The Big Chill
Deciduous fruit trees need a period of time for cooling off, some varieties require "The big chill" and other varieties like the ones we grow around here are low chill varieties. A certain number of cold winter hours are needed by the tree in order for it to produce fruit in the summer. A tree that requires more chill hours than the winter gave it will break dormancy in a hap hazard manner, and bloom sporadically, usually later than other trees of it's genus, causing chancy pollination, and low or no fruit production. The chill factor is simply a calculation of hours of temperature that fall between 32 and 45 degrees. Any hours less than 32 are excluded, and hours above 60 degrees are subtracted from the total. Our chill hours fall between 400 to around 800, sometimes less, sometimes more, depending on the weather pattern of any particular year. When choosing a tree, it's best to avoid any requiring greater than 800-900 hours of chill. We are really very lucky in the number of varieties that we are able to grow. An Anna apple for example has a low chill factor requiring only 200 hours, while the Gravenstein apple needing 700 hours is successful as well. Some of the northern varieties requiring 1200 plus hours are usually disappointing. Without adequate chill the fruit will not develop, that's why many varieties of cold region fruits don't do well here. When you buy a fruit tree the tag will usually say how many hours of chill is needed. The chill factor will vary within the same sunset zone. Cool valleys and ravines, and homes on the north slopes of hills will get chiller than south or west facing homes. Even within your own yard you will find variation in chill. To increase the chill factor for a tree you think may need to have as much cool as possible, plant it where it will be in the shady side of the house during winter months. It needs to have sun in the summer, so you must know how far the shadow falls during the seasons for this to work well. Avoid planting a tree that has a high chill factor right next to the house, or it will be warmed and insulated by it's close proximity. The backside of the hills in Southampton, will be cooler than homes in downtown Vallejo or Benicia. Concord and Fairfield, and Vacaville are cooler than we are in Vallejo/Benicia. You may have to do a little calculating to figure the true number of chill hours you have. If we have 3 weeks of cold, fog, and rain and temperatures in the low 40's, then that is 21 days times 24 hours/day for a total of 502 chill hours. We usually have temperatures that sneak up into the 50's for the day, so that 502 hours probably should be reduced some, and cold snaps come in spurts, so it's hard to keep track of exactly. It would not be unusual to have over our winter period a total of 2 months of nighttime temperatures in the 40s. By that reasoning, we come up with 60 nights at 12 hours per night to equal 720 chill hours. It would be fun for a study to be done in various areas throughout our towns to really determine the chill factors for different locations. By rough calculations however, you can go to the nursery equipped with a fairly good idea of the maximum chill hours your yard gets, and how to apply that knowledge.
Some of my favorites for this area are: Apples- Anna, Golden Delicious-700 hours, Granny Smith-600 hours, and Gravenstein and Pink Pearl-600 hours, Braeburn-500 hours, and Golden Dorsett-250 hours. Apricots - I like Blenheim (Royal) with 500 chill hours, and Autumn Royal, the only late fruiting apricot known- 500 hours. There are the unusual Peachcot, Plumcot, and Pluots to consider. They are Luther Burbank introductions with combined traits and flavors of peach, plum and apricot. -300 to 550 hours. Cherry- Compact Stella is my choice for this area. Bing-900 hours, and Black Tartarian-900 hours, need pollinators, and Stella is an excellent choice. If you don't get one to fruit, you may get the other. Cherries are an iffy crop here, but fun to try if you have the space, and the flowering tree is spectacular. Stella is self-fertile, requiring no additional pollinator and can be planted alone. Peach- I prefer the tiny Bonanza Peach, a true dwarf making it accessible to pruning, spraying and harvesting of full sized fruit while staying very cute, and adaptable to container planting. There are several new varieties out now of nectarines and peaches that are reported to be low chill varieties, and worth trying. Babcock-350-400 hours, is a standard, as is Elberta at 800 hours. Pears-Comice-300 hours, Seckel-300 hours, and Bartlett-800 hours. Asian pear do great here, and there are several varieties to try, as well as trees that have more than one variety grafted onto one tree. A few names to look for are 20th Century Pear AKA Nijisseiki, Tsu Li, Ya Li, Chojuro and Shinseiki, and Hosui, one of Japan's leading sellers. All require 200 to 440 chill hours. Persimmons require very little chill, and are a fall favorite for their fruit as well as the fall coloration of the leaves. Look for Fuyu, Hachiya and Chocolate Persimmon, and Izu. Plums-Santa Rosa plum-400 hours, and Weeping Santa Rosa plum that can be trained onto a trellis or espaliered makes a tree that is both elegant and delicious. The 'Wonderful' or 'Grenada' varieties of Pomegranate are the best. Try a Jujube ("Chinese Date") for something different. It will need to be planted on the hottest side of the house to ripen into a shiny reddish brown date-like fruit which have a sweet apple flavor when candied and dried.
Carol de Maintenon is a member of Garden Writers of America and owner of Benicia Garden & Nursery.
