Historically, killing freezes make an assault on our region every five to 10 years. In 1989 an 11 degree freeze in our area temporarily eliminated citrus production. Since then a 20-year grace period has restored and encouraged the expansion of our citrus acreage to over 15 commercial acres and the backyards of many homeowners.
Large evergreen citrus trees loaded with hundreds of pounds of fruit towering over the roof-lines of homes are now a common site in the Vermilion Parish landscape.
Weather in the teens is sending a chill up the spines of every citrus grower in the parish. Most growers are aware that citrus is not able to withstand temperatures in the 16 to 18 degree range for extended periods of time.
Calls are flooding the parish’s extension service office with growers questioning what steps or precautions they can take to help prevent a total frosty devastation to their cherished orange trees. However, even the mythical crystal ball would have trouble predicting the outcome of this impending cold weather due to the highly variable nature of the assault. Trying to give an accurate answer will require that we absolutely, positively, know answers to the following questions.
1) How cold will it get? Vermilion Parish is often several degrees warmer than reports coming from the northern media in Lafayette. In addition, urban trees often benefit from the heat radiating from a nearby building. Microclimates exist and often freezes are patchy with some areas being spared the coldest temps. Plants also don’t get caught up in the wind chill frenzy. It is simply the actual temperature next to tree that will determine if it will freeze. Citrus trees are not 100% water. Therefore, they don’t freeze at 32 F. It takes temperatures below 24 degrees to 26 degrees for 6 to 8 hours before freeze damage occurs in a citrus tree. Consecutive nights of freezing temps dipping into the teens usually create conditions ideal for citrus killing weather.
2) What type of citrus are you growing? Not all oranges are created equal when it comes to freeze resistance. The hierarchy of freeze tolerance from best to worst is as follows: kumquats, satsumas, round oranges(LA sweets & navels), grapefruits, lemons and lastly limes. This explains why the parish does not see widespread lime production in the area, and why kumquats and satsumas are so plentiful. They naturally have more cold tolerance. Kumquats and satsumas have been known to take down to 16 to 18 degree temperatures.
3) Is the citrus tree grafted and if so, what rootstock was used? Poncirus trifoliata rubideaux is the best rootstock when it comes to transferring cold tolerance. Often chain stores have been known to sell trees grafted onto rootstocks like swingle, rough lemon, sour oranges etc. Although these trees may grow fast, they are not as dormant during winter conditions and are not as cold tolerant. Some citrus trees are not grafted but are started from seeds or cuttings. These plants will have the advantage of being able to sprout back from the roots after a freeze, but will not have overall cold tolerance of a grafted tree.
4) How old are your trees? The first year a tree is planted it is very sensitive to freeze. It has very little stored food in its root system and is going through transplant shock. Over the next few years, as the tree becomes established the tree has a more extensive root system, a larger diameter trunk and a denser canopy. It takes temps in the low teens to kill big 20 year old trees. If you just planted a citrus tree, you might consider digging it back up and bringing it into a protected until the freeze event is over.
5) How healthy is the tree? If the tree is in poor health because of foot rot/root rot, spider mites, mealy bugs, scale, whiteflies etc. it will have less food reserves to help it withstand a freeze.
6) What weather have we had prior to the freeze event? If we are 80 degrees the week before a freeze, citrus trees might be encouraged to start budding since sap will be flowing in the tree trunks and branches. These sprouting trees will freeze much easier than trees in a good dormant state that have been consistently exposed to cold winter without warm temp fluctuations. Fertilizer applied late in the year after July will create a similar situation by promoting late season tender growth that will be more susceptible to freeze damage.
7) How much fruit is on the tree? Fruit left on a tree during a freeze will draw down some of the reserves of the tree. Trees without fruit generally withstand a freeze better.
8) How much moisture is in the ground? Wet soil holds more heat than dry soil. Years ago citrus growers often flooded citrus groves to save trees during freezing episodes.
Here is what a homeowner can practically do to try and protect trees.
1) Try and create a warmer microclimate around the tree. This can be done by using a small heat source like Christmas lights, light bulbs, etc . near the base of the tree. A tarp or plastic sheeting can be used to drape over the tree to try and trap in electrically generated heat along with ground heat. Be careful with the use of clear plastic sheeting around trees. The rate that the tree thaws out after a freeze also influences the survivability of the tree. When the sun comes out the next morning after a freeze, a tree under plastic will undergo too rapid a thaw and this will cause additional damage to the tree. Therefore, take off plastic covers before the sun comes out the next morning before temps start to rise under the plastic. The beneficial effect of a slower thaw is often observed in citrus trees growing in the shade. Although they are not as productive as trees in full sun, they generally survive freezing better because of the slow thaw that occurs in the shade. Bent PVC pipe and plastic can be used to make small high tunnel greenhouses over small trees. Trash cans, drums, boxes can also be weighed down and placed over trees. Since trees are not able to generate their own heat, it is essential that you provide a heat source or that ground heat is present.
2) Use water to protect trees. Wet soil holds more heat than dry soil. Make sure the soil is well saturated before the freeze. Watering the foliage before a freeze won’t be of much help. Trees covered in ice often undergo a supercooling effect and get to temperatures lower than their dry foliage counterparts. Water can be used to save the trunks and scaffold branches of trees only if it is constantly applied the entire time the temperature goes below freezing. This is because water gives off heat as it goes from a liquid to a solid. Setting up a sprinkler protection system to an orchard can be expensive and impractical for most homeowners. Investment in a backup power system is necessary to insure a constant flow of water for potentially several days. The cost of thousands of gallons water to spray during a freeze is prohibitive for most growers.
3) Is it better to have mulched trees or bare ground? Actually bare ground absorbs more solar rays during the day and this heat radiates from the soil back into the canopy of tree during the night. Mulched ground or ground covered with weeds actually keeps the soil cooler. Mulch left around the base of tree will keep the trunk wet and can contribute to issues with foot rot/root rot the next year.
4) Do I focus my efforts on covering the roots or the ground?
Most trees are grafted, therefore the root system of the tree should have good cold tolerance. Growers need to be concerned with the more cold susceptible portions. Saving the tree from the graft union up to the scaffold branch area is the utmost priority. This region begins 4 to 6 inches above the ground. If all else fails the tree can regenerate as long as the graft union is spared. On small trees the graft union can be wrapped and protected with pipe insulation.
Years ago, growers, would bank up the trunks and graft unions of trees with a thick layer of wet soil before the onset of a freeze. This wet soil was a good insulator and radiator of heat. It could be applied to the trunk and would save the graft union. Although time consuming and labor intensive this method is a very effective method of saving trees.
4) To pick or not to pick? Unless temps dip into the low 20’s fruit should be left on trees. Once it is harvested you have to do something with it. It doesn’t store very long unless you can keep it refrigerated. The best way to store citrus long term is to leave it on the tree. However, if temperatures dip into the teens you need to take the fruit off the trees, because the fruit will zap energy that might be needed for survival.
5) What should I do after the freeze? Nothing. Don’t get in a big hurry to prune damaged wood. You may be removing branches that might end up resprouting. Wait until May to determine if which branches are still alive. Often leaves will drop off of freeze damaged trees, but the larger branches and trunks end up surviving. You aren’t doing the tree any favors by pruning the tree right after the freeze.
6) When should I replant? Don’t get discouraged and give up on citrus just because a freeze wipes you out. Some seasoned citrus growers boast you can’t call yourself a true citrus grower until you have replanted 3 times. Freezes should be an expected part of growing citrus in Vermilion Parish. Try and replant in late February. If you happen to get discouraged and take a few years off before replanting, you may miss out on the important frost free years that you needed to get restarted. So, if you want to stay in the citrus business you always need to be prepared to immediately replant.
In summary, my gut feeling is that unless the weather gets down below 14 degrees our larger citrus trees will survive this onslaught of cold temps. We probably will see extensive damage to the outer limbs and canopies of large trees, but most trees will survive.
If you have any questions about protecting your citrus trees or if after the freeze you would like to commiserate or form a support group for people suffering from the loss of freeze damaged citrus, give Stuart Gauthier, County Agent a call at 898-4335 or come by and visit at 1105 West Port in Abbeville.