a cold protection aid
on Florida citrus
AGRICULTURAL EXTENSION SERVICE
University of Florida
~~ '': tr
Use of MH-30 as a
COLD PROTECTION AID1
Dormant citrus trees can withstand lower temper-
atures than actively growing trees. Fluctuating winter
temperatures in Florida often cause non-bearing
citrus trees to break dormancy before spring. This new
growth greatly increases the sensitivity of the trees
to freeze damage and is responsible for loss of many
trees during subsequent periods of cold weather.
Citrus Commission and Experiment Station work-
ers in Florida have shown that MH-30, a plant growth
inhibitor, will induce dormancy in non-bearing citrus
and prevent regrowth of defoliated mature trees dur-
ing winter months. The resulting cold protection is
believed to be from this dormant condition and not
from the MH-30 itself. Generally one application is
sufficient, but a second may be necessary under some
MH-30 is not a cure all! If temperatures fall be-
low the critical point for dormant trees, killing to the
soil bank will probably occur. MH-30 is a tool to
put the tree in a dormant condition when desired
and keep it that way even during warm, growth-
promoting spells. Experience has shown that the
results may be highly variable, from no effect to
extended periods of induced dormancy.
What is MH-30?
Maleic Hydrazide (active ingredient) is a plant
growth regulant which inhibits growth by preventing
cell division. It is formulated as a 30 percent liquid
and is completely soluble in water. MH-30 is ap-
plied as a foliar spray, absorbed and translocated
'Research conducted by Dr. C. II. Hendershott, assistant
plant physiologist with the Agricultural Experiment Stations
and the Florida Citrus Commission at the Citrus Station,
Circular prepared by J. David Martsolf, Jr., assistant horti-
culturist with the Agricultural Extension Service, Gainesville.
within the tree to the most actively growing areas
where it inhibits further cell division and growth.
For optimum results from the use of MH-30, fol-
low these instructions carefully:
(1) Time applications correctly.
(2) Use proper equipment.
(3) Apply complete coverage.
(4) Check for indications of regrowth.
(5) Retreat before regrowth.
Correct time to apply MH-30:
Young non-bearing citrus-Apply MH-30 between
November 1-15 or at least two weeks before freez-
ing weather generally occurs in your area. Studies
have shown that trees actively growing may not be
protected by the chemical if sufficient time has not
elapsed between treatment (induction of dormancy)
and freezing weather. This time interval allows ten-
der wood to "harden-off". The absorption of MH-30
is reduced when applied during periods of low soil
moisture even though the trees are not in a visible
Mature trees recently defoliated by a freeze-Apply
MH-30 as soon after defoliation as possible and be-
fore new growth occurs. MH-30 will be absorbed
directly through the bark of the twigs, branches and
Under some conditions, MH-30, applied to de-
foliated trees, may delay or reduce bloom the follow-
ing spring and may cause abnormal (thick peel) fruit.
Therefore, each grower will have to decide on the
basis of his own situation whether the cold protection
from MH-30 induced dormancy is more important
than the possible reduction of yield or quality in
next season's crop.
MH-30 does not have FDA approval for use on
citrus fruit. Therefore it is not recommended for
use on trees where harvestable fruits are present.
Proper Equipment and
How to spray:
To spray young, non-bearing citrus, apply MH-30
using a single head hand-gun with coarse cone type
nozzle. Thoroughly wet upper and lower leaf sur-
faces, branches and trunks of trees.
To spray mature, defoliated trees, use a single or
multi-headed hand-gun with coarse nozzle and thor-
oughly wet all twigs, branches and trunks.
To avoid excessive foaming, fill spray tank 90
percent with water before adding MH-30. Thoroughly
mix solution with stick or mechanical agitation and
add remainder of water. MH-30 goes completely
into solution and further agitation is not necessary.
Compatibility studies are incomplete, therefore,
do not mix MH-30 with any other chemicals at this
For initial applications on young, non-bearing and
mature defoliated citrus trees apply:
2 quarts of MH-30/100 gallons of water
For repeat applications use either 1 or 2 quarts of
MH-30 per 100 gallons of water depending upon the
length of control desired (Rule of thumb):
1 quart of MH-30/100 gallons 30 days
2 quarts of MH-30/100 gallons- 60 days
Generally one application is sufficient but a second
may be necessary under some weather conditions. If
winter temperatures remain mild for several weeks
or more, dormancy induced by MH-30 may wear off
before spring and growth will resume. It is advisable,
therefore, to periodically observe buds for signs of
swelling (an indication of regrowth), and to re-treat
with MH-30 Before new growth appears. (See dos-
Tests have shown that citrus trees with small tops
and large root systems (i.e., old trees previously
frozen) break dormancy during warm spells much
more rapidly than young trees. These trees, there-
fore, require more frequent observation for deter-
Under some conditions, the MH-30 effect from
the November application may last all winter, and
regrowth will not occur until late spring.
No permanent injury to citrus trees has been ob-
served in the three years of testing. However, MH-30
sprayed trees may show a delay of growth in the
spring for several weeks. The first and possibly
second flushes of growth may also be slightly
"Frenched" or strap-leafed, but subsequent growth
will be normal.
DO'S AND DON'T'S
1. In the fall apply III-30 at least two weeks before
freezing weather is expected and during the winter
Before new growth appears.
2. Use IMH-30 as a tool not a cure-all!
3. Rain occurring 12 to 24 hours after application
may reduce effectiveness.
4. Compatibility studies are incomplete, therefore,
apply MH-30 as a separate operation.
5. MH-30 is not hazardous to farm animals or
6. IMH-30 is not volatile and offers no drift problems
on nearby crops. It has no residual in the soil.
7. MIH-30 washes easily from either wooden or metal
spray tanks and is not abrasive to pumps, lines or
nozzles. Rinse spray tank and equipment thor-
oughly with plain water when finished.
8. Read the label and observe precautions.
The use of trade names in this publication is solely
for the purpose of providing specific information. It is not
a guarantee or warranty of the products named and does not
signify that they are approved to the exclusion of others of
COOPERATIVE EXTENSION WORK IN AGRICULTURE AND
IActs of May 8 and June 30, 19141
Agricultural Extension Service, University of Florida,
Florida State University and United States Department of Agriculture,
M. 0 Watkins, Director