Frost damage can show up days later

Did your garden dodge damage from recent frost? Look again.

Frost damage may not appear immediately, say UC agriculture experts and master gardeners. Browned leaves and dieback may appear days, even weeks later.

“Often injury is not apparent until days after a freeze and when temperatures rise,” say the UC IPM pest notes.

Downtown Sacramento hit lows of 32 degrees on Dec. 17 and 18. Seven December nights (so far) dipped down to 35 or below. Surrounding areas saw lows below freezing.

It’s not just how low temperatures dropped but how long those cold periods last. Most plants can withstand a few minutes of freezing temperatures – but not a few hours.

Frost damage looks like other kinds of damage, notes the UC experts. “Symptoms resembling freezing and frost injury are also caused by anthracnose and other leaf and shoot diseases, gas or mechanical injury to roots, phytotoxicity, and water deficit.” (That last category is very common after years of drought.)

Frost damage is often described as “burn,” because that’s how the plant looks – as if it was torched.

“Cold temperature damage causes buds, flowers, and shoots to curl, turn brown or black, and die,” say the UC experts. “Foliage appears scorched because low temperatures severely dehydrate plant tissue. Bark and wood can crack or split, and whole branches or entire plants may be killed if temperatures are below those tolerated by the plant.”

Frost and freezing aren’t the same thing, note the experts.

“Frost and freezing produce the same damage but occur under different conditions, and some of their management strategies differ. Freezing occurs when air temperatures are 32 degrees F. or colder. Frost occurs when air is warmer than 32 degrees F. but plant tissues drop to 32 degrees F. or below because plants radiate (lose) heat into the atmosphere, especially during cool, clear nights.”

So, a tender plant – such as succulents or tropicals – can suffer frost damage even if the temperature stays above freezing.

UC experts recommend removing mulch from under and around frost-sensitive plants to increase the bare soil’s ability to absorb heat and warm plants.

Moist soil holds more heat. When frost is in the forecast, irrigate topsoil so it has a chance to absorb more warmth – preferably at least three days before frost is expected, say the experts.

What can you do in a hurry? Use cloth – not plastic – covers, UC experts say; cloth is better at retaining heat. “When frost is expected, cover sensitive plants overnight with cloth or similar material other than plastic to reduce heat loss to the atmosphere, but leave covers open at their bottom so heat from soil can help warm plants. Remove covers during the day.”

Just covering a plant may not be enough to save it, note the experts. “During freezing, covering plants is of little help unless a heat source is provided. Placing incandescent lights designed for outdoor use in the canopy may generate enough heat to prevent plants from freezing if plants are also covered. Be sure not to create electrical shock or fire hazards.”

When frost damage does occur, leave it – at least for a while. It will help protect the plant from more frost damage this winter. And it might not be totally dead.

“Do not prune freeze-damaged plants until after you are certain what tissues are dead, preferably by waiting until spring or summer after new growth begins,” say the UC experts.

Instead, prune damage in spring after all frost danger has passed.

For more tips: https://ipm.ucanr.edu/PMG/GARDEN/ENVIRON/frostdamage.html