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Dealing with Freeze Damaged Landscape Plants
Rick Gibson, Jack Kelly and Glenn Wright
The University of Arizona, Cooperative Extension
Even though the low deserts of Arizona are known for warm winter temperatures, from time to time a harsh winter storm will blow through and lower temperatures to levels that are dangerous to sensitive landscape plants. It is important to know how to deal with freeze damaged plants and to minimize the potential for damage during future weather events.
The Difference Between a Frost and a Freeze
It is important to remember that there is a difference between frosts and freezes. Frost is the most common type of cold weather injury. It occurs when low temperatures and humidity combine with calm and clear nights to cause surfaces such as leaves, soil and car windshields to cool faster than the surrounding air. The moisture in the air condenses and freezes in place. This condition is called a frost.
Freezes occur when temperatures drop below 32° F. All plants have a specific temperature at which they will begin to suffer damage. Once that temperature is reached, damage begins. Lemons will begin suffering damage right around 28° F. while oranges and grapefruit usually do not start showing freeze damage until 26° F. or below. Both the duration and temperature of the freeze are important. Four hours or more at 28° F. or less will cause damage to citrus. Typically frosts cause less damage to sensitive plants than a freeze.
Cold Weather Damage
Damage to plant tissues from a frost or freeze occurs when temperatures fall below the point of a particular plant’s resistance. Frost, the formation of ice crystals on the surface of a plant, or freezing, the actual solidification of liquids within the plant cell, both can create sharp edges and points that can puncture cell walls and cause the cell to dry.
Damaged plant tissue can display many symptoms, but most commonly the affected parts will first appear water- soaked, wilt and turn black, and turn brown or gray after they dry. Leaves are normally the first tissues to be affected but heavily damaged plants may also have stem and trunk damage. Management strategies depend upon the type of plant.
In general, it is best not to cut out the damaged parts of plants like cacti, agave, aloe, and other tender plants such as Bougainvillea and citrus until after the danger of frost has occurred. The average date of the last killing frost varies by elevation throughout Central and Southern Arizona. Watch the weather trends and long term forecasts to determine the proper time to prune out dead and damaged wood.
There are two ways to distinguish dead wood from that which is alive. One is to wait until bud break in the spring and watch for new growth. Wood or plant tissue that does not grow when the rest of the plant is actively growing will be dead. The other method is to scratch the bark with a knife, fingernail, or other sharp object to see whether the tissue under the bark is green and moist, or brown and dry. Brown and dry tissues are dead. When pruning, try to avoid making heading cuts, and especially do not shear the plants off. It is better to cut each branch back to a bud or shoot that is alive and growing instead of cutting between the nodes or bumps on a stem. Such cuts lead to the growth of many branches from one spot which can damage the plant. Cutting back to a bud or shoot that is pointed towards the exterior of the plant helps preserve the natural appearance of the tree and prevent unwanted growth.
Cactus and Agaves
Even if agave plants or cacti look severely damaged, watch the plants for a few weeks to see if they are getting better or worse. Do not be in a hurry to prune. For agaves, take the heart of the plant and gently tug on it. If the heart comes loose, the plant is dead and can be removed. If the tissue is still firm and cannot be pulled out, the plant is still alive and will most likely recover.
Many agaves have underground rhizomes that may grow with the onset of spring and recover. Several 'solitary' or non-pupping species, however, do not have this ability and if the plant heart is damaged these will most likely die.
Cold weather damage to cacti is not necessarily fatal either. Many with only tip damage may survive. They appear blackened soon after injury but after a few months this damaged tissue will turn tan or gray and become crisp. If the damage is 18 inches or so in length, simply cut just below the damaged area and discard the removed parts. At this time there is a choice to be made. One can either wait until a new bud begins to grow or the whole stem can be removed and cuttings made from the unaffected parts.
If the choice is to make cuttings, the propagating piece should be cut into 12 inch long pieces and stored in a filtered light area. The cuttings must remain upright because if left on their side they will begin to curve toward the sun. Cuttings can be safely stored upright in filtered light for a few months. Rooting is more rapid when the nighttime temperatures are above 60 degrees Fahrenheit.
Another way to propagate new plants from the large cutting is to take the removed the stem, place it on its side, and bury it so half of the stem is exposed. Latent buds will develop from the large cutting when roots begin to form later on this spring.
Aloes will come back and may even flower even though they will look horrible. It is important to wait until the tips of the leaves turn gray or black and crisp and then remove the desiccated dead parts.
Indian figs drop branches in a freeze and will require a bit of pruning to remove the jagged point of attachment of the pads. The pads can be rooted.
It is not a good idea to remove damaged stems from the slow growing organ pipe cactus because the cuttings take 2 to 3 months to root. It is better to use healthy tips for propagating new plants. In the case of cold weather damage on organ pipe cacti, it is better to just prune back the tissue after the plant has time to adjust to the damage and then prune out the damaged tissue. A new latent bud will eventually begin to grow and restore plant growth and development.
Citrus varieties are affected by cold injury in various ways. The fruit is generally the first part of the plant to be affected by cold and sometimes will be the only part of the tree that is damaged. The first typical cold injury symptom of cold damaged fruit occurs when the fruit juice sacs are punctured by the formation of ice crystals. This gives the fruit a “slushy” appearance. Next, open cavities appear in the fruit flesh where the juice sacs have been destroyed. Finally, there is the drying effect. As fluids evaporate from the damaged fruit, the inside of the fruit becomes pithy and dry. After a frost or freeze, it is generally best to quickly pick the remaining fruit and juice them for later use. Otherwise the fruit will simply dry out and end up on the ground.
The next parts of the tree to show damage are the leaves. Frosted or frozen citrus leaves will show a water- soaked effect, then curl, dry, and eventually fall from the tree. Just because a citrus tree loses its leaves, this is not an indication that the plant is severely damaged. If the buds in the stems are still alive, they will begin to put out new growth and new leaves later in the spring.
While it is likely that twigs and small branches will freeze back and die, limb loss due to freezing is uncommon in Arizona except perhaps at higher elevations where citrus production is always a risk. At lower elevations, trees rarely die from freeze damage. It is more common, however, in the more susceptible varieties like lemons and limes, for fruiting wood to freeze back to the bud union, especially in younger trees. Growth subsequently in these cases will come from below the bud union and produce generally undesirable rootstalk- related fruit. When this occurs, it is best to remove the old tree and replant.
Fertilization of Damaged Plants
It is important to not over fertilize cold damaged plants. Nutrients from the fertilizers will probably not be efficiently used by the plants and could be wasted. It is better to wait until the damaged plants are actively growing to provide nutrients. It will be important, however, to watch the plants for signs of micro nutrient deficiencies, especially iron, and be ready to take corrective action.
Protecting Sensitive Plants from Cold Weather Damage
So how do we protect our plants against damage? When thinking of cold weather protection, most people immediately think of covering their plants at night, and that is exactly right. Improperly done, however, covering will actually afford little protection and may actually harm tender plants.
When covering plants, it is best and safest to use a fabric material. Plastic tarpaulins and table cloths will not provide sufficient insulation to keep plants warm during heavy frosts and freezes. Here is the reason why.
Natural cold descends from the atmosphere during the night. Natural warmth radiates upward from the earth, which is heated during the day by the sun. When the amount of cold overpowers the amount of radiated heat and temperatures go below what sensitive plants can endure, cold damage can occur.
Coverings shelter plants from the cold air that is descending down upon the plants and trap the radiated heat that is moving upward from the earth. This minimal protection often is quite sufficient to keep tender plants from harm.
In order for coverings to be successful, they must accomplish each of these two tasks. To do this, they must extend all the way to the ground. Full coverage will keep the warmer air trapped inside from escaping. They must also be put in place before it begins to get cold, which usually means late afternoon. They must not be removed until temperatures rise to a safe level, usually well into the following day. Covers are removed to allow the sun to reheat the soil underneath the plants. This will provide warmth for the next night’s protection.
Cloth, painter’s cotton drop cloths, heavy mover’s quilts, cardboard, or paper coverings insulate better against the cold than plastic coverings. However, plastic could work for frost protection if the temperatures do not dip too low. Plastic tends to radiate heat faster than these other coverings and are a little more risky to use. In a pinch, and if plastic is all that is available, rig a frame to hold the covering off of the plant foliage. The cold temperature of the plastic itself could damage tender plant tissue. Extra warmth can be provided for the most tender or most valuable plants, by placing a low watt light bulb inside the covering. The extra heat from the light bulb can help keep the air temperatures inside the tent high enough to avoid plant tissue damage. In doing this, do not forget safety. Do not let the light bulb touch the covering or the leaves or stems of the plant. Coverings could catch fire and tender tissues can be damaged by the heat.
Use an outdoor extension cord and make sure that there is no standing water that could cause an electrical hazard. Do not forget to turn the light off during the daylight hours to save money and avoid the possibility of plant damage.
Irrigation is often practiced before a cold event because wet soil holds the heat better than dry soil. Flood irrigating also works for frost protection during the event on the principle that water must give off heat to freeze and the slight amount of heat released can moderate the sharp plunges of temperature during a frost. However, flooding during the event is risky because water must be present for the complete duration of the freeze or frost or the colder temperatures resulting after heat release may worsen the damage. It is not worth the risk to sprinkle trees with water during a frost or freeze. It is too dangerous because if you do not have good water coverage or if the sprinkler quits running during the time when temperatures are below freezing, significant damage could occur, since the branches may become colder than the air temperature. Accumulation of ice will also break branches.
The easiest form of frost protection is to create and use microclimates in the garden. Citrus trees, for example, do well in the narrow spaces between houses because the close proximity of the walls protect them from plunging temperatures. Heat-loving and frost sensitive plants like bougainvillea and hibiscus seem to do best on south-facing walls with an overhanging roof.
Cold weather protection requires planning and careful observation of weather patterns. Now is the time to make plans and preparations to protect our tender plants once the forecast tells us that a frost is coming. Then, with just a little effort, we can put our plants and our preparations into action. Unfortunately, there is little that we can do when catastrophic cold temperatures make prevention difficult or even impossible. We just have to deal with the aftermath. However, a little know-how, patience and common sense will help us get our landscapes looking good again.
The University of Arizona is an equal opportunity, affirmative action institution. The University does not discriminate on the basis of race, color, religion, sex, national origin, age, disability, veteran status, or sexual orientation in its programs and activities.
For more information on citrus cold damage please go to: http://ag.arizona.edu/pubs/crops/az1222.pdf
About the authors:
Extension Agent Agriculture, Pinal County
Associate Extension Agent, Agriculture, Pima County
Associate Research Scientist, Citrus