Agriculture Reference
In-Depth Information
number of fruits per tree. Growers determine how many fruits to leave based
on the size of the fruit at a specific reference date, typically 10 days after pit
hardening. Pit hardening is determined by slicing through the fruits with a
sharp knife. With a little practice, you can detect the hardening pit.
If the peaches are smaller than a certain diameter at pit hardening, you
will need to thin heavily. Slightly larger fruits require less thinning and fruits
that are above a given size require no thinning. According to
Modern Fruit
Science
, diameters of 33, 36, and 38 mm, respectively, at the time of pit
hardening serve as starting points. With experience, growers may adopt dif-
ferent threshold diameters based on climate, cultural practices, and varieties.
Plums and Prunes
Plums need to be heavily thinned in order to develop large, uniform-sized
fruits with good color. Thinning also helps ensure that the trees are vigorous,
and it reduces limb breakage due to heavy crops. Hand and pole thinning
work well for plums. Leave European varieties an average of 2 to 4 inches
apart (9 to 18 fruits per 36 inches of fruiting wood). Japanese plums set more
heavily, and an average of 3 to 4 inches between fruits (9 to 12 fruits per 36
inches of fruiting wood) is better.
Some varieties, including 'Burbank' and 'Friar', often set heavily when
cross-pollination and weather conditions are good. These varieties may need
somewhat heavier thinning. Varieties that tend to self-thin throughout the
growing season include 'Climax' and 'Santa Rosa'. Wait until just after June
drop to thin by hand.
Commercial growers have long used mechanical trunk shakers designed
for harvesting cherries to thin plums. Thin about 7 to 10 days after pit
hardening to remove green fruits. Fruits will continue to drop naturally after
this point, so be careful not to thin too heavily.
The mechanical shaker described for apples and peaches shows promise
for plums as well, although little research has been published on the practice.
Trials, however, show that large, rigid branches suffer more injury from the
shakers than do smaller, more limber branches. For commercial orchards,
