We have all watched fruit trees grow and change through the seasons.
Fruit and nut trees grow most rapidly in the spring and summer months. As summer progresses, the growth rate slows until it all but stops in the fall. Days shorten, temperatures drop and the trees drop their leaves. Finally, the trees go dormant for winter.
It might not look like much is happening outdoors during the next few cold months of dormancy, but fruit and nut trees' hormonal systems are hard at work.
The longer, cooler days trigger hormonal growth inhibitors. Trees produce these hormones to prevent them from growing in winter and being damaged by cold. During its dormancy, if a tree has not received sufficient chill hours, even a warm spell with perfect conditions will not awaken it. Some plants do not have this hormonal protection. Many of us have lost plants that were fooled by an early warm spell and coaxed to bud, only to be killed by a later frost or freeze.
Napa County is luckier than many parts of the country. Winters are warm enough here that plants are not usually killed by extreme cold, yet we have enough chilling hours to meet the needs of our fruit trees and promote an abundant crop.
Trees have different requirements for the number of hours they need to break dormancy. Almonds require at least 250 to 500 hours at less than 45 degrees Fahrenheit to break dormancy. Pears, including Bartlett and Bosc types, need 700 to 800 chilling hours. Asian pears, which do well in my garden, need only 350 to 400 hours.
Most red-skinned apples need a cool climate to develop good color. But while some apple varieties require 500 to 1,000 hours of chilling, others need only 400 to 600 hours. When you shop for apples and other bare-root fruit trees, look at the plant tag to see if your choice has high or low chilling needs. Local nurseries tend to choose varieties suited to our climate, so you will usually find choices that are going to be successful. This is a good reason to buy locally instead of from catalogs that carry plants suited to zones and growing conditions different than ours.
We certainly have enough winter chill to grow many kinds of peaches, pears, plums and other fruits and nuts. Take a stroll down the bare-root aisle of your local nursery or home improvement store next month and survey the selection. Just be sure to read the plant tags and ask a nursery professional if you are not sure whether a variety is suited to your microclimate.
If your garden does not get much frost, you probably won't have much luck with sour cherries, which need 1,200 hours of chill. But you still have many options. Figs of all types, Hachiya and Fuyu persimmons, almonds, olives, pomegranates, chestnuts and pecans usually produce abundant crops with a minimum of chill.
A mild winter may be pleasant, but it can diminish our future harvests. Warm winters can result in delayed foliation and prolonged blossoming, but the buds deteriorate or drop, yielding few flowers or fruit.
December and January are the most important months for meeting chilling requirements. If the temperature falls below 45 degrees Fahrenheit for at least 400 hours in each of these months, and those cold hours are fairly evenly distributed, most of our orchard trees' needs will be met.
Once a fruit tree has accumulated the chilling hours it needs, a period of warm weather will signal to its hormones that it is spring, time to break dormancy and safe to grow.
Napa County Master Gardeners (cenapa.ucdavis.edu) answer gardening questions Monday, Wednesday and Friday, 9 a.m. to noon, at the UC Cooperative Extension office, 1710 Soscol Ave., Suite 4, Napa, 253-4221.