How Frost Damage Happens, and When and How to Prune

Phoenix Landscape Architects & Landscape Designers Noelle Johnson Landscape Consulting
Freezing temperatures can cause brown, crispy growth on plants that are susceptible to frost damage. Frost damage can occur when temperatures dip to or below the freezing point, 32 degrees Fahrenheit (0 degrees Celsius). And both cold-hardy plants and tenderer plants that do best in warmer temperatures can, depending on the circumstances, be affected by frost damage.

Growth damaged by frost can look ugly, and your first impulse may be to prune it — but don’t. Believe it or not, you can do more damage if you remove the damaged growth too soon. In fact, those ugly brown leaves actually help to protect the interior of the plant from future freezes. So, let’s dive into frost damage — how it affects our plants and, more important, how and when to prune frost-damaged growth.

Frosts and freezes. Before we talk about the damage that freezing weather can cause to plants, let’s talk about what frosts and freezes are and the damage they both can cause, which is called frost damage.

Frosts occur on clear nights when the temperature dips to 32 degrees Fahrenheit (0 degrees Celsius) or even a few degrees above. At these temperatures water vapor freezes on the surface of plants, creating a frosted appearance. Unless you have a garden filled with tropical plants, most plants can handle a light frost with little to no damage.

Freezes happen when very cold weather arrives and the temperature dips below 32 degrees and even into the 20s (negative Celsius) and below. Damage to plants from freezes is more severe than that from frost and can cause significant injury to plants.

How do freezing temperatures damage susceptible plants? When the outside air temperature hits 32 degrees Fahrenheit (0 degrees Celsius) or below, it causes the water both inside and outside plant cells to freeze. The ice within the plant punctures the plant’s cells, which causes the death of that part of the plant.

What are the signs of damage? The sudden wilting of the outer growth is the first sign your plants have suffered frost damage — leaves are usually affected first. This happens because the plant’s cells have been punctured and their contents have leaked out. The next sign of frost damage is when the wilted growth begins to turn brown and crispy or even black, which indicates that that part of the plant has died.

Cold-temperature injury to plants is first seen on the outer growth, while the undergrowth may still remain green. However, more severe frost damage can affect the entire plant and cause most, if not all, of the above ground growth to die.

What types of plants are affected by frost damage? Different types of plants are susceptible to frost damage. Tender plants, such as tropical plants, are one type. These plants do not tolerate freezing temperatures and can suffer significant damage, or even death, when the temperature goes to 32 degrees Fahrenheit or below. Semitropical plants, such as bougainvillea, lantana and yellow bells (Tecoma stans var. stans and Tecoma stans var. angustata, zone 8), can survive temperatures below freezing, often into the 20s, but will suffer significant frost damage and will need to be pruned.

Plants that have not hardened off before freezing temperatures arrive can also suffer frost damage. Hardening off is the process by which plants gradually acclimate to temperature changes over time. Unseasonable freezes, like after plants have emerged from dormancy in late spring, that arrive before the plants have had a chance to adapt to cold temperatures, can damage even cold-hardy plants that can normally handle subzero winter temperatures.

When to prune. While frost-damaged growth can protect the lower parts of your plants, the good news is that you don’t have to keep it around forever. After the average last frost date for your area has passed is the earliest time to prune frost-damaged plants. (Check The Old Farmer’s Almanac to see when your average last frost date is.)

You may cause the death of your plant if you prune too early and more freezing weather arrives. Because pruning stimulates new growth, any freezing weather that occurs afterward can cause significant damage to the new growth, which is extremely susceptible to injury from freezing temperatures, and to the plant itself.

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