Janet Hartin

It is the time of year when we all become spooked, not just from ghosts and goblins but also from the overwhelming array of Halloween candy and costumes intertwined with Thanksgiving displays and Christmas decorations in stores all at the same time. I do not know about you, but I am barely able to stay on top of fall garden chores, let alone think about the holidays. Nonetheless, fall is upon us and it is personally my favorite time of year. It is also a reminder that with the beauty and stillness (outdoors at least!) of the season comes the reality of the rapidly changing weather and potential frost damage to some of our most beloved plants.

As Master Gardeners, you hear a lot of harping about the influence of microclimates as well as macroclimates and the importance of site selection when planting. Keep in mind that low temperature sensitive plants perform optimally under the protection of a large tree or next to a wall with an eastern or southern exposure.

Below are other tips to share with your fellow gardeners: 

-      Ensure that the soil has adequate moisture. This increases its ability to retain heat.

-      Cover sensitive plants with a sheet or newspaper. Remember to remove the cover before late morning once the danger of frost has past.

-      Wrap trunks of sensitive young trees in cloth or cardboard for insulation.

-      Consider using the older kind of heat-generating electric holiday lights (a real bargain these days in second hand stores due to the changeover to energy conserving varieties!) to generate warmth. Place lights at the base of plant since heat rises.

-      Avoid pruning plants until all danger of frost has occurred in late spring when new growth appears. Damaged trees should not be heavily pruned for a year. Frost-damaged plants are more susceptible to biotic (eg: disease and insects) and abiotic (eg: more frost damage!) damage so keep a watchful eye over them until spring has sprung.

-      Learn from your mistakes. Consider planting less frost-sensitive species the next time around, or at least place tender plants in more protected locations!

Recognizing freeze-damaged citrus (thanks to Dr. Ben Faber for the following advice)

During periods of low temperatures, citrus leaves can appear wilted or flaccid. This is the

citrus tree’s natural protective response to freezing temperatures, and does not always

imply permanent frost damage to your tree. If a hard frost does occur, frozen leaves will

appear firm, brittle and sometimes curled. Leaves will become flaccid after thawing, and

if the injury is not too severe, they will gradually regain turgor and recover. Dark flecks

on the leaves can be an indication that a freeze and thaw has occurred. Leaves that have

been exposed to a prolonged frost will mostly likely shrivel, die and remain on the tree.

New and tender foliage are most susceptible to frost damage. If twigs or wood have been

seriously damaged, the frozen leaves may remain on the tree for several weeks. If the

twigs and wood have not been damaged severely, the leaves are rapidly shed. Contrary to

what many growers believe, it is actually a good sign when a tree sheds its leaves rapidly.

This usually indicates mild frost damage to the trees.

Cold damage to twigs appears as if the twigs have absorbed excess water and also

become discolored. In older branches and on the trunks, cold damage will be indicated as

splitting or loosening of bark, where the cambium has been killed. Bark may curl and dry

with many small cracks. Dead patches of bark may occur in various locations on limbs

and trunk. Sensitivity to frost is dependent upon many variables. In general, mandarins

are the most cold hardy followed by sweet orange and grapefruit. Lemons are very frost

sensitive with Eureka decidedly more sensitive than Lisbon. Limes are the least cold

hardy. Healthy trees are more tolerant than stressed ones.

The rootstock also imparts sensitivity onto the scion. Injury to the foliage and to young trees may be immediately recognizable, but the true extent of the damage to larger branches, trunks, and rootstocks can take up to four months to show damage following the freeze. No attempt should be made to prune or even assess damage from the frost until spring when new growth

Rehabilitation of freeze-damaged citrus

Whitewashing

The only treatment that should be done after a hard freeze is whitewashing. Often the

most severe damage following a freeze results from sunburn of exposed twigs and

branches after defoliation. Temperatures do not have to be extremely high to cause

sunburn. A white latex paint that has been diluted with water (making it easier to spray),

is the best way to whitewash the exposed tree parts. The whitewash needs to dry white

on the tree, so careful not to overly dilute the paint.

Pruning

Pruning should be carried out to prevent secondary pathogens and wood decay organisms

from slowing down tree recovery. Again, as previously mentioned, there should be no

rush to prune. Premature pruning, at the very least, may have to be repeated and at the

worst, it can slow tree rehabilitation. It should be remembered that when pruning, all cuts

should be made into living wood. Try to cut flush with existing branches at crotches. Do

not leave coat hooks (branch stubs) or uneven surfaces. Tools should be disinfected in

bleach or other fungicide before moving on to the next tree.

Irrigation

Irrigate carefully! Remember that when leaves are lost, evaporation from

leaves is greatly reduced, and therefore the amount of water needed should also be

greatly reduced. A frost-damaged tree will use the same amount of water as a young or

smaller tree. Over irrigating will not result in rapid recovery. Instead, it may induce root

damage and encourage the growth of root rotting organisms. Irrigation should also be

applied less frequently, with smaller amounts of water until trees have regained back their

foliage that was lost due to the freeze.

Fertilization

Fertilization of freeze-damaged trees should be carefully considered. There is no

evidence to indicate that frozen trees respond to any special fertilizer, including fertilizers

that are supposed to stimulate growth. If trees are severely injured, with large limbs or

even parts of the trunk killed, nitrogen fertilizer applications should be greatly reduced,

until the structure and balance of the tree become re-established.

Trees should be monitored for evidence of micro-nutrient deficiencies. Deficiencies of

zinc, manganese, copper, and iron are most likely to develop. For citrus, these nutrients

should be applied as sprays, and they should be used as often as symptoms are observed.

Two or more applications may be required the first year.

--