One of the most important things to do when thinning is to have proper records of crop load and
thinning date.It is necessary to count fruit remaining on trees after thinning for each variety and
for every orchard.This is more difficult in plums and some growers thin trees to what they
consider appropriate levels and then strip the remaining fruit to get a totally accurate fruit count.
Over a number of years, such records become very important in helping to identify correct
thinning levels and problem sites.

Chemical Thinning
For years, researchers have looked for some type of chemical thinner appropriate for stone fruits.
This is made more frustrating because of the great success of chemical thinners for apples.It is
important to remember that apples have a separate “king”bloom that opens earlier than the other
flowers on the tree.Stone fruits have no such type of bloom and this makes finding materials
and application timing that much more difficult.

Many types of thinners have been investigated, but current research efforts focus on two types:
1) flower thinners (which generally act by burning or drying out flowers) and 2) flower
inhibitors.Research with products of the first type shows that both Wilthin® and Armothin®
have promise.These products “burn”flowers.The more product applied, the more flowers
removed.Current research is focusing on determining the best rate and best timing for each.
Small commercial plots have been applied, but results have been erratic (Johnson et al., 1997).

The product Release® or, as it is now known, Ralex® is a gibberellic acid compound that is
applied during the summer to prevent flower initiation and thus reduce bloom the following
spring.Again, results have been promising, but erratic.Research is ongoing to develop better
information regarding rates and timings for this product.

It is also important to keep in mind that all chemical thinners are intended to supplement—not
replace—hand thinning.Any use of these products must be based on that premise.

GIRDLING
Purpose
Girdling involves removing a thin strip of bark containing the cambium and phloem from around
the stem, branch, limb, or scaffold of a plant.Doing so is thought to prevent the downward flow
to the roots of sugars and other organic compounds, thereby making them available to support
and augment fruit growth.

There are three primary reasons to girdle fruit trees:1) advance fruit maturity, 2) increase fruit
size, and 3) reduce the number of harvests necessary.Girdling is usually practiced only on early-
season peaches and nectarines.This is because they are usually quite small and difficult to size
and because prices for early fruit are more volatile and earlier harvest usually means greater
profit.Varieties of peaches and nectarines that ripen after about the first three to five weeks of
the season usually are not girdled.

Drawbacks
There are a number of problems associated with girding.The greatest potential problem is tree
death.Girdles that are too deep, wide, or heal too slowly can severely damage or kill trees.
Extreme care should be taken to ensure that the xylem of the tree is not damaged when girdling,
otherwise tree health may suffer.

Another major problem associated with girdling is split-pit fruits.These occur when fruits grow
too quickly just as the pits are hardening.Proper timing of girdling can reduce this problem, but
varieties prone to split-pits often cannot tolerate girdling.Associated with this are internal split-
pits.These cannot be seen from the outside of the fruit but still adversely affect fruit quality.
Fruits with internal split-pits often ripen erratically and/or quickly, making harvest and packing
very difficult.

Date of Girding
Peaches and Nectarines
Proper timing of girdling is essential to maximize advantages and minimize difficulties.An
understanding of the growth curve is again essential to understanding how girdling works.Just
as with thinning, girdling provides a “boost”in fruit growth rate.However, with thinning the
boost is permanent, but with girdling it is temporary and lasts only until the girdle heals.For this
reason, fruit should be girdled just prior to the beginning of stage II of fruit growth.Girdling at
this period reduces the amount and duration of the lag phase in stage II and allows for increased
fruit size.

Many methods have been proposed for predicting this period, days after bloom, degree-day or
heat unit accumulation after bloom, and onset of pit hardening.The best way to predict optimum
timing is by using seed length.Seeds grow at a rate dependent upon heat accumulation and act
as plant-based integrators ofdegree-days.Additionally, seed length is independent of fruit size
and is not affected by annual crop load variations.

Table 3 shows the results from a girdling trial that investigated using seed length as a marker for
determining girdling date.In this trial, girdling effect was optimal when performed when seeds
were 9 to 12 mm long.Subsequent studies have verified this 10 ±2 mm seed length timing as
being optimum for other varieties as well.

Table 3.The effect of date of girdling on tree and fruit performance of Mayfire nectarine (after
Day and DeJong, 1990).

Soluble
solids
concentra-
tion (%)

Fruits
harvested
in first
pick (%)

Seed
length
(mm)

Fruit
count
(#/tree)

Fruit
size
(g/fruit)

Split-pit
fruits
(%)

(Yield
(kg/tree)

Treatment

24 March
31 March
7 April
18 April
Ungirdled

7.1
9.6
13.9
17.6
-

12.31
12.94
14.22
13.25
11.67

136.7
132.7
156.6
158.0
146

91.5
97.8
94.2
84.3
79.5

14.1
14.2
13.7
12.2
10.0

4.8
4.7
6.6
8.2
5.7

79.8
89.6
79.0
75.6
67.9

Timing of Girding
Plums
Girdling of plums is less risky than peaches and nectarines if only because plum fruits do not
generally have split-pits.Also, most plum varieties are firm and are not as likely as peaches and
nectarines to soften rapidly on the tree.However, we have observed that the results are often
very erratic.In some years there is little to no effect on the fruit, and in some years the effect is
very dramatic (Day, unpublished data).

The key to girdling plums is to apply the girdles early.Many early trials showed girdling to be
ineffective on plums because girdles were applied too late after bloom.The best time to girdle
plums is during or just after petal-fall.Girdling should never be done later than three weeks after
bloom.The problem with such timings is that it is impossible to tell that early in the season if
the trees will have a normal crop.If so, girdling will be helpful but, if the crop is very light,
girdling will have no effect on fruit size or maturity.

Plum trees seem to be more affected than peach or nectarine by improperly applied girdles.Tree
death can occur if plums are girdled too deeply or a wide knife is used that causes slow wound
healing.Also, weak trees should never be girdled; it is too debilitating.Care should be taken
when girdling plum varieties that sometimes soften rapidly, such as Santa Rosa.The presence of
a girdle can cause fruit to ripen very quickly.

CONCLUSION
Improving fruit size is best achieved through proper crop load adjustment.Pruning is the
quickest, easiest, and least expensive method available for so doing.Hand thinning is essential
and is most effective when performed as early as reasonably possible.Chemical thinning has
potential but only as a supplement to hand thinning.Girdling can improve fruit size and advance
maturity of fresh market stone fruits but can permanently harm trees.Peaches and nectarines are
best girdled when seed length is 10 ±2 mm.Plums are best girdled during or just after petal-fall.

LITERATURE CITED
Crisosto,C.H., R.S.Johnson, T.DeJong andK.R. Day. 1997.Orchard factors affectingpostharvest stone
fruit quality.HortScience 32:820-823.
Day,K.R., C.H.Crisosto,T.M.DeJong andR.S. Johnson.1994.Preharvest factorsaffecting fruit quality
at harvest andin storage.CaliforniaTree Fruit Agreement Annual Research Report.8pages.
Day,K.R. and T.M.DeJong.1990.Girdling ofearly season‘Mayfire’ nectarine trees.J.Hort.Sci.
65:529-534.
DeJong,T.M., K.Day,J.F. Doyle and R.S.Johnson.1990.Evaluationof thephysiological efficiency of
peach,nectarine,and plumtrees in different orchard systems.California TreeFruit Agreement
Annual ResearchReport.10 pages.
DeJong,T.M., R.S. Johnson,K.R. Day and R.Beede.1991.Feasibilityofincreasingcling peachyields
by early thinning.California ClingPeach AdvisoryBoard Annual Research Report.7 pages.
Grossman, Y.L.and T.M. DeJong.1995.Maximum fruit growthpotential following resource limitation
during peachgrowth.Annals of Botany.75:561-567.
Johnson, R.S.,K. Day and H. Andris.1997.Chemical blossom thinning ofpeaches and nectarines.
CaliforniaTree Fruit Agreement Annual Research Report.pp.73-78.