In autumn, as nights grow longer and temperatures plunge, many trees and plants go dormant. What does this mean aside from gorgeous fall foliage? Dormancy is a defense mechanism that allows plants to survive winter’s tough environment. It is brought on by what is called abscission and leaf senescence, when leaves, fruit and flowers change color, wither and drop. These processes are triggered by cold temperatures and decreasing photosynthesis, which releases inhibitory plant hormones to stop the growth and protect the plant. Dormancy sounds harsh, but it is a wonderful rest period for plants. When stored, carbohydrates keep the plant happy on low-level respiration until it can burst forth when conditions are more favorable in springtime. During dormancy, growth will not occur even under ideal conditions, preventing plants from prematurely budding during unseasonal warmer weather and then suffering damage from frosts that may come after.
For many trees and plants, some “magic” has to occur during dormancy to enable them to break out of this resting period into full vigor come springtime. This phenomenon, called vernalization, requires that certain plants be exposed to cold temperatures for a sufficient time during winter to reverse the dormancy process and continue the expected growth, flowering and fruiting process. The chilling requirement is the cumulative number of hours exposed to temperatures — between 32 and 45 degrees Fahrenheit — required to break dormancy. When the chilling requirement is met, dormancy is technically broken, but plant growth may not appear until environmental conditions improve with the approaching spring.
If a plant or tree does not receive the required number of cumulative chilling hours to interrupt dormancy, problems can develop. The yield can be reduced with fruit trees, and the fruit quality is subpar. Other plants may see sparse growth and fewer or delayed blooms or no blooms.
Chilling requirements to break dormancy vary widely. Each species has a unique need to maintain healthy development. For example, some fruits trees range from as low as 200 hours for pomegranate, 300 hours for quince and between 350 to 1,200 hours for pear varieties and between 400 to 1,500 hours for apple varieties.
If you want to plant a new fruit tree, looming climate change makes the chill factor a major consideration. The huge importance of fruit and nut production to the economy of California is causing the scientific community at the University of California to look closely at how projected climate change will impact the industry’s future. Studies have shown that winter chill in California has declined substantially over the last 70 years, and project that by 2100 winter chill hours in major growing areas may be as little as 500. The good news is that in recent years many low-chill varieties of popular fruit and nut trees have been developed. (Low chill hours are considered to be less than 300.) As a Mediterranean climate, Marin has always been conducive to low-chill varieties, so perhaps your existing trees will stand the test of time. But if you want to plant new, productive trees, consider your microclimate and be aware.
The UC Davis’ Fruit and Nut Research information Center and UC ANR’s California Backyard Orchard will have the information you need to choose the right fruit tree for your garden, including chill requirements. You can also speak with your local nursery or call the Marin Master Gardeners’ help desk.
Sponsored by UC Cooperative Extension, the University of California Marin Master Gardeners provides science- and research-based information for Marin home gardeners. Email questions to helpdesk @marinmg.org. Attach photos for inquiries about plant pests or diseases. Please phone 415-473-4910 to see when a Master Gardener will be at the office. There is a sample box outside the office so Marin gardeners can drop off samples 24/7. Visit our events calendar to attend an upcoming gardening workshop at marinmg.ucanr.edu/ASK_US/EVENTS. Subscribe to the Leaflet, UC Marin Master Gardener’s free quarterly e-newsletter, at marinmg.ucanr.edu.
