Frost protection for sensitive plants depends on the species, how cold it will get, and how long it will be that cold. For the Tucson and Green Valley areas, so can expect temperatures in the winter to dip below 25 degrees a number of times. December’s low temperatures were near 20 in Green Valley several days.
While some plants can take temperatures below freezing without damage, some are nipped significantly, such as citrus. This damage is permanent and may lead to loss of fruit. If temperatures stay well below freezing for over 8 hours, even more damage will occur. Sometimes even plant death.
If you are covering tender plants, remember these rules. Always extend covers to the ground as you are trying to capture warmth coming up from the soil, not trying to prevent the tips of the plant from frost. Always remove covers the next day to let the plant conduct photosynthesis and so the ground can warm up in case the next night is also cold. Placing Styrofoam cups over columnar cactus does little to protect the whole plant from freezing. If you are using these, always remove them during the day as plant tissue damage can occur if you leave them on for extended periods.
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