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Pruning of Citrus and Pruning after Cold Damage
Citrus trees prefer tropical or subtropical climates and grow best in regions with mild winters and few nights with temperatures below freezing. Citrus trees come in all sizes and shapes to fit in any landscape. Citrus trees include orange, grapefruit, lemon, lime, Satsuma, tangerine and kumquat varieties. Citrus trees produce ornamental foliage and beautiful, sweet-smelling blossoms before the juicy fruits. Select varieties suited for your location for best results, and prune citrus trees as needed.
Pruning is a common task necessary for best production of many common fruit trees. Most types of deciduous trees are pruned to invigorate the tree, to improve branch configuration, and thus make branches less likely to split under a heavy crop, to improve fruit quality, and/or to reduce the crop load which will improve the potential size of individual fruits.
Homeowners with previous experience pruning deciduous trees often assume that citrus trees should be pruned similarly. However citrus wood is naturally strong and is not as likely to break under the stress of a large fruit load. Furthermore, citrus trees can produce fruit in all but the most shaded part of the tree, and need not be regularly pruned to allow more light into the interior of the canopy. Even when the crop load is heavy, individual fruit size is large, so pruning to reduce the crop load and improve fruit size is not necessary except occasionally with tangerines. Finally, citrus fruit quality is typically just as good or better from a minimally pruned tree as compared with one that is heavily pruned.
Nevertheless, citrus trees should not be left completely unpruned. Proper citrus care for the young tree should include sprout removal, and cautious elimination of weak limbs within the tree canopy. For mature trees, sprouts should be removed regularly, deadwood should be pruned out, and diseased or crisscrossing limbs should be removed. If there is no interior fruit, the center of the canopy may need to be opened up to improve light penetration.
Skirting Up
Satsuma mandarins tend to have pendulous branches that hang to the ground. These are called skirt branches, and they can impede weeding, fertilizer and compost application, and provide pathways for ant populations to use the trees.
With heavy fruit loads, these branches bend and fruit may touch the ground. Fruit may then be contaminated by soil borne pathogens. These pathogens may be plant disease-causing such as Brown rot, or potential food safety risks, depending on the practices in the orchard. Trees should be skirted up to 18-24 inches above the ground every couple of years.
Pruning to Mitigate Alternate Bearing
Alternate bearing is the phenomenon where a tree carries a very heavy crop load one year and then a very light crop the next. In heavy years, the tree’s resources are depleted and it produces less new growth and is unable to set as much fruit the next year, giving only a minimal yield.
Pruning can mitigate alternate bearing to a degree. Trees should be heavily pruned after a light crop year, to reduce bearing wood and potential fruit load. Major branch pruning as well as canopy thinning should occur at this time. Pruning heavily after a light year allows the tree to replenish its reserves and move to a more balanced bearing habit.
When to Prune
Regular pruning for citrus trees should take place in the spring, between February and April. Pruning may be done prior to bloom, and it is important to note that although flowers may not be seen, they exist in a microscopic stage and will be lost due to pruning. However, unless pruning is drastic, yield loss will be minimal. Major pruning activities should take place after risk of a freeze has passed, but well before summer heat. If trees are pruned too early in the spring, the pruning can stimulate a growth flush which is susceptible to frost or freeze damage. Any maintenance pruning done in winter should only be small branches, one-half inch or less in diameter
The best time for severe pruning is after the danger of freezing temperatures is past and just before the spring growth flush. At this time new foliage will grow rapidly to cover exposed limbs. Bark that has grown in the shade is easily sunburned and may be killed in severe cases. Never prune trees drastically when they are suffering from drought.
Prune citrus to eliminate sprouts, remove weak, crossing or dead branches, or to allow more light in the canopy.
- February through April are the best months to prune.
- Remove all sprouts originating from the trunk. Most sprouts are best removed by hand when they are small.
Cold Tolerance of Citrus
Although citrus trees are cold-tender plants of subtropical and tropical origin and have not developed the effective cold hardening processes typical of temperate, woody, deciduous species, they have the capability for acquiring considerable cold tolerance. Citrus does not enter a deep dormancy (“resting” condition) characteristic of temperate-zone deciduous tree species such as apples or peaches. Rather, citrus enters a period of “nonapparent” growth (quiescence) as cooler temperatures (approximately two weeks of 40-60 degrees F.) occur.
Cold tolerance develops most readily when trees are not flushing (producing new leaves). Severely pruning dooryard trees during the late fall or winter months can reduce the size of the canopy, limiting the canopy’s heat retaining capacity and stimulating untimely growth of tender flushes of new foliage. The healthier, less injured, and less stressed trees are, the more they respond to cooler temperatures that induce quiescence.
In general, the degree of cold tolerance acquired by a tree is influenced by environmental conditions - mainly cool temperatures - as well as by tree health, its rootstock, and scion. Maximum cold tolerance ordinarily develops in citrus in the cooler parts of the marginal citrus growing areas because of lower average winter temperatures, compared with citrus growing in the far southern areas and along the coasts. But warm temperatures at any time during the winter may cause citrus trees to resume growth and reduce their cold tolerance. Such trees are very vulnerable to cold damage and should be protected or severe damage can occur.
How Trees Freeze
Trees are most vulnerable to cold damage during their first 5 years, especially during growth flushes and when the trees are recovering from stress caused by lack of water, drought conditions, diseases, insect pests, nutritional deficiencies, and previous cold damage. Parts of the tree exposed to the atmosphere and the smallest parts of the tree (twigs, leaves, developing fruit) cool the fastest and are the most vulnerable to cold damage. Flowers are the first tissues to freeze, followed by tender, new growth (leaves and twigs), then older fully mature growth, small-diameter wood, and then large-diameter wood, with the trunk being the last to freeze.
Young, developing fruit tends to freeze before mature fruit and smaller size fruit before larger fruit of equal maturity. Fruit with thin peel tends to freeze sooner than fruit with thick peel. As a rule of thumb, citrus trees generally freeze from the top to bottom and from the outside to the inside of the tree.
Ice formation in citrus tissues - not low temperatures as such - kills or damages citrus trees and fruit. However, tissue where ice forms does not always die. The critical temperature for ice forming in citrus tissues is approximately 28 degrees F., except when trees are in bloom and/or visible frost (frozen dew) occurs. Because of a phenomenon called supercooling, citrus flowers, fruit, leaves, and wood sometimes have the ability to supercool (to exist in an unfrozen, undamaged state below critical freeze temperatures) to as low as 16 degrees F. in leaves and 10 degrees F. in fruit, depending on the severity and duration of the freezing temperatures.
Pruning After Severe Cold or Freeze Damage
After a severe freeze that causes damage to major limbs, wait several months to prune. During the spring flush following a freeze, leaves on freeze-damaged limbs may grow but then will wilt soon after. After this wilt occurs on the spring flush, you will have a better idea about which limbs to prune. Realize that limbs with minor cold damage and split bark can linger in a poor state of growth for months, and even years, after a freeze. Loose, split bark and oozing are immediate signs of injury, but damage may be more extensive so one should wait to prune until they know the full extent of the damage. New growth (or the lack of such growth) will show the damage after several months. Once any damage is evident, typically by early summer, it is time to remove dead branches by cutting back several inches into healthy, green wood, then protecting the large limbs that are cut with diluted latex paint if the returning shoots are few. Typically the regrowth is vigorous and paint will not be needed.
Sometimes when a tree is weak, frozen back or broken off, a sucker or shoot will grow from the rootstock. The fruit from this rootstock shoot will usually be different than on the original tree. (The tree may produce two kinds of fruit if a portion of the scion (top, desirable part of the tree) remains. Fruits from rootstocks may be sour orange, rough lemon, trifoliate orange, Carrizo citrange, or Swingle citrumelo or other rootstocks. Large thorns are common.) Cut the sucker off to allow the desired variety to become dominant. Some selections such as ‘Orange Frost’, ‘Arctic Frost’ and ‘Bumper’ are propagated on their own roots so they will come back true to type after a freeze. Some commercial nurseries such as Greenleaf Nursery propagate all of their citrus on the plant’s own roots so they will come back true to type after a hard freeze. Greenleaf Nursery propagates Meyer Lemon, Mexican Lime, Calamondin, ‘Seto’ Satsuma, ‘Miho’ Satsuma, ‘Okitsu’ Satsuma, ‘Kimbrough’ Satsuma, ‘Mr. Mac’ Satsuma, ‘Rio Red’ Grapefruit, ‘Meiwa’ (round) kumquat and ‘Navel’ orange.
It is very important to always protect the main trunk of all citrus from being killed by a severe cold so the top of the tree can regenerate itself. When a severe freeze (temperatures less than 20 degrees F. for several hours) is expected, the main trunks of all citrus should be protected. A simple, practical way to provide enough protection is to stack and/or lean several bags (2 cubic foot bags) of bark mulch around the lower trunk. The bags should be tightly nested together as a solid wall all the way to the ground. Then cinch or tie them together with rope to create a solid, thick barrier to protect against the low temperatures damaging the main trunk. A well protected lower trunk and the mature root system assure quick regrowth of the top of the plant and the tree will return to productivity within several years. There is no other practical means to protect a mature citrus tree when the freeze-from-Hell (1983/1989 when temperatures below freezing temperatures occurred for over a week and low temperatures were in the teens.) occurs again. Want even more protection? Inject water into the bags to make the bark wet causing it to be even harder to freeze. In the spring when all danger of hard freezes has passed, remove the bags of bark mulch from the tree trunks and use them to mulch around the base of the trees.
If your tree is completely destroyed, it is usually better to plant a new tree of the desired variety than to try to bud the rootstock. If you are thinking of moving a mature tree to a different location, it is also usually more economical to plan
Defoliated top of Changsha at Becks in 09
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Dr. Larry Stein harvesting Satsumas on top of protective roof
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Mrs. Betty Nethery with Satsumas planted near a house for protection
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Satsuma over protection of roof
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Tip Freeze Damage on Citrus
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Untouched Changsha tree at Becks
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Mounding with soil to protect base of tree -- gardeners use bark mulch
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Regrowth from limbs after a hard freeze
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