Dormant Oil Spray
by Jerry Parsons, Ph.D.
Horticulture Specialist, Texas Agricultural Extension Service
in San Antonio
How would you like to have your limbs cut off?
How would you like to have your body beaten, bumped and bruised
periodically? How would you like being stripped naked by ravaging
hordes? How would you like to have your root cut, compacted
and covered? And worse yet, how would you like to be an invalid
totally dependent upon another uncaring, complacent individual
for your hygienics? If you find all of the above situations
distasteful you certainly don't want to be a tree.
Trees can tolerate and actually appreciate a
limb removal periodically - it opens things up and lets the
sun shine in. Trees are understanding of careless lawn mowers
who bump their bark and bruise their bodies. Even though it
damages them internally and scars their appearance, most trees
are capable of compensating for the abuse. In Texas most trees
expect to be periodically defoliated by hordes of pests, whether
insects or disease, and keep reserve leaf buds and life elements
in order to survive such trauma. People cannot see and consequently
ignore one of the most vital parts of a tree - the roots.
Consequently roots are indiscriminately cut by digging around
trees, compacted by heavy traffic patterns and machinery and
covered with suffocating soil by builders. All of these are
bad but by far the worse and deadliest is human neglect of
tree hygiene. A tree is basically an invalid - it cannot care
for itself! When it is contaminated by Texas' pestilence it
needs your help. Now is the time to act to rid your defenseless
trees and shrubs of inconspicuous killers by using a prophylactic
The spray which should be used is referred to
and sold under many brand names as dormant oil or scale emulsion.
These are highly refined oils (not motor oils!) which spread
uniformly on the bark of trees and shrubs to which it is applied
and coat non- mobile, dormant insects on the tree smothering
them to death. Heavier oils may have to be applied with a
tank (pump-up) sprayer which can apply the fully diluted product
rather than with a hose-end sprayer which may become clogged.
Applicators should frequently shake sprayers to agitate the
water and chemicals mixed since plant damage can occur if
a concentrated oil spray, caused by solution separation, is
applied. Mix dormant oil at the recommended rate on the product
label. This usually involves mixing one-fourth pint of oil
per gallon of water.
Insects possibly controlled are phylloxera on
pecan and grape (causes leaves to be curled and distorted),
San Jose scale and cottony cushion scale on fruit trees, ornamental
shrubs and trees (whitish, snow-like covering on branches)
and some mites which might be asphyxiated too.
If you want to "sweeten the pot" and
kill more for your effort, add a copper fungicide such as
Kocide 101 (one tablespoon in one-half gallon). If a copper
fungicide is added, the spectrum of control is increased to
include peach leaf curl (puckered leaves in spring), brown
rot (soft brown spots on ripening fruit), coryneum blight
(tiny holes in leaves which are too numerous and concentric
to have been caused by insects), anthracnose on blackberries
(lesions on canes and discoloration of leaf areas) and some
scab (black lesions on pecans and shriveled, drawn areas on
If you really want to insure pest annihilation
fortify the dormant oil mix with an insecticide. The addition
of such an insecticide kills active scale as well as dormant
scale which is suffocated by the oil coverage. Entomologists
(bug killers) recommend the chemical Diazinion. The recommended
amount to add is 1 1/3 tablespoons per gallon of water in
which one-fourth pint of dormant oil and two teaspoons of
Kocide 101 has been mixed. All of these elements are compatible
and can be mixed together with no adverse effects.
It is best to spray before buds begin to swell.
If buds of trees and shrubs have begun to swell slightly,
go ahead and spray. Although some of the buds may be damaged,
the benefits of spraying dormant oil plus a copper fungicide
far outweigh the possible repercussions. Besides, fruit trees
have many more fruit buds than they need to produce an adequate
crop anyway - - you won't have to thin the fruit as much thus
avoiding the moral issue of abortion! Do not spray trees which
are in full bloom however. Applying a dormant oil spray after
the pruning season will also serve to cover pruning cuts which
have been recently inflicted and can serve as a second attack
on stubborn pests which were not killed by an earlier oil
Spraying of dormant oil should occur on a clear
day when the temperatures are expected to remain over 50 degrees
F. for at least twenty-four hours. The ideal temperatures
for application is between 40 and 70 degrees F. Try to avoid
applying dormant oil when severe freezing trends are expected
in the 3-4 days following application. Some plant damage might
occur if freezing occurs before the spray water evaporates.
Now is the time for annihilation of much of
the pestilence which can cause a lot of a trees' hygienic
problems this spring. However, you must act with a blind faith
(prophylactically) that you may have some of these pest problems
on your trees and the treatment is effective since few of
the intended victims are visible. Pest control at this time
of the year is for prevention of potentially damaging situations.
If these particular pests are not controlled now with this
sight-unseen technique, they may become painfully apparent
this spring when their control will be difficult if not impossible.
You must also become a sadistic killer of pestilence
for the good of your plants. Pests will be suffocated by a
thin film of oil! Better that they are "wasted"
now than your plant this spring! A word of caution: The use
of a dormant oil mixture will not only kill, but annihilate,
annual flowers such as pansies, bluebonnets or snapdragons
growing under or near plants to be treated. To insure domestic
tranquility, completely cover such tender vegetation before
spraying nearby trees and vines with dormant oil. The use
of dormant oil spray is anorganic pest control technique which
will control the overwintering stages of harmful organisms
with sprays which are safe to handle and non- toxic to people.
But better yet, application of this spray is cheap insurance
to make sure that you as a caretaker of nature's invalid trees,
shrubs, and vines are never reported to plant- abuse-central
as a contributor to plant life demise.