After the Freeze Pruning
February 2007, Southern California

Once in awhile we Southern Californians get a really cold winter storm. We had one of those chillers this winter. The night time temperatures went below 32 almost everywhere except protected spots along the coast. Inland areas had sustained temperatures down into the low 20's, even 18 or lower in the really cold spots. The other factor that made this storm damage more severe is the fact that it stayed cold for more hours of the night than a typical frost.

This is an image of a frost damaged hibiscus plant in a local Carlsbad garden. Note that there is a little green at the base. Waiting to grow again.

What we hear from gardeners everywhere is "What do I do now? Shall I cut off the burned frozen parts now or wait?"

IMPORTANT:

The answer depends on when you are reading this. February readers need to wait for warmer weather. Even though the brown frozen plants look terrible you need to wait, wait, wait. It seldom hurts to wait and pruning woody plants too soon can cause all sorts of problems.

If you are reading this in April or May you have probably waited long enough.

If you live in our higher elevations or the coldest inland valleys you need to wait until all danger of frost is past. Any where else if you are not sure ask your local Garden Center expert or contact your local Master Gardeners Agriculture Agriculture extension office.

Here are the reasons why you should not prune too soon. We could still see another Alaskan storm come roaring through in February or March. Right now all those freeze-damaged vegetation looks really bad but it may still do some good. The frozen dry, dead leaves, shoots and branches can still give some protection if we have another freeze. Wait until mid March or even April or even later to prune. Then you can get in there and cut back your frozen plants. Here are some easy tips for determining how much damage was done.

Scratch the Bark Test

Scratch the bark with a sharp knife or your finger nail to see if it is still green inside. If it is green then it is very likely that your plant will grow again. In this plant there is some green at the base. Everything up higher was dead. See the paragraph under Woody Shrubs later in this article for more information.

Know Your Plants

You need to know what kinds of plants in your garden were damaged before you can understand what to do. I put this together using some of the advice from UC Cooperative Extension (UCCE) horticulture advisor Ed Perry. His advice is good but almost totally for citrus and avocados. I have copied and adjusted some of this advice and will add my own experience.

The frost injury to plants depends upon a number of factors, including species, age, health, soil moisture and location. Frost injures plants by causing ice crystals to form in plant cells, making water unavailable to plant tissues and disrupting the movement of fluids.
When we have many hours below freezing the damage is much more severe. I tried to think of a good analogy and this is what I came up with. When I fill my freezer's Ice Cube tray and I check it an hour later, it will be frozen around the edges but the inside is still water. If I check it 3 or 4 hours later the ice cubes are solidly frozen. Your plants and trees are similar. Long hours of below freezing do more damage.

Impatiens and Begonias etc.
Let's look at the soft tissue plants like impatiens, begonias and other garden bloomers. Everywhere else in our fair country these are all considered Annuals. They grow quickly and look great all summer and as soon as the first real frosts appear they are dead. Dead forever!
The next spring these gardeners plant all over again. Here in "LaLa Land Southern California" we often call this class of plant "Frost Tender perennials". That means that if it doesn't freeze, they can and do grow and look good for 2 to 4 years and then they have worn themselves out and need to be replaced.

What about all those bougainvilleas that froze? They certainly do freeze easily. One day they are gorgeous, the next they are a mass of brown dead leaves and branches. Unless your bougainvillea was newly planted it should come out again when warm weather arrrives. When you first plant a bougainvillea the roots are extremely sensitive and if disturbed your plant can easily die. However, established plants have roots that can't hardly be killed, no matter what.
When your plant comes out in the spring the leaves will be young and tender and the new Bouganvillea looper could attack and actually defoliated your plant. That could bring it close to death. You should begin to watch for those notches along the leaf edges as soon as new growth arrives. If you see that and they are not being caused by snails---then it is time to get out the Spinosad or BT type of caterpillar control spray. Weidners has them both. They are not poisonous and will work if you keep at it. To learn more about Bougainvillea Looper

With these types of plants don't bother to wait for spring to decide whether to cut back or not. Even if there is some life left it will be too long before they look good. Face it. They are dead! Have a nice funeral and dispatch them off to plant-compost heaven. Then you can replace them quickly with spring color.

Woody Shrubs.
The next group of plant material are shrubs. Woody shrubs that you use around the garden to give permanent green with some seasonal flowers. Some of these types of shrubs are quite frost tender. Bougainvillea, Mandevillea, Dipladenia, Hibiscus, Plumeria and some of the newer petite Oleanders.

Some palms and all of the bananas and bird of paradise plants were damaged. Most of these will come back quite quickly. Think about where a plant is from in nature. If they are native to the tropics they are more tender and thus more likely to freeze. Sunset Western Garden book will give the frost tolerance of many of these. However we all believe that the real frost will never happen in our garden. If you want to check to see how far down the frost damage has been you can do the fingernail or sharp knife test. Scratch the bark. If there is green underneath there is a chance for life to come. If no green, then that part of the tree is dead. Growth may come from the trunk, crown or root but not from that branch.

With this group of plants you want to wait to prune until late March or April. My husband used to say let the plant tell you when it is spring. You will see new growth coming and that is when you should prune. My husband was usually right. An important factor for when plants put out new spring growth is soil temperature. We all know we can get a flush of warm weather in February and see a flush of new growth in early March. This is probably still too early. Wait just a little longer
If no more frost is expected prune out any damaged growth, cutting to an undamaged side-shoot or bud.

Remember this is advice for February, March and April in Southern California! Colder parts of the state will be different. A popular place to go for information is to the Internet. If you go to Google or other gardening sites it is a good idea to look to see the geographic source to know where it is coming from. Advice from Ohio will be far different from advice from a California Ag extension office advice.

Trees like Citrus and Avocados and other treesAdvice from UC Davis advisor, Ed Perry.

Unprotected, sensitive young trees may be killed, but frost rarely kills mature trees in California. "While you may be tempted to prune out damaged branches right away, it's best to wait until spring when new growth will show you the extent of the injury," said Ed Perry in his often quoted Press Release. He advises to "Always allow plenty of time for new growth to take place, so that the damage is clearly defined. Earlier pruning often results in leaving some limbs which might continue to die back, and the removal of limbs which might recover." If trees were severely damaged, you may have to wait until May or even June before new growth appears on large limbs or the trunk.

Irrigation

Irrigate carefully! Remember that when leaves are lost, obviously evaporation from leaves is greatly reduced, and, therefore the amount of water required is also greatly reduced. A frost-damaged tree will use the same amount of water as a much younger or smaller tree. Over irrigation will not result in rapid recovery. Instead, it may induce root damage and encourage growth of root rotting organisms. This is particularly true for avocados. Irrigation should be less frequent, and smaller amounts of water should be applied until trees or large shrubs have regained their normal foliage development.

Fertilization

Fertilization of freeze-damaged trees should be carefully considered. There is no evidence to indicate that frozen trees respond to any special fertilizer that is 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.

Perry goes on to say that there can be deficiencies in some of the micro elements but I decided that that information was perhaps too complicated for this piece. There are a number of good products with micro nutrients that can be sprayed on. If your citrus tree comes out with leaves that don't look like your normal green then contact your local Ag extension office/ Master Gardeners for special advice on how to correct this. You can find their phone number by going to the white government pages to county and then look under Farm and Home advisor.

Sunburn

Avocados, citrus and other trees with thin bark can suffer damage when protective foliage is damaged by a freeze. Heat from the sun can kill the thin layer of growing cells, called cambium-located just beneath the bark. Damage usually occurs on the south and west side of the trunk and vertical branches as well as the top sides of horizontal branches. You can apply white wash to protect exposed bark from sunburn damage. Mix white, flat interior latex paint with an equal amount of water and paint any exposed bark.

Succulents: agaves. aeoniums and plumerias that have been frost damaged

This is advice is from Patrick Anderson who has a huge collection of succulent in his garden.
Take a good look at your plants. Generally you want to wait for warmer weather because you can't always tell if the growing crown has been damaged or not.
Patrick says, For agaves & aeoniums, I would advise waiting, unless the central crown is clearly blackened - then the plant is a goner.
There are others, though, where I would suggest some preventive surgery. On some of the stemmed succulents, a damaged or killed growing point could introduce mold or fungus, and the plant might start to rot. That would kill more of the plant than the freeze did. If people see a soft, shriveled tip - in my case on a couple of alluaudias and a pachypodium - they should watch it carefully, and if it begins to progress down the stem, immediately cut below the rot and try to get the cut to dry.

Plumeria the same, if the end of the stem starts to rot and is soft to the touch. Cut it back to sound wood. This is to prevent further decay. Clean your prunning tool periodically with alcohol. This will help to prevent spreading decayed bacteria.

One last bit of advice. We all hate to see the damage from mean old Jack Frost but many of your shrubs and vines will benefit by natures pruning. What's gone is gone. Get over it. After all these are only plants, not your cousin Nellie.
Have a good spring and see you all at Weidners Gardens one of these days.