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Frost damage can show up days later

Problems may become evident when temperatures warm

Is this frost damage or something else? Since it's an ornamental ginger plant -- a frost-tender tropical -- it's probably been damaged in the recent frosty weather.

Is this frost damage or something else? Since it's an ornamental ginger plant -- a frost-tender tropical -- it's probably been damaged in the recent frosty weather.

Kathy Morrison

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.

Citrus branch with leaf damage from frost
This citrus tree shows frost damage. UCIPM photo

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.

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Dig In: Garden Checklist

For week of March 26:

Sacramento can expect another inch of rain from this latest storm. Leave the sprinklers off at least another week. Temps will dip down into the low 30s early in the week, so avoid planting tender seedlings (such as tomatoes). Concentrate on these tasks before or after this week’s rain:

* Fertilize roses, annual flowers and berries as spring growth begins to appear.

* Knock off aphids with a strong blast of water or some bug soap as soon as they appear.

* Pull weeds now! Don’t let them get started. Take a hoe and whack them as soon as they sprout.

* Prepare summer vegetable beds. Spade in compost and other amendments.

* Prune and fertilize spring-flowering shrubs after bloom.

* Feed camellias at the end of their bloom cycle. Pick up browned and fallen flowers to help corral blossom blight.

* Feed citrus trees, which are now in bloom and setting fruit.

To prevent sunburn and borer problems on young trees, paint the exposed portion of the trunk with diluted white latex (water-based) interior paint. Dilute the paint with an equal amount of cold water before application.

* Cut back and fertilize perennial herbs to encourage new growth.

* Seed and renovate the lawn (if you still have one). Feed cool-season grasses such as bent, blue, rye and fescue with a slow-release fertilizer. Check the irrigation system and perform maintenance. Make sure sprinkler heads are turned toward the lawn, not the sidewalk.

* In the vegetable garden, transplant lettuce and kale.

* Seed chard and beets directly into the ground.

* Plant summer bulbs, including gladiolus, tuberous begonias and callas. Also plant dahlia tubers.

* Shop for perennials. Many varieties are available in local nurseries and at plant events. They can be transplanted now while the weather remains relatively cool.

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