First published in California Rare Fruit Growers Journal, vol. 20, 1988,pp. 14-16 (Updated 6/1998)
The Christmas weekend freeze of 1987 had a sobering effect on my enthusiasm
for growing many of the conditionally adapted exotic fruit trees here in
Southern California. My recorded temperatures dropped to 27° F and stayed
below the freezing point long enough for a pail of water to form a quarter-inch
thick cap of ice (freeze damage depends on the temperature and duration).
The frost-scorched leaves of the rose apple, mango, guava, babaco and pepino
dulce reminded me that to succeed in growing and fruiting these plants,
protection must be provided against severe cold spells.
This is something commercial avocado and citrus growers are well aware of.
They protect their trees using wind machines to keep cold air mixed with
warmer layers, or do so using orchard heaters. While these technique aren’t
practical for most hobby growers, I know of at least a dozen things one can do
in a small-orchard situation. They can be grouped as preconditioning
procedures and freeze-response procedures.
Preconditioning prepares the plant to withstand cold temperatures without
damage. To toughen plant tissues in the late fall, do not apply nitrogen
fertilizer after October 15 [in southern California]. Any fertilizer
application should be high in potassium, as this element is known to promote
thick cell walls. Another nutritional technique is to spray all plant surfaces
with kelp seaweed solution. I use Maxicrop dry concentrate at one teaspoon per
gallon of water. Cytokinins present in the seaweed toughen the plant cell
Experienced commercial growers are currently frost-protecting citrus, avocados
and strawberries with anti-transpirant sprays. All the plant surfaces are
sprayed prior to anticipated freezes, and the protection afforded is suggested
to be from two to eight degrees (°F). Here is a list of the ones I know of:
- Frost Away — by Bonide. Available from Mellingers.
- Wilt-Pruf — by Wilt-Pruf Produces, P.O. Box 469, Essex, CT 06426.
- Frostguard — by Custom Chemicides, P.O. Box 11216, Fresno, CA
93772. Available from local farm suppliers.
- Frost Shield — by Maz-Zee S.A. International, P.O. Box 82717, San Diego,
CA 92138. Available from Peaceful Valley Farm Supply.
While I haven’t been able to pin down any of the manufacturers as to how their
products operate, it’s safe to say that these products reduce the cooling
effect of wind on leaf surfaces (chill factors).
Another approach to increasing the low-temperature tolerance of plants is by
eliminating ice-nucleating bacteria (INB) from leaf surfaces. Recent research
has demonstrated that some plants, which usually succumb at 30° F can be
supercooled to 24° F without damage if these bacteria are eliminated from the
plant’s surfaces. As explained in the literature, these bacterial promote the
formation of ice crystals that pierce plant cell walls, leading to desiccation
and death. While not a completely accepted method, these bacteria can be
killed by spraying plant surfaces with a bactericide such as Kocide 101
Having preconditioned your trees with all these sprays, don’t forget to
protect the trunk and major branches. This is vitally important for trees in
the ground only a year or two. The trunk, especially near the graft union, can
be effectively protected by covering it with one of the special products made
for this purpose. One type of tree wrap comes in a roll and is made of treated
paper; another is a spiral ribbon of white plastic. Both products are
available through mail-order farm and garden companies.
Exploiting the Heat of the Earth
On a cold night with air temperatures in the twenties, the earth’s surface at
52° F is like a giant heater. To fully exploit that heat the ground near the
tree should be cleaned of both mulch and growing weeds. It should be wet and
uncultivated to maximize heat transfer from earth to air.
You may have noticed that plants placed under the leaf canopy of a large tree
or a building overhang escape serious freeze damage, while those out in the
open are killed. These covers contain the earth’s heat.
I build a protective cover for most of my sensitive trees using a a wooden
framework covered by shade cloth–beware of using clear plastic. The poles can
be 2″x2″x8′ lumber with the one end sharpened., and the wood treated with a
preservative such as Waterseal. Four poles are driven into the ground in a
4-5 foot square pattern. I always use a steel pipe to make
holes for the poles so
as to avoid splitting the pole tops. The top ends are then joined with four
1″x2″ boards using nails. I cover the framework with 50% shade cloth on top
east and west sides and 75% shade cloth on the north side. I leave the south
side open. This protector can be left up in summer for those plants needing
it. The north side protection reduces the chilling and drying effect of winter
winds. A similar approach which reportedly protects plants is to enclose the
entire plant with a product called Agrinet. This material is very light and is
easily supported by the tree’s limbs. It is also available from mail order
Exploiting the Heat in Water
We were taught in high school physics that water stores heat better than any
solid material, and that it releases 80 times as much heat (heat of fusion) in
changing from liquid to ice at 0° C (32° F) as it does when
cooled one degree Celcius. That is why farmers water their orchards during a freeze,
and so should the backyard grower. The water coming out of the pipe is probably at
about 52° F (11° C) and gives off 11 calories per gram when it
cools to 32° F (0° C), and then gives another 80 calories per
gram when it solidifies.
Another way to exploit the heat held by water is to surround the tree with
five-gallon plastic pails filled with water. This simulates the protection
given plants near a body of water, such as a swimming pool.
© Copyright 1988,1998, California Rare Fruit Growers, Inc.