Why Prune?As bamboo stems age, they lose their vigor and eventually die; at the same time, new shoots emerge to replace them. As the older and dead stems become interspersed with vigorous stems, the foliage can appear tattered and worn. Also, most of the smaller bamboo are susceptible to mite infestations, which can also mar the appearance of a planting. Periodic pruning will keep your plants looking fresh and healthy, and will greatly reduce mite populations.
ToolsFor ground cover bamboos, I use a pair of bypass pruners to snip each stem off at its base. For large patches, hedge-type shears will work as well and are faster. I haven't tried it, but some people use a lawn mower for larger areas. For the larger, shrub bamboos, loppers or a sawzall work best.
TechniqueRegardless of the tool, the technique is the same: you cut all of the stems completely back to the ground, a process called coppicing. Below, are the before and after pics of a coppiced shrub bamboo. Click on the picture to view a larger image.
TimingCoppicing is best done just before new shoots emerge. Depending on the bamboo, that's some time in March or April in Western Washington. By waiting until just prior to shoot emergence, you will minimize the time spent viewing the hole created in the landscape. In addition, the delay prevents the exposed soil from warming too quickly, (in which case, shoots may emerge early and be damaged by frost) and reduces the opportunity for weed seeds to germinate. How Often? Once the planting is established, coppicing may be performed every 1-3 years depending on how the bamboo is performing. In general, you can mow the ground cover bamboos annually; the shrub bamboos (3'-10') every 3 years. As you can see from the pictures below, the Sasa palmata 'Nebulosa' has recovered to its original height in two growing seasons. This bamboo still looks quite lush, but I will still cut it back to the ground in spring 2009 to control mites. That's all there is to it - cut back to the ground and wait for the beautiful new shoots to emerge. Wait a couple of years, and start over again!