Preparing Your Garden for Frost

It’s been almost a year since “The Big One” struck down plants all over the Valley and indeed throughout the entire region. Large trees suddenly browned out, shrubs looked like they had been torched, and cacti and succulents slowly turned into mush as the effects of the worst freeze in 15 years became more and more evident. If you were like most of us, you probably figured your plants had been through this the year before and did okay, so why take extra precautions? The next night is when we realized that this was no ordinary freeze, and that we had better do something, but by then the damage had been done. This winter, I predict a spike in frost cloth sales and the hoarding of old sheets and blankets, because we won’t get caught a second time, will we? I expect not.

If you have frost-tender plants in your yard, you can begin preparing them for the colder months starting in fall/early winter. It is unlikely that we will have another killer frost like last year’s, but it is reasonable to expect an average of seven nights of frost in the Phoenix area based on long-term records. The citrus growers I worked with in Mesa used to say to expect the first freeze between Thanksgiving and Christmas. This will happen after a storm blows through; leaving a clear cloudless night that allows temperatures to drop significantly.

Before this happens, you can do a few things to give your plants the edge they need to minimize damage in a typical winter frost. The first involves foresight. Do the research before choosing plants for your landscape. Once again, plants from the immediate region or similar areas of the world will fare better than ornamentals from the tropics and sub-tropics, which see little or no frost. You may get away with using those for several years, but they will get nailed in a severe frost like the one we just had. Typical examples are Ficus, Jacaranda, and Hibiscus, all of which thrive in San Diego, but need extra protection here. Trees and shrubs from the inner deserts see a wider range of temperatures and so are better adapted to withstand frost.

Following the late summer/early fall growth spurt that occurs here, begin hardening off your plants for the winter. Any fertilizing should be done by early fall and no later, and cut back on the water as soon as the temperatures have gone down to bearable levels. This will keep the plants from producing any new growth too late in the year, which is more sensitive to frost than the older more mature portions of the plants. After things cool down here, I turn my system off for the winter and only occasionally run it on semi-auto. The ground is cold and stays moist much longer, and the plants aren’t growing anyway. Over-watering at this time can stress the root system and lead to rot. Save your water for when the plants really need it.

When the frost does come, you should have a few things handy to protect your tender plants. Sheets and light blankets have been used here for years as an economical way to cover the plants. Secure them with clips and weight the lower portions with rocks or stakes. The more coverage you have from top to bottom, the better. Frost cloth is the newer high-tech way to go, because it is much lighter and allows more light transmission. Some frosts may repeat over two or more nights, and with this cloth you can leave everything in place until things warm up.
Denser sheets and blankets will block more light and should be taken off
sooner than frost cloth.

On columnar cacti the terminal growing points are the tenderest parts, which you can protect by covering with Styrofoam cups. Larger bodied cacti such as saguaros, cardons, and barrels will need frost cloth over the top. For large cacti or trees with canopies that are out of reach, use propane heaters placed at the base to provide a few degrees of protection. Be aware that during a hard freeze, some terminal damage is still likely, with the survival of the plant itself the objective. You can even use Christmas lights in plants to add a little warmth and appear festive at the same time. Be careful not to let the larger bulbs contact any delicate tissue.

So, after the frost has passed, how do you tell if your plants have passed as well? Frozen tissue can show up immediately in leafy plants as brown or black branch tips and leaves, with the terminals bent over due to ruptured cells. Succulents and cacti aren’t always as noticeable at first, but rather will reveal the damage gradually in the days and weeks to come. Look for a slight discoloration at first in the terminals and ribs of cacti and euphorbias, and changes in color in the exposed outer portions of other succulents. The damage to frozen areas will progress, and will either stop at some point on the plant or rot will continue to move into healthy tissue. You will have to prune this out in order to save the plant. Observation is the key after a frost to determine when to prune. Leave all damaged parts of herbaceous and woody plants intact until well into spring, as these will protect the living interior parts during any subsequent frosts. This works very well on bougainvilleas and lantanas. Plants in good condition will begin to grow and leaf out in the spring, at which time the dead tissue can be taken away.

Desert trees from our southwestern states (and the northern states in Mexico) generally do very well during freezes. Also recommended are trees from the Australian and African deserts. These trees will sustain little to no damage in a moderate freeze such as we have in a typical year, but may still have significant damage after a hard freeze, depending on whether they were hardened off properly and if they were growing in a low-lying area where colder air tends to settle.

I have had very good luck with trees purchased at our plant sales over the years. They don’t come in any size larger than 15-gallon, but that’s an advantage in the long run in establishing a healthy root system. After the roots have had time to develop, these trees can take off and reach the size of their larger counterparts in a few years. Ask one of us to help you decide at our next sale. Many of the shrubs we offer are also frost hardy, as well as water-efficient. Many people find it convenient to shop at the Garden’s plant sales because they can find a much wider selection than at any one nursery.