Gordon Johnson, Retired Extension Specialist; gcjohn@udel.edu
There has been significant storm damage in orchards over the last two years on Delmarva. Severe weather is expected to increase in the coming decades and orchardists should be prepared to take measures to reduce damage to tree fruits. High strait line winds and rotational winds (most severe as durachos and tornados), heavy rain, and hail can all cause major damage to trees. Damage losses come in several forms: limb breakage, fruit drop, tree uprooting, fruit damage by hail, foliage loss, and flooding.
Hail Damage
Hail can cause complete loss of fruit crops. In young trees, severe hail can damage branches and stems by injuring the bark and may cause tree decline or death. Bruised stems will also be susceptible to infection by opportunistic fungi and bruises may turn to pox marks. Older fruit trees can withstand the force of hail strikes because bark is thicker and protective, and damage will consist of injury to new growth, leaf loss, and fruit damage. Trees will produce new leaves from dormant buds if loss occurs early in the season but will not later in the season. It is advised to apply a broad-spectrum fungicide mixed with a copper product to protect against fungal and bacterial infections after a hailstorm. In the dormant season selective pruning may be needed to remove severely damaged growth and fruit loads may need to be reduced by additional thinning to speed tree recovery and prevent decline.
Fruit damage by hail soon after fruit set will cause fruit drop. Later fruit damage with small hail may consist of dimples, small dents, bruises, or discoloration of the fruit. This fruit will remain on the tree but will be distorted. With large hail, severe injury including breaks in the fruit skin, cratering, or severe bruising will occur with injured fruit often dropping off the tree as ethylene is released after wounding.
Hail damage on apple fruits.
In hail prone areas, growers are investing in hail netting. Hail netting is an effective tool to protect orchards from both hail damage and major insect pests when draped and secured around trees. It also can improve the quality of fruit such as apples by reducing sunburn. Netting is draped over rows of trees and secured around the base of trees to limit insect entrance. Apply draped hail netting as soon as possible after petal fall, once flower pollination is complete. Netting can be applied and removed using a tractor-operated applicator or by hand in small orchards.
Along with hail protection, draped netting has been shown to exclude pests such as codling moths and apple maggots reducing the need for insecticide applications. It also can improve grades of apples, particularly in varieties such as Granny Smith that can discolor and other sunburn susceptible varieties.
Draped and tied hail netting around apple trees.
Limb or Branch Breakage
Storms can also cause limbs or branches to break. This is most severe later in the season when trees have heavy fruit loads. Limbs with narrow branch angles and ingrown bark are also susceptible to limb breakage.
In trees with heavy fruit loads, if fruits are sufficiently mature, fruits should be harvested before a storm arrives. This is possible where advanced storm warnings have been issued such as with tropical storms or hurricanes. Props made of notched 2 x 4’s can be placed under lower branches to prevent breakage and in high density orchards, extra ties can be added to support wires on longer branches.
Thinning and pruning practices can also limit branch damage by storms. Thin trees to lower fruit loads in storm prone sites or wind exposed orchard edges and prune branches to shorter lengths. Train branches to wider branch angles when trees are young (using clothes pins, branch spreaders, or weights) and avoid having branches with ingrown bark which leads to greater potential for breakage.
Once storm damage occurs, broken branches should be removed immediately by cutting to the branch collar which allows for proper healing. Do not leave stubs or cut flush to the stem, both can lead to decay organisms infecting the tree.
Proper pruning of damaged limbs to the branch collar and not flush. Also note the bark ridge where ingrown bark can weaken limbs. This should be avoided by creating a wide branch angle when training young trees.
In open pruned trees such as peaches, branches can split at the trunk and the tree must be removed entirely.
Nectarine tree spilt at trunk that will need to be removed.
Tree uprooting
Tree uprooting occurs with high winds such as with hurricanes. Most trees can tolerate heavy winds if they are well rooted and uprooting often is a sign of poor root structure, waterlogging, or improper planting. Deep or shallow planted trees do not develop good root systems initially. Plant so that the root collar is at ground level and the graft union is above ground. Break up hard pans and install drainage systems prior to planting for proper rooting and use irrigation patterns that encourage wider rooting.
When uprooting does occur, If trees are fully downed with more than 75% of the root system exposed, they cannot be reset and must be removed.
If trees are leaning or have minimal exposure of roots (less than 50% exposed) they can be reset if done immediately after the storm. Trees should be pushed upright without damaging the trunk and props should be placed to support the tree until new roots can grow. This can be done with notched 2 x 4’s or 2 x 6’s or wooden pallets. Put soil over any depressions and tamp soil around the roots around on the side that was exposed. Trees may need to have fruits removed the following year to allow for recovery. Avoid heavy pruning. It can take 2-3 years for trees to fully recover.
To reset a fruit tree, push upright, place props, cover exposed roots with soil and tamp the soil. Prune off broken branches but leave undamaged wood.
Fruit drop
I am often amazed at how much wind fruits can stand without dropping. However, if fruits are near harvest, particularly in apples, storms can cause significant fruit drop. As stated previously, if heavy storms are forecast, and fruits are ready, harvest ahead of the storms. Do not harvest dropped fruit for sale or for processing even though they look good. When dropped, they often will have hidden bruises and they are a high risk from a produce food safety standpoint.
Flooding
Some tree fruit crops, such as peaches and many of the stone fruit, have a very limited tolerance for saturated soils during the growing season (they are more tolerant in winter dormancy). During the growing season saturated soils for as little as three days can result in significant tree death in peaches and nectarines.
Modify the orchard site to allow for proper drainage. Avoid planting in poorly drained areas. In sites prone to drainage problems with heavy rainfall pattern tile fields or plant on wide raised beds, shape fields to allow for quick runoff during heavy rains and provide good outlets for drainage.
Storm Preparedness
Late summer and fall are when tropical storms can hit Delmarva causing severe damage to orchards and severe thunderstorms are to be expected during the summer. Tree fruit growers should prepare for these storms with all the methods described previously. When new orchards are planned, storms should be considered. Consider planting wind breaks along field edges. Choose varieties and rootstocks with storm damage in mind. For example, some fruit varieties are more prone to fruit drop than others.