of Rhode Island GreenShare Factsheets
Chemical thinning is an established and essential practice
performed by fruit growers each spring. In recent years, the
apple market has caused chemical thinning to become the single
most important cultural practice that many growers undertake.
The purpose of this leaflet is to summarize thinning recommendations
and to discuss the conditions that may influence your chemical-thinning
Fruit thinning is done to increase fruit size and enhance
repeat bloom. The fewer fruit that are allowed to develop on
a tree, the larger those fruit will be. In general, reduction
in the number of fruit to 4 to 6 fruit per cm of limb circumference
is required for good size. Good return bloom usually can be
assured if fruit density is reduced to 8 to 10 fruit per cm
within four weeks of bloom. Therefore, more severe thinning
is required to obtain good fruit size than to get adequate
Growers must consider thinning all mature,
bearing trees. Trees that carried a light crop last year will
have a heavy
bloom and will require extensive thinning this spring. Furthermore,
trees with a light bloom this year will set a higher percentage
of flowers than those with heavy bloom, thus they may require
Carbaryl or Sevin is the workhorse of thinning agents.
It is the mildest and safest thinner that we have. It generally
is used at 0.5 to 2 lbs 50WP or 0.25 to 1 qt XLR per 100 gal,
assuming dilute application. The XLR formulation of Sevin is
preferred over the 50WP, because of the particle size of the
50WP formulation. Particles of 50WP Sevin are approximately the
same size as pollen grains, thus increasing the likelihood that
these particles will be carried to the hive by bees. Sevin XLR
has a much smaller particle size, reducing, but not eliminating,
the likelihood of bee damage. Sevin XLR contains a surfactant,
so it may be absorbed more readily than other Sevin formulations.
It also may improve the penetration of other chemicals with which
it is applied, such as NAA.
Naphthalene acetic acid is the most potent thinner
that we have. It is used at rates between 2.5 and 20 ppm. Because
it is a more active compound than Sevin, there is a greater chance
of overthinning, especially when high temperatures (85o or higher)
follow application. NAA should be used, but it should be treated
with respect. Fruit size may be reduced or pygmy formation may
increase with late or high-concentration applications or when
it is very hot.
Naphthalene acetamide is less active than NAA.
It frequently is used in situations where flagging of foliage
is a problem. It generally is applied at 25 to 50 ppm. NAD never
is used on Delicious, since small seedless fruit (pygmies) will
result and persist to harvest. It often is used in a petal-fall
Apples can be thinned satisfactorily any time from bloom
to 3 to 4 weeks after bloom. The exact length of the thinning
period will be determined by the weather. If warm temperatures
occur during the 3 weeks prior to treatment, results of thinning
may be disappointing. However, if cool conditions prevail before
treatment, thinning may be accentuated and the thinning window
may be extended.
Over the last number of years, the preferred time, assuming
that the weather cooperated, was when fruit were 8 to 10 mm
in diameter. As mentioned above, the market has caused thinning
to become the most important cultural practice for many orchardists.
Thinning must work! Inadequate thinning will result in significant
losses of crop value, much more than will be experienced by
light to moderate over thinning. You must create every opportunity
possible to thin your fruit chemically.
Because of this need, we recommend that you begin thinning
at petal fall or shortly thereafter. At this time, we suggest
the use of Sevin alone, or where more severe thinning is needed,
a combination of Sevin and NAA (5-10 ppm).
Assess the response and initial fruit set when fruit are
7 to 12 mm in diameter (about 7 to 10 days after petal fall),
and treat again if necessary with Sevin alone or Sevin plus
When fruit are over 15 mm in diameter (after another 7
to 10 days), assess the response again and determine whether
or not any stress periods have occurred, and treat if necessary
with Sevin only.
This approach will improve your results possibly in two
ways. Multiple applications should provide more thinning response,
and multiple applications will spread the applications through
a number of possible weather windows for optimal response.
The effectiveness of chemical thinners is influenced by
the weather in several ways.
All chemical thinners, to be effective, must diffuse across the cuticle, which
covers all aerial portions of the plant, including leaves, flowers, and
young fruit. Waxes on the cuticle provide a major barrier to penetration.
When the weather is cold and cloudy, little wax is secreted on the leaf
surface, thus permitting greater penetration of the chemical. Conversely,
when the weather is sunny and dry, wax is secreted and deposited at an
accelerated rate, thus restricting penetration of thinning chemicals. Therefore,
chemical thinners, generally, are less effective after warm, dry periods
and more effective after cool, moist periods.
Regardless of the mode of action of each thinning
chemical, all require some type of stress to be imposed by weather
for an ideal thinning response. Competition among flowers, fruit,
leaves, and growing points occur for water, nutrients, metabolites,
photosynthate, and growth regulators. Chemical thinners increase
this stress on fruit, causing weak fruit and those with a small
number of seeds to abscise. It is our opinion that in years where
chemical thinners have worked poorly, weather-imposed stress
has occurred before thinner application, or stress has not occurred
until after the time that fruit abscision can be altered. Satisfactory
thinning results may be obtained if thinning is followed by 3
days of sunny weather with temperatures in the mid to upper 70's.
Ideal results, however, require 3 days of sunny weather in the
80's. Cloudy, rainy weather following chemical thinning has been
shown to result in enhanced thinning in the Mid Atlantic region;
however, this response has not been confirmed in the Northeast.
If weather during pollination was good and considerable
bee activity occurred, then a heavy initial set of fruit with
many seeds is likely. Thinning will be required and may be difficult.
If the weather was less than ideal for bee activity and developing
fruit have few seeds, set may be reduced and thinning may occur
Frost which may kill only a few flowers low in the tree also may injure spur
leaves throughout the tree. Absorption of thinning chemicals into these
injured fruit and leaves is likely to be greater than into noninjured fruit
and leaves, and therefore the thinning response may be greater. Further,
impaired photosynthesis and stress caused by reduced carbohydrate supply
also may accentuate thinning.
Winter injury to either the tree top or the roots
can accentuate thinning activity.
Trees that have been damaged by voles will be weakened and will thin more easily.
Root damage caused by pine voles is not always apparent, and effects may
not be seen until it is time to thin chemically.
Heavy crop the previous year
Trees bearing a heavy crop store less reserve food than trees bearing a light
crop. These less vigorous trees will be more responsive to thinning sprays
the following year.
Trees that have had insufficient nitrogen or have a deficiency of another essential
element will thin more easily.
Care must be taken to chemically thin young trees very carefully. They thin
very easily, and at most, reduced rates of chemical thinners are all that
Thick, dense trees
Interior spurs, those located on shaded (or shaded prior to pruning) portions
of the tree, and spurs on lower limbs usually are weak and low in carbohydrate
reserves. These spurs thin easily. If trees were pruned to expose a number
of previously shaded spurs, realize that these spurs will be thinned easily.
Chemical thinning is a difficult process, often thought
to be more art than science. Regardless, it is one that orchardists
must master to be successful. Poor thinning results in loss
of more crop value than all other problems together.
Adapted fromDuane W. Greene
and Wesley R. Autio, Department of Plant & Soil Sciences,University
of Massachusetts Extension, 2001
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information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
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