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Poor fruit set

Leaf buds and flower buds on 'Bali' tart cherry. The flower buds are on the sides while the leaf buds is in the middle

Thaddeus McCamant, Central Lakes College

Poor fruit set is a common problem in plums and tart cherries. Poor fruit set can be caused by winter injury, frost damage in the spring, poor pollination or plum curculios. If poor fruit set has been a problem in previous years, inspect the trees before, during and after bloom.

Poor fruit set due to winter injury and frost

In late winter, look for fruit buds on the tree. Flower buds are larger and usually more round than leaf buds. They swell faster in the spring than leaf buds. Flower buds can be killed during winter cold snaps even when the rest of the tree shows no sign of winter injury. It is possible for a severe winter to kill all flower buds, resulting in no flowers and no fruit in the following growing season.

If the flower buds survive the winter, there is still a possibility of a late frost killing the flower itself before it is able to set fruit. When the blossoms open, center of a healthy blossom has a green ovary that will turn into the apricot, plum or cherry fruit. After a mild frost, the ovary can be black, while the rest of the flower remains perfectly healthy. Once the ovary dies, the fruit will not form and the flower will drop from the tree.

Poor fruit set due to poor pollination

If the blossoms are healthy, they will attract insect pollinators. Look for bumble bees, honeybees and other insect pollinators on mild, sunny days. A blooming plum or cherry tree should be buzzing with insect activity on a sunny day with temperatures in the 70's. Conversely, if the weather is cool and cloudy, insect pollinators are less active and these trees may not be sufficiently pollinated. Poor pollination can also be due to not having the right mix of varieties. Tart cherries are self fertile, and should form fruit if the blossoms open and there are insect pollinators. Most plums and apricots need two or more varieties to assure adequate cross pollination. Many plums hardy in Minnesota are difficult to pollinate. Sometimes, two plum varieties next to each other will bloom at different times. Other times the pollen types are incompatible. If pollination is the reason why a tree is not producing fruit, the blossoms and small fruit will fall off a few days after petal fall. The best practice to improve pollination is planting multiple varieties of plums in a small area. For recommendations on compatible cultivars, see the publication Stone Fruits for Minnesota Home.

Poor fruit set due to plum curculios

If the blossoms are adequately pollinated, then the fruit will start growing immediately. About one week after petal fall is the shuck splitting stage. The shuck holds the petals and surrounds the green center of the blossom or ovary. When the ovary turns into a small, green plum or cherry, the fruit quickly grows and breaks through the shuck. If the fruit goes through the shuck splitting phase and then the small green fruit fall off the tree, the most likely cause for the crop failure is the insect plum curculio.

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