FROST PROTECTION FOR TROPICAL &
Phoenix and the lower Salt River Basin is historicaly subject to to at
least one but sometimes several frosts that occur in the later half of
December and early January killing tender plants and damaging many
subtropical plants like citrus. But, in 1967, a severe, Valleywide
freeze killed to the ground every living thing in my yard.
Microclimates occur all over this valley. If you are lucky enough to be
in a warm-spot that allowed you to escape unscathed last winter,
different atmospheric and climatic conditions in the next few months
could wipe out years of hard work devoted to your laqndscape. The best
defense is to start early to develope a plan of defense.
The Salt River Basin, the area that roughly includes Phoenix and
the flatlands that surround it, experienced three episodes of
mild frost in December of 1997. The winter of 1998 had an almost
identical record. 1999 had no frost except for a few “Outlying
pockets” that, if you are in one of them, may have formed an ice
film on your birdbath. These years are typical of the winter
climate in the Salt River Basin. History warns us, however, that
rare, killing freezes are possible through March. The seasoned
rare fruit grower assumes this “rare” disaster will hit again
during the present year and he plans accordingly for a worst-case
A course of action: Study every plant with a critical eye,
determine what precisely needs be done to protect each one and
record the information in a spiral notebook. Make a list of
essential materials. Buy the stuff and be ready to put your plan
into action on very short notice.
Do for trees and bushes the same thing you would do for yourself
if you had to be outside on a cold night; don a warm hat and
enough clothing to prevent your body heat from escaping into the
atmosphere. Likewise, you must preserve the heat that has soaked
into the soil and foliage during the day. The basic strategy for
that is to cover all plants that you can reach with a material
that can reflect the heat waves back into the tender, vegetative
mass you are covering. Exactly the same mechanism takes place
under a cloud cover. There are several types of effective frost
cloth carried by plant nurseries. Covers that you can scrounge
from your habitat include bed sheets, burlap and anything similar
that can be placed over the plant to extend all the way to the
ground. If the foliage and branches can’t support the weight, put
a stake or a couple of garden tools in the center to hold it up.
Plastic covering is okay but it should not contact the foliage.
The objective of cover is to conserve the heat already there. An
extended freeze may kill the foliage anyway, covered or not, but
for added protection, place a heat source beneath the temporary
If you can’t cover the plants, the second line of defense is to
import heat from any source available. As a last resort, if you
can drive the hood of your car underneath a valued tree, start
the engine when the temperature approaches 32F and let it idle
until danger passes. Seriously! You may want to weigh that
action, however, against the price of gasoline. But, use any heat
source you can find: Electric heaters, Christmas tree lights are
quite efficient, any light bulbs, flood lamps, heat and sun tan
lights that you can safely lay or hang will help. If feasible,
cover the plants as well for double protection. When it freezes
in the Salt River Basin, the air is always dry and electrical
shorts caused by moisture are generally not a concern. It will
never freeze in the Phoenix area when the humidity is high or
when it is overcast and raining. When using electricity, observe
carefully all safety precautions at all times.
During our freezes, the air is usually dead still except on
slopes where the denser, heavier air drifts by virtue of its
weight to a lower level. On that slope where the air is actually
flowing, it will be warmer. Friction created from movement of air
molecules generates heat. Where it puddles in a basin, against a
wall or building, it will be colder. A third main line of defense
is to create air movement with the use of electric fans. Direct
the wash into the foliage. In many commercial citrus growing
areas, trees and crops are protected with large propellers
usually powered with diesel fuel or aviation gasoline. Good
oscillating industrial shop fans on a stand cost $40 to $80.
There are other precautions that hardly need mentioning.
Containerized plants can be dragged inside or parked under the
dense foliage of another tree. Locate them beneath extended
eaves, under a patio or ramada, adjacent to south facing concrete
structures. Beware of cold air traps described above. Remove all
mulch. Dry, hard ground and dry foliage--not saturated with
water, that is, increase chances of survival according to some
Mist systems and overhead, oscillating sprinklers and foggers can
help but the mechanics need clarification and research and may
mot be feasible in a home garden. This grower’s only such
experience resulted in four adult citrus trees and an area of
about 4000 square feet covered in places with an inch of ice but
not a leaf nor a piece of fruit suffered damage. The oscillator
eventually seized but the water continued to flow.
Most of us grow bananas and papayas having very tender leaves.
More than one year old, most are too tall to cover. A freeze
severe enough to damage roots is not likely. I don’t waste time
protecting them except to keep the root zone as dry as possible
to prevent rot. Tender stems will suffer damage but are easily
protected with wrappings of blankets, jackets or old sweaters.
You can also wrap the trunks with Christmas tree lights. Do
anything you can to save the stems and foliage except irrigate:
Cold, wet soil will kill them.
If other woody evergreen plants are defoliated by frost, direct
sun will damage previously shaded bark. Paint the trunks and main
branches immediately with white latex to protect against sunburn,
even in the winter. Leaves will return when weather warms up. I
have never seen stem sun damage on defoliated bananas or papayas.
Finally, we need to investigate the relatively new field of
sprays. The literature claims the material will
prevent some frost damage but my own back yard tests have not
been conclusive. A degree or two of frost protection is, however,
worth a lot in most of the Salt River Basin where the low is
rarely below 31 for only short periods. Get a bottle of it at
your local nursery and tell us how it worked for you. The
directions I’ve read insist that all surfaces of foliage be
coated. I find that task to be virtually impossible for me to
accomplish without a high-pressure sprayer or fogger.
Hardiness Zone 10 is defined as an area with lows from 40 to 30
degrees F with rarely more than 6 freezes of short duration. That
places the Salt River Basin squarely in that category. But,
growth in most warm climate plants native to areas that never or
rarely ever drop below 70 nor rise above 80 is practically shut
down when soil temperatures drop into the 40s during winter
months when excessive moisture in the root zone from excessive
irrigation kills more trees than the low temperature itself.
Material written by Dick Gross, Secretary, for the Arizona Rare
Fruit Growers Newsletter.