PTP_2008_11_Bare

Plants That Produce

The Bare Essentials, Part II

By Richard Frost

The term “chill hours” refers to the need of some plants to have sustained cool temperatures in the late fall in order to produce a viable crop in the coming year. It applies to most deciduous and some evergreen perennial fruits, nuts, berry vines, and berry bushes. It does not apply to tropicals or most subtropicals, such as avocado and citrus.

Growing any perennial plant in your garden to produce fruit is a serious investment in time. It is often 3 to 5 years before you obtain a “crop”. Knowing that a particular plant – for example True Black Currant, needs many more chill hours than your yard can provide is a worthy piece of information.

When you see a plant in the nursery with a chill hour requirement listed on the tag, it means that the variety has produced reliable crops of fruit under those conditions. It does not tell you if the plant will fail with less chill hours – a sorely lacking piece of information. A related problem is that the labeling on many fruits is out of date. This is a constant frustration to nursery professionals who know that reliable studies for a particular plum (e.g., Green Gage) have shown better performance but the supplier is too cheap to update their labels.

A historical approach to measuring chill hours in northern climes is to count the number of hours in late fall and early winter below 45° F. A more robust approach used by fruit professionals in our climate zones is the “Utah method”. It is commonly computed by counting between Nov. 1 and Jan. 1, the # of hours below 45° F minus the # of hours above 65° F. So for example, if you have a week in December where it is a cool 37° F at night but 70° F during the day, then your location did not accumulate any chill hours that week. Notice how different the measurement would be if the hours above 65° F were not subtracted.

This leads to the second major complaint about chill hour ratings: not all growers use the same method and in particular, don’t record which method was used. This is especially true with apples and apricots. For example, an apricot variety listed at 700 chill hours might actually be rated 250 chill hours by the Utah method. Your local nursery staff should be aware of these differences.

Many people know that San Diego urban areas are “low chill” environments. By the Utah method, the immediate coastal areas (e.g., Imperial Beach, Solana Beach) rarely exceed 100 chill hours. The coastal bluffs (e.g., Chula Vista, Kearny Mesa, and Western Penasquitos) rarely exceed 250 chill hours, with the exception of ravines that receive cold air drainage from surrounding hills. The coastal interior valleys of Otay Lakes, Lemon Grove, El Cajon, Santee, Poway, and San Marcos contain various terrains. The slopes rarely exceed 350 chill hours, while the valley and stream bed bottoms have 400-500 chill hours. Julian achieves 1200 chill hours.

One inexpensive way to determine the chill hours of your locale is to walk around and find out what is working for your neighbors. Another is to find a customer service representative at a local nursery who actually grows fruit and has experience with your area. Whatever you do, don’t delay -- it’s time to order bareroot fruits!

SDHS member Richard Frost is also member of the California Rare Fruit Growers. For more information, please see www.plantsthatproduce.com.