Fall and Winter Pruning
by Allison Levin
This is the fourth in a series of articles from aesthetic pruner Allison Levin. The previous articles can be found here: (Part 1 | Part 2 | Part 3). We hope they help you to achieve a healthier and more beautiful native garden. Happy pruning!
P. radiata (Monterey Pine) photo by Ted Kipping
A special note about 2011: This year's unusual weather reminds us that assessing a tree's health throughout the year is a crucial prerequisite to pruning. This year, an abundance of rain and fog was accompanied by a paucity of sunny days in the Bay Area. You may have noticed trees putting out unusually large amounts of spring growth. That seems like a good thing, but the tree expends extra energy to make all that new growth. If insufficient sun follows, the energy is not recouped. In fact, in some areas around the San Francisco Bay, some of our trees are weaker for all the rain, instead of stronger. Perhaps in your area of the state you have noticed something similar.
As a general rule, just as in summer we pruned and tidied shrubs and trees whose growing season was winter and spring, in winter we'll prune trees whose growing season was spring and summer.
In fall, many trees may be looking shaggy, overly dense, or leggy from a second, late summer's growth spurt. What kind of pruning can be done for this?
Remember that larger pruning cuts should be made when a tree is dormant. But that rule doesn't preclude tipping cuts of long whips -- wait to do this until the growth spurt is complete, or you'll have to do it a couple times in a season. If this wild, exuberant (some might call it coarse) seasonal growth is removed or shortened now, it won't have a chance to become more established in spring.
Pruning Basics Review
Winter Dormant Species
While California pines are not generally thought of as garden-size trees, their dimensions can absolutely be controlled, and in an aesthetically pleasing form, if pruning begins while they are small. Some that are especially suitable include Pinus contorta (Beach or Lodgepole Pine), P. radiata (Monterey Pine), and Tsuga heterophylla (Western Hemlock). Other Bay-friendly California conifers that can be pruned in winter include Torreya californica (California Nutmeg) and Sequoia sempervirens (Coast Redwood).
California cypress also make beautiful and dramatic garden trees. These should be pruned in summer.
Acer macrophyllum photo by Ted Kipping
Acer circinnatum photo by Ted Kipping
Deciduous plants from the Rosaceae family – Amelanchier (Serviceberry), Cercocarpus (Mountain Mahogany), and Prunus virginiana (Chokecherry) -- should be pruned in the coldest part of winter. Non-deciduous Heteromeles (Toyon) can be pruned now, or, if only light work is needed, in summer.
In the Betulaceae family, the two Alnus (Alder) species and Corylus cornuta (Hazelnut) should also be pruned in January. Betula (Birch), however, would be pruned in October, to avoid infestation by Bronze borers. In fact, Betula take pruning very poorly, and the wise tree lover should not plant this tree when it can't grow naturally.
Cornus nuttallii (Pacific Dogwood) is a slow growing tree, and rarely needs structural pruning; make those rare larger cuts in January, and touch up for form and airiness in summer.
Acer (Maple) species, from A. macrophyllum to A. circinatum, should be pruned for structure in February. Touchup work including thinning of small branches, twigs, and leaves can be done in late spring, once the leaves have hardened off.
Platanus racemosa (Western Sycamore), like Acer, is healthier for a late winter pruning. These trees are very susceptible to the fungi anthracnose and powdery mildew. To minimize the spread of these diseases, sterilize tools between each cut on an infected tree. Annual garden hygiene can also help curb these infestations: shake out, pick up and dispose of all the tree's leaf litter at the end of fall. Because these spores are airborne, many sycamores will never be rid of the disease; the species does put out a second round of leaves in summer, which don't typically show symptoms.
Allison Levin is an aesthetic pruner and native plant consultant living in Sausalito and working in the greater SF Bay region. You can send her comments or questions for this series at grownatives.cnps.org