willow hedge would be in early spring or late fall. You should cut it almost to the ground and it will grow back very quickly, usually up to 1.2 m (4 feet) in a single season. In your situation, annual coppicing will probably be necessary.