The Christmas tree is in the recycling bin and the lights will be coming off the house as soon as we get a dry weekend day. And soon we will see our winter-blooming trees beginning to show color. Let these be reminders for us to take care of tree work in step with the rhythm of the seasons.
That rhythm includes a winter pause in the phenology -- periodic cycles of change -- of trees and their associates: fungi, bacteria, insects and other organisms both beneficial and injurious.
Good gardeners and arborists know to step in and take care of certain chores when the time is right. This may include pruning when trees are in a period of rest so as not to interrupt active growth, and when pests and diseases are less likely to take advantage of wounds.
Here are a few examples:
Monterey pines (Pinus radiata) are susceptible to fatal attack by engraver beetles (Ips paraconfusus and other species) when the beetles are attracted to stressed and wounded trees. The beetles actively seek and infest pines in spring, summer and fall. To minimize the risk, prune pines in winter.
White birch (Betula pendula and all other species of white birch) is susceptible to fatal attack by the bronze birch borer (Agrilus anxius), a small beetle. It has gotten so bad with white birch that I no longer recommend planting them in our area. White birch typically requires very little pruning, usually for overhead or roof clearance or to reduce crowding with other trees. If you are among the few who have pest-free white birches and they need pruning, take care of it in January or February.
Elm trees (Ulmus, several species) are susceptible to attack by two species of bark beetles, which are attracted to stressed and wounded trees. The beetles infect the trees with Dutch Elm Disease, the fungal disease that has killed millions of elms across America and continues to kill trees here in Napa Valley. Elms of Asian origin, and some of the new hybrids such as Frontier and Emerald Sunshine, are immune or less susceptible. Again, it is critical to prune American and English elms only in winter, while the beetles are inactive.
Fruitless mulberry, Siberian elm, coast live oak (Morus alba, Ulmus pumilla, Quercus agrifolia) and a few other trees are very susceptible to wetwood infections and a subsequent condition called “slime flux” when pruned in spring. The disease rarely causes serious harm, but the seasonal dripping from the infections is unsightly and can stain surfaces under the trees. There is no cure so prevention is the key: Avoid making large wounds and prune in winter or summer, not in spring.
Fruit trees: Annual pruning is essential for crop and disease management, and easy harvesting. Ideal timing varies from one region to another and according to variety. Here in Napa Valley, January and February are good months for most fruit tree pruning, but wet and rainy conditions can increase the chances for spreading tree diseases as well as making the work uncomfortable.
Plan on it for a dry spell if we get one!