Some Plants are Damaged if Temperatures are Above Freezing

Some plants can get damaged at temperatures above freezing. Learn how and which ones.

Chilling damage occurs mostly to tropical fruit if temperatures are like a refrigerator.

Freezing Temperatures

This is the time of year, the second week of December, we normally experience the start of freezing temperatures. The likelihood of freezing temperatures increase through the winter. Sometimes freezing temperatures occur in November, as it did a few years ago, but that's rare and not “normal”. What I mean by “freezing temperatures” is plant damage that occurs anytime the air temperature drops below 32 degrees Fahrenheit (F). Those familiar with Celsius or centigrade, may realize this temperature is the same as 0 degrees on the Celsius scale.

What is Chilling Injury?

As a reference point, the temperature inside most refrigerators is set to around 40 degrees F, or 8 to 10 degrees above freezing; too cold for most tropical fruit and plants but not too cold for temperate fruit like apples and peaches. Chilling injury (plant damage that occurs because air temperatures are too cold for the plant but not yet freezing) is one reason many ripe tropical fruits, like tomatoes and (more obviously) bananas, should not be exposed to the 40-degree F temperatures of a refrigerator. All parts of tropical plants such as tomatoes and bougainvillea, experience “chilling injury” when temperatures drop a few degrees above freezing and may extend to 50+ degrees F. Chilling injury (as opposed to freeze or frost damage) occurs at different temperatures and depends on the plant.

Chilling Damage

Chilling injury damage to tropical and subtropical plants include small stem and leaf discoloration, leaf roll, poor growth, and susceptibility to some diseases like root or collar rot. Symptoms of chilling injury include a change in color such as yellowing or bronzing of leaves that ultimately result in leaf scorch or drop, the slowing or halting of growth, leaf drop, water-soaked patches in soft and semi-hard tissues, susceptibility to diseases, and wilting. Chilling injury is due to cooler or cold weather (above freezing) temperatures to tropical plants growing outside of, or close to, the fringes of their normal range. As a side note, I noticed leaf and stem discoloration (closer in color to leaf “bronzing”) in mesquite, palms, citrus and a wide range of plants growing at different temperature ranges.

Warm Season Vegetables Usually Have More Chilling Damage

Vegetables can exhibit chilling injury and freezing damage as well. So-called “warm season vegetables” such as tomatoes, peppers and eggplant can show chilling injury anytime the air temperature drops into the damaging temperature range I mentioned earlier. Cool season vegetables, on the other hand, may sail through the same temperatures, or lower, or require a crop cover when temperatures are below freezing. Vegetable varieties may differ in their chilling injury by a few degrees. The ‘Dragon’s Tongue’ variety of bush bean is more susceptible to collar rot (chilling injury) when grown in garden soil a few degrees cooler than other bush bean varieties.

Refrigerator Temperatures Result in Chilling Damage

Temperate fruit like apples and pomegranates, unlike tropical fruit like tomatoes and bananas, are not damaged at refrigerator temperatures (around 40 degrees) because fruit from these trees can handle these lower temperatures. The ideal storage temperature for these types of fruit is somewhere close to freezing (0 degrees F) and combined with high humidity. A high humidity slows water loss and helps delay some fruit from shriveling.

Freezer Temperatures Result in Freezing Damage

The freezer part of our refrigerator is set to around 32 degrees F, or about 10 degrees below the “refrigerator temperature.” Our nighttime winter temperature frequently drops to a “refrigerator temperature” range at night during the late fall, winter and early spring and occasionally into the “freezer temperature” range during the early morning hours of December, January and occasionally early February. When nighttime temperatures reach the “freezer range” is when we often times see plant damage or experience fruit loss, but we may not know it yet.

Open Flowers are All Subject to Some Type of Damage

Open flowers of any fruit (citrus, peach, apple and others) can’t handle temperatures below freezing (32 degrees F) even though most plants or trees might show no damage at all! When flowers are simply buds and not yet open, there is a small amount of freeze protection provided to the developing flower. This freeze protection starts disappearing as the flower buds mature into open flowers. As the flower begins opening, and the frost-sensitive ovary is surrounded by the freezing night air, is when we experience damage or fruit loss. Fruit loss due to a frozen flower ovary can happen in a few seconds. This is why sprinklers, ultimately resulting in applied water turning to ice on the flowers, are used in orchards to prevent freeze damage to flowers (ovary). The act of water freezing releases a small amount of heat that protect flower ovaries from death.

If you are curious if the ovary of a flower from your fruit tree was damaged during a freeze, pull the flower apart a few days after a suspected freeze and inspect the ovary for death. Ovaries that eventually turn into fruit will be robust and green. Dead flowers drop from the tree early or have a dull, water-soaked appearance if they are still attached. Just because the flower you inspected was “dead” doesn’t mean there will be no fruit produced at all. It takes about two or three weeks for all the flowers to open in spring flowering plants. Several consecutive light freezes in a row (or only one hard freeze) are needed to totally wipe out a crop of fruit from a mature tree.

 There is a temperature difference between the freezing death of open flowers and the freezing damage or death of the plant or tree. For citrus this difference can range from the same temperature as flower death (32 degrees F; limes and true lemons) to lower temperatures (mid 20 degrees F; Myer lemon, grapefruit, and kumquat). Much of this depends on how many minutes or hours a killing freeze lasts and at what temperature.

Temperature, and duration of that temperature, are what is critical. With open flowers, freezing temperatures are needed for just a minute. With plant or tree death, the same temperature is needed for a longer time period. With some fruit trees (such as citrus), the temperature difference between flower and tree death can be small (0 to 7 degrees). In other fruit trees (such as apples, pear, peach and apricot) it can be much greater (20 degrees or more).

What to do about freezing temperatures and fruit or plant loss? Track and use weather station apps during times when freezing temperatures might occur. Buy a recording thermometer for your backyard and hang it in your important trees. If you are into growing vegetables, use a soil thermometer to judge when to precisely plant.

Move fruit trees to a new location. If your fruit trees are less than three to four years old, now is the time of year to move them. South and West-facing sides of walls and buildings are tough to grow plants because of the heat they reflect. Some citrus are in landscape “wind tunnels” that remove flowers during strong winds. North and east sides of buildings and walls as well as backyards have a gentler environment. In cooler, gentler areas of the landscape expect flowering to be two to three weeks later. Group plants together so they can share water, fertilizer, and protect each other. If they can't be grown together, consider constructing wind barriers.

Try kumquats which are a cold hardy, subtropical citrus trees. The small fruit can be freshly harvested and eaten from the tree but still gives you that citrus “kick”. Kumquat flowers several times during the summer instead of just once and yet is small enough to grow in a container. Flowering more than once during a single growing season increases your chances of having a successful crop if you have late or multiple freezes.

Grow fruit trees that flower continuously. Pomegranate, fig and jujube are temperate cold and heat tolerant fruit trees that flower continuously through the growing season.

Always group plants and fruit trees together. Improve the soil at the time of planting and every couple of years. The water, soil improvement and shade that they provide and share is a big benefit to them when grown in the desert.

Why Not Encourage Winter Planting in the Warm Desert?

Q. Why do you encourage people to avoid planting during the winter months? I can understand it in Minnesota but why southern Nevada?

Oleander winter freeze damage during a particularly cold winter.

A. Simply because I don’t know what is going to happen. I tell you spring and fall are the best times to plant. If it is a warm winter. then planting during the winter is okay. But if it is a cold winter, such as the lower teens at night, then be careful! Plants need soil at least at 45 -50F to grow roots. Otherwise the roots just sit there until the soil warms. 

Winter Freeze?

Cold or freeze damage to myoporum (locally known as Australian racer).

Will it freeze this winter or not? If the plant is winter tender, like bougainvillea or most citrus, then you are playing Russian roulette with the winter temperatures. It is best to plant those (that might freeze during the winter) in the spring for that reason.

Winter freeze damage to bougainvillea.

Oleander Freeze Damage Can Be Fixed By Cutting Plant Back

Q. I have an oleander that suffered extensive freeze damage this past winter. It is still brown but coming back slowly. I would like to improve the looks of the bush without killing it. Should I prune the dead looking branches or just let it go

Repairing Lemon Trees From Winter Cold Damage

Q. Our lemon tree shows damage after our recent very cold spell.  Some of the leaves are dry and dropping.  Is there anything we can do to assist the tree?

Potted Meyer Lemon Flower Drop

Q. You helped me before with my Meyer Lemon, so I hope you have an answer for this one.

My tree is in a huge pot. It is about 4 years old.

            Last Spring, it had massive flowers (yum) and a lot of little green buds followed. Then every single one of those buds turned black and dropped off.  Not one remained.

I want to figure out why and change what I am doing so this never happens again.

            The plant is fertilized with granular fertilizer 2x a year -- early spring and late summer. It gets moisture and hasn't dried out.  However the leaves could look more beautifully green.

Sometimes, some of them curl and are not quite bright green.

            I do not know if the two issues are related, but I sure hope you have a suggestion.

A. Sounds like you had post bloom fruit drop. Fruit drop can also occur during summer months and just before harvest. The usual reasons for post bloom fruit drop is usually some sort of stress.

            Four years is getting up there for being in the same pot without repotting. You might consider repotting and adding some new soil to the mix.

Meyer lemon flowers

            I know you said it had adequate water but if it went through just a few hours of drought during or just after pollination, fruit drop may occur. If we have some freezing weather during or just after flowering, that can cause the fruit to abort too.We had some on January 6 and 8 in parts of the valley.

            When watering, make sure about 20% of the water that you apply runs out the bottom of the container each time you water. This is important for flushing salts from the soil.

            Another possibility in containers is overheating them. If in direct sunlight and the outside of the container gets too hot and transmits this heat to the soil, this can cause stress and cause fruit drop.

            Proper fertilization is important. Over fertilizing fruit trees, excess nitrogen, can cause fruit drop. And finally less commonly some insects such as scale or mealybug infestations can cause fruit drop as well.

            What to do? Make sure your container, the soil volume, is big enough to handle wide swings in temperature and water. Monitor both closely. You might find a houseplant moisture meter to be helpful.

             Keep the outside of a plant container out of the hot sun. Double potting a container is  helpful to keep the soil temperature down. Watch for freezing temperatures at bloom time and cover the plant.

            Water the soil just before the heat of the day. Wet soil heats up more slowly than dry soil. If we have any frost during bloom it will affect fruit production.

Avocado Very Iffy in Cold Desert

Q. I ordered an avocado tree online and followed the directions for care.  I understood it was compatible with the Las Vegas climate but it kept loosing leaves.  New leaves would grow but I finally gave up on it.  Just for fun, I presoaked a pit from a fresh avocado and I planted it outside and it's doing well.  Now that the weather is cooling down, should I leave it in the ground or transplant it to a pot and bring it inside? What do you recommend I do with it in the spring?  Would it ever bear fruit?

Super Avocado weighing in at over 2 lbs. (Photo courtesy Rare Fruit Society of the Philippines)

A. A lot of people would really love to grow avocados here but they are very "iffy" in our climate. For the most part, I would discourage most gardeners from planting them. If you could get it to survive here it would bear fruit.

            The usual reason for dropping leaves is a watering problem; going from dry to wet and back again. If you are going to experiment with an avocado I would suggest putting down a 4 to 6 inch layer of wood mulch around the trees but keep the mulch a foot away from the trunk in the first five years. I think you will see a big difference.

Avocados of Zimbabwe on display at local fair in Harare

            If you just HAVE to grow an avocado tree, focus on the more cold hardy types such as the variety ‘Mexicola”. Find a warm spot in the yard out of the wind. Store bought avocado fruit, such as Hass or Fuerte, are not cold hardy varieties. These would be destined as houseplants or grown in greenhouses.

            Avocados are really big trees. Another possibility is to try a dwarf avocado and put it in a container. There is one true dwarf avocado called ‘Wurtz’ or marketed sometimes as ‘Little Cado’. This variety does not tolerate any freezing temperatures at all so you must move it into a spot that will not freeze at the first hint of a frost.

            If you just want to play around and experiment then by all means do that. Just remember that avocados are typically not tolerant of our winter cold.

Bougainvillea Protection From Freezing

Q. While most of our bougainvillea are in large pots which we bring in doors for the winter; this year we planted one in an above ground planter built around the Jacuzzi. My question is what should I do to help it survive the winter without having to dig it up and transplant it again in the spring. Should that even be an option?

A. I like your idea of wrapping the bases of the plants before any freeze. Unless you have a very warm microclimate in your yard, and there are some in Las Vegas, you will see damage to them at the first light freeze with temperatures below 32° F. Wood mulches will also protect the base of bougainvillea but wrapping it is more effective. This way when it freezes it will kill the top of the bougainvillea to the mulch or wrap and not much further. 

Of course this works to temperatures below freezing for short periods of time. If the temperatures are extremely cold or at these temperatures last a long time that it may freeze the plant to the ground. How well you protect the base of the plant will determine how strong it will come back, or not, next spring. Having bougainvillea in the spot in the yard which does not have a lot of wind and is very close to a warm, South or even better yet, West-facing wall will improve its chances for survival.

Plants that freeze to the ground will come back like gangbusters next spring because of their established root system. Plants that are planted in the spring have to grow both the roots and the tops and so growth is divided between the two. Plants that have an established root system only have to grow the top back and so all of that growth goes to top growth and we see huge amounts of growth in the spring.

You do not want to feed them anything this time of year. This goes for any winter tender plant. These plants should get no fertilizer applied to them any time after mid-August. Fertilizers which stimulate new growth will cause the plant to become more succulent and lower its chances of surviving winter freezes. This is not true of plants that have no problem surviving winter freezes. In fact, those plants can receive mid fall applications of fertilizer with no problems. A mid fall application of fertilizer can substitute for a Spring application but not winter tender plants.

Avocado Not a Good Choice for Las Vegas

Q. I saw an Avocado potted tree at Lowe's hardware and was thinking of planting one in my back yard. My question is: When is the best time to plant this tree? Is fall okay? I'm thinking of the winter freeze that might occur. I hope I hear from you soon, in case they sell them out.

Freeze Damage on Acacia Can Be Hard to Fix

Q. Any advice on Acacia trees would be great.  These trees took a hit when the temps dropped to the 20's in December.  I see new growth, but mostly on the suckers.  Should I leave the suckers there or remove them? What is the best thing to do to help these trees recover?
 

A. This is more complicated. First you will remove any wood you know is dead. By now (June), any part of the tree which is still alive should have thrown out some growth. Remove any limbs that are dead (no growth coming from them) by cutting at point of attachment to another limb or the trunk.  You should not leave any stubs when you are done.

            If the limb is large and heavy, you should remove it by either removing sections of the limb at a time that are manageable or use a technique that we sometimes call the 1-2-3 method. This is demonstrated pretty good on Wikihow http://www.wikihow.com/Cut-a-Limb-from-a-Tree

            Next, remove any broken branches. They will not repair themselves. Remove any wild or “sucker” growth. This type of growth usually has weak attachment to the trunk and not support itself in years to come. Remove any growth coming from the trunk that is not high enough in the future.

            This growth will not get any higher and as it gets bigger will “sag” or bend downward perhaps into places where you can bang your head. Remove these by making a “flesh cut” in other words remove it all and don’t leave a stub.

            Finally thin out the remaining branches so that any dead wood is removed (again by making “thinning cut” which is the same as in the third sentence, removing it at a point of attachment without leaving a stub.) Try to have the remaining branches going in different directions to help balance the crown visually.

            I hope this helps. By the way, I would do any major limb removal next early spring after the worst temperatures have passed. Minor cuts (with a hand shears) can be done any time.

Using Ficus as a Hedge/Screen Around a Pool in Phoenix

You scared me for a minute. I'm in Las Vegas so when I saw your Ficus mentioned I was concerned. I am less concerned with it in the Phoenix area but the pool area puts it in a different twist and you still have to worry about freezing damage.

            But I am going to copy this to my good friend Terry Mikel who was your Extension Specialist in Maricopa County with the main office in Phoenix. He is better at answering this one and I will post his answer. He is in retirement but I am SURE he will come out of hiding on this one!

Q. I live in Glendale Arizona. I have a 45ft section of 6 foot brick wall that I would like to plant Ficus nitida along. There is a 4 foot width between the wall and a plaster underground swimming pool. No problem watering the hedge?  But will this plant seek the pool water and cause a problem. I am not worried about frost. I want a 10 foot hedge when done.

A. Your thoughts about having a hedge sound good.  Ficus microcarpa sub species/or cultivar 'Nitida' can fit the situation; its clean (no real messes). evergreen and makes a dense wall of foliage.

            I have to use the term 'can' with a couple caveats. 

1. This plant can grow to a very large size.  Frosts every few years help keep them in check.  And pruning can, to a certain degree keep them in check.  But, frosts and pruning will be a continuous battle against their genetics: it wants to be a 60' tall and 80' wide tree.

2. This species of Ficus will after time develop huge surface roots that will lift, push or barge anything in their path.  It’s their genetics and watering will have little, if any affect so the wall to the one side and any pool decking on the other side will be vulnerable to the large lifting roots.

            Your one concern about them 'seeking' water is a commonly misunderstood trait of any plant. . . Plants do not search out water, period.  Plants send out roots randomly in all directions and roots that run into a water source will proliferate.

            Pool sides, if sound will be the same a rock in nature, the roots will try to grow up, under, or around any solid object.

            If, and this is a big if there is the slightest oozing leak from the pool's wall  and a root meanders there then it will grow and proliferate in response.  That's where the issue of roots and pools becomes a problem.

            Personally, the Ficus is overused and most people who grow them quickly tire of all the problems with them.  If anyone who knew much about them would warn folks against using them except for large evergreen tree.

            Your setting is a little bit of a challenge. You might think about something much less vigorous with fewer potential problems and some have blooms (a potential 'mess' issue). Look at: citrus, Hop Seed Bush (Dodonea), Xylosma (both common and botanical name), Arizona Rosewood (or any in that genus of Vauquelinia), one of the many different blooming colors of Arizona Yellow Bells (Tecoma and various species and varieties), Petite Oleanders have been used in that setting for generations.

            Every person's 'likes' and 'dislikes' vary. Check with your water department and pick up some of the nice booklets about planning and plants for the landscape. These were produced by the Arizona Municipal Water Users' Association, more lovingly called AMWUA to be distributed in the different communities.

            Another resource might be going to the Mountain States Wholesale Nursery (MSWN.com) site for a truly complete list of plants that are well adapted in the lower Sonoran desert. They also produce some for the higher deserts but their main goal is for lower deserts. .

Respectfully,

Terry H. Mikel

Yellowing in Palms Can Be Lots of Different Things

This is California fan palm with cold damage from
temperatures in the low 20's or high teens F

Q. We have a fan palm where the palms are turning yellow and I'm assuming dying. One or two would be acceptable but we are have 5 and 6 that are going bad.

A. Let's cover some ideas about why your palms could have yellow fronds. These are the main reasons: older fronds are dying from natural causes and should be removed; fronds were damaged during winter freezes; too much water applied too often or too little water; palms planted too deeply; palms planted in heavy soil that doesn't drain well; palms planted with pure sand around the rootball; fertilizer problems such as iron, manganese or zinc.

Not totally sure on this one but it was probably
light cold damage on this date palm during the
 winter in low 20's

Palms should be planted with the soil taken from the hole plus 50% compost and a starter fertilizer high in phosphorus added. Palms should NOT be planted with their root ball surrounded by pure sand in the planting hole which is commonly done in Las Vegas. The idea of doing that is absolutely crazy.

Palms are high water users even though they tolerate high temperatures and our desert climate. Different types of palms require different amounts of water. The larger the palm, the more water it will require. Some of the highest water users will be date palm with their huge canopy spread. The amount of water will vary but most fan palms would be happy receiving about 20 gallons every time they are irrigated.

This is Queen palm, not a good palm for the hot desert
and probably lack of soil preparation or watering too often
or both

One of the common problems is irrigating palm trees with small amounts of water, like ten or fifteen minutes of drip irrigation, daily or even twice a day. Watering like this can fill the soil with water and suffocate the roots causing them to rot, diseased or both. So if you are irrigating your palms daily, don't do that anymore!

During the heat of the summer they can be irrigated two or three times a week, using 20 gallons each time you irrigate but the soil must freely drain the water away from the tree. In the wintertime you might be dropping your irrigation to 20 gallons every 10 days or perhaps even as long as two weeks.

Sometimes the soil lacks certain types of minerals that palm trees need. Deficiencies like iron and manganese usually appears as a discoloration in the fronds at the center the canopy, the most recent growth. This can range from light green to nearly yellow. If these inner fronds are yellowing then we can usually narrow this to watering too often, poor drainage or a lack of minor elements such as iron.

Center fronds, youngest, demonstrating yellow growth
which could be due to watering issues, soil issues
or possibly disease

Next February when you make your annual application of fertilizer use a complete fertilizer such as a Miracle Gro, Rapid Gro, or Peters. You can also use fertilizer stakes. Make sure it is well balanced and try to select a fertilizer with the three numbers the same or close to the same value like a 16-16-16 or 10-10-10. Add an iron chelate that contains the EDDHA chelate in the ingredients. Let's see if that works along with irrigating with a large volume of water but doing it less often.

Cold damage on palms usually results in the older fronds turning a bronze color first and then browning later as they die. General rule of thumb is if more than half of the frond has turned brown, remove it. The new fronds at the center of the palm should be healthy and green when they emerge in mid-spring.

If your soil is heavy and holds water a long time then plant on a mound 2 to 3 feet high and 6 to 8 feet across so the water drains away from the roots.