Pruning Native Plants – Part 4

By Allison Levin
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

When it comes to structural pruning in winter dormant trees, review the basic rules of pruning: develop and refer to a multi-year plan, and don’t take too much off at once. Solve problems first, like crossing branches or those moving into the center of the tree. Prevent future problems by removing problem branches while they are still twigs.

Winter dormant species

California pines are in deepest dormancy during the coldest time of year. Pruning in December and January will minimize sap run, and, in turn, minimize the hazard of attracting beetles to the tree.

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 beach pine (Pinus contorta), monterey pine (Pinus radiata), and western hemlock (Tsuga heterophylla). Other Bay-friendly California conifers that can be pruned in winter include California nutmeg (Torreya californica) and coast redwood (Sequoia sempervirens).

California cypress (Hesperocyparis) also make beautiful and dramatic garden trees. These should be pruned in summer.

Deciduous trees

To many of us, the glory of a tree in leaf is almost surpassed by the beauty of a winter silhouette. To polish that graceful form, spend an hour or two in fall plucking or shaking off the dead leaves off a focal point tree, and remove the coarse summer growth that I referred to earlier. Wait until the appropriate time in winter for structural work.

Deciduous plants from the Rosaceae family – service berry (Amelanchier utahensis), mountain mahgony (Cercocarpus), and chokecherry (Prunus virginiana) — should be pruned in the coldest part of winter. Non-deciduous toyon (Heteromeles) can be pruned now, or, if only light work is needed, in summer.

In the Betulaceae family, the two alder (Alnus) species and hazelnut (Corylus cornuta) should also be pruned in January. Birch (Betula), 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) 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) 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), 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.

Pruning series
Part 1: Basics
Part 2: Spring time 
Part 3: Woody plants
Part 4: Fall and Winter

Allison Levin is a member of the CNPS Marin chapter. She is an aesthetic pruner and native plant consultant living in Sausalito and working in the greater SF Bay region.