The average first frost date in the Fresno/Clovis area is Nov. 15. Our winter dormancy period usually begins rather suddenly as the first Alaskan cold front of the season causes temperatures to drop 20 degrees overnight.
A winter dormancy period during which plant growth stops temporarily is essential for good crop and flower production. Our very brief, often mild winter that lasts only six to eight weeks does not always provide enough chill hours (when daily temperatures average less than 45 degrees). This factor, as well as the summer drought stress that affects many plants in our climate, can interfere with plants' and trees' ability to fully enter into a winter dormant state.
Excessive summer heat and drought stress cause plants to enter into a protective survival state of semi-dormancy during which growth slows and nutritional and water needs are reduced. Mature landscape trees and fruit and nut trees that don't receive supplemental deep irrigation during the summer months will still be in this semi-dormant state in mid-November when shorter days and colder weather sends them into full dormancy. The extended dormancy period, from mid-summer to early spring, can cause late flowering, bud and blossom drop, and poor fruit and nut set.
If the soil around your big landscape trees has been bone dry for months or if your fruit and nut trees weren't regularly irrigated during the summer, they will benefit from really deep irrigation now. We can't depend on rainfall to deep irrigate our trees. Use a bubbler attachment on a hose or construct an earthen watering well or berm around the edge of the tree canopy and slowly soak the soil for at least 3 or 4 hours or until the water has penetrated a foot or more into the soil.
When temperatures remain moderate in November and December, some flowering perennials, especially roses, never go dormant. They'll continue to produce buds and new growth all winter long. To force roses into dormancy, collect a last bouquet for the Thanksgiving holiday table, then cut off all new buds and new growth. While you're doing that, also prune out any branches that cross into the interior and any weak, spindly growth (this gives you a head start on winter's hard pruning). In a couple of weeks, the rose leaves should start turning brown and falling off. Speed up the defoliation by hand stripping the leaves or (my trick) by blasting the leaves off with a hose.
Winter's cold temps will kill many overwintering pest insect eggs and fungal spores, but, just in case our winter is mild with few freezing nights, diligently clean up all fallen leaves and debris, especially around plants that have had insect (aphids, whiteflies, mealybugs, etc.) or fungal problems.