When Winter Frost Arrives

It might seem a strange topic for our climate here on the Sunny Coast, but with the early arrival of winter last week we had plenty of queries about dealing with frost in the hinterland and on the range. Here are a few global tips on dealing with frost in your garden.

One day it's hot and sunny, and the next it's 10 degrees with frost on the ground. Yep, that's the early autumn/winter transition that we get sometimes in QLD.

For many of us, tis change presents one of the year's greatest gardening challenges: protecting tender new growth from damage due to cold. Frost damage, freezing death, root damage, and frost cracks on bark are four primary negative effects of severe drops in the temperature.

In early winter (and sometimes again in early spring), when the threat of frost is especially great, closely monitoring weather conditions via weather radio, TV, and websites for reports of expected cold spells is imperative. That way, when frost is predicted, you can prepare for it. It's also a good idea to periodically check the temperature at ground level near your plants to see how cold it is for them and whether or not you need to do something about it.

This article will explain what frost is, how freezing temperatures affect plants and what you can do about it. It will also provide easy and effective suggestions for protecting plants from frost, methods that can be applied to tender food crops like tomatoes and citrus trees, delicate potted plants like succulents and begonias, as well as other plants susceptible to extreme cold.

Here are 10 easy, practical methods I've used to reduce frost's impact on my garden:

1. Choose Cold-Hardy Plants

Some vegetables and flowers are hardy souls that thrive in spite of (or sometimes because of) the cold. These kinds of plants are known as "hardy," because they can tolerate some amount of short-term freezing. By contrast, plants that are killed or severely injured by freezing temperatures are known as "tender."
Crocuses often push their way through snow to bloom, and a spring storm rarely gives narcissus, tulips, grape hyacinths, or pansies pause.

There are also a wide range of tasty edibles that are resistant to frost, including

• broccoli
• cabbage
• calendula (pot marigold)
• carrots
• chives
• lettuce
• leeks
• peas
• radish
• spinach
• Swiss chard

Experts at your local nursery are great sources of information about hardy plants appropriate to your zone. Native plants, particularly native perennials, will most likely be the best choices.

Which plants are sensitive to frost?

• Tender plants such as avocados, fuchsia, bougainvillea, begonias, impatiens, geraniums and succulents
• Edibles such as citrus trees, tropical plants, tomatoes, pumpkins, sweet potatoes, cucumber, okra, eggplant, corn, and peppers
• Spring-blooming shrubs and trees such as cherry, azalea, and rhododendron
• Tender perennials like canna, elephant ear, caladium, and dahlia. 

Don't Strand Plants - smart placement near other plants, benches, or walls—especially if these structures are south- or west-facing—will go a long way toward protecting plants from being damaged by frost.

2. Place Plants in Frost-Resistant Spots

It's as true for plants as it is for real estate: location, location, location. Set out seedlings and store-bought spring plants in areas that are less likely to experience damaging cold.

As cold air moves to lower ground, it will pass by plants located on high ground or slopes. That's why it's best to place seedlings and other plants that are susceptible to frost in these elevated locations.

Placing plants by benches, fences, and walls—particularly if they are south- or west-facing—can provide additional protection, especially if the structures are dark in color. During the day, the structures absorb heat. Throughout the night, they radiate that heat, keeping plants warmer than they'd otherwise be. Nearby shrubbery also provides protection from light frosts.

What is frost?

Frost generally occurs on clear and calm nights, where there are few to no clouds to reflect warmth back to the ground and little to no wind to disperse warmer patches of air. The cold air then settles down to the lowest point, while the hot air rises up and away from the ground. On these nights, frost can happen even if the temperature on your thermometer does not read below freezing. As long as the air temperatures at ground level dip below 5 degrees, ice crystals can still form on plants. This in turn disrupts the movement of fluids within the plant, depriving its tissues of water and drying it out. This is why leaves damaged by frost shrivel up and turn dark brown or black. If left in freezing temperatures for long durations of time without much protection, plants can easily die from desiccation.

Note: Frost can also occur when there is wind, but it is a chilling wind that then brings in even colder air, making matters worse.

3. Avoid Frost Pockets

Frost pockets are depressions in the ground. Cold air drains into these "pockets," and it can't get out. When this happens, plants located in the depressed areas can suffer frost damage. Avoid sowing seeds and bedding new plants in these low places.

Check the Ground-Level Temperature - Temperatures higher up may vary from those lower to the ground. In other words, just because an elevated thermometer reads above freezing doesn't necessarily mean it isn't below that at ground level.

4. Harden Off Seedlings

Before setting out seedlings, acclimate them to the outdoors by gradually exposing them to conditions outside. This process, called hardening off, will help you grow stronger plants that are more likely to withstand the vicissitudes of early spring.

Begin the hardening off process about 14 days before transplanting. When the weather's mild and above 20 degrees, place the seedlings outside during the day in a warm, shady spot that's protected from the wind. At night, bring them indoors.

After two weeks, the seedlings will be stronger, sturdier plants, ready for transplanting.

5. Cover Plants Before Nightfall

If you’re going to cover up your plants before a hard frost, do so before dusk. If you wait until darkness falls, most of the stored heat in your garden will have dissipated.

No matter what type of cover you use, make sure that it extends down to the soil on each side. Do not leave any openings for warmth to escape. If you can, it's also advisable to use stakes to keep material, especially plastic, from touching the foliage. Do not affix or gather your cover to the trunk, however, as this will prevent the heat radiating up out of the soil from reaching the plant. 

In the morning, after the frost has thawed, remove the covers. Failing to do so could cause the plant to break dormancy and start actively growing again, which would make it even more susceptible to frost damage in the future.

What can I cover my plants with to protect them from frost?Here are just some of the items you can use to cover your tender plants:

• bed sheets or blankets
• drop cloths
• an inverted flower pot or bucket
• milk jugs/bottles with the bottom cut out
• frost cloths
• garden blankets.

It's also important to remember that covers don’t have to be elaborate or expensive in order to work. A row of sticks with newspaper, cardboard, or sheets and towels tented over them will do just fine. If you don’t have sticks, lay the covers directly over your plants. This too will prevent heat loss.

6. Protect Plants With Cloches

Strictly speaking, cloches are removable glass or plastic covers that protect plants from cold. Sometimes called bells or bell jars, most fit over individual plants, but some are large enough to cover a row. Like other covers, cloches should be placed over plants before the sun goes down and removed in the morning after the frost has thawed.

Glass cloches are highly ornamental. When you're not using them outside for frost protection, you can use them indoors over humidity-loving houseplants like violets.

You can also use plastic cloches, which are generally less expensive than glass ones. But because they are lightweight, they must be staked into the ground to prevent them from blowing away in high winds.

7. Warm Plants With Water Jugs

Fill plastic milk jugs with water and place them in the sun, allowing them to soak up heat during the day. Before dusk, set the jugs around your plants and throw a cover over them. The water in the jugs will lose heat more slowly than the soil and the air, and the warmth it emits will help protect your plants from the cold.

8. Water Before a Frost

It may sound crazy, but watering around plants the night before a spring frost can actually protect them from freezing. During the night, the wet soil will release moisture into the air, which will raise the temperature and keep plants warmer.

Ground Hanging Baskets

Place hanging baskets on the ground before covering them so they can benefit from heat rising up from the soil.

9. Bring Potted Plants Indoors

When frost is predicted, bring planters and hanging baskets inside. The roots of potted plants experience more severe temperature fluctuations than those planted in the ground. They'll reach lower temperatures, too. That's why potted plants are especially susceptible to root damage due to cold. It can cause their roots—particularly those near the edge of the pot—to turn spongy and black. Although root damage may not kill the plant, it will stunt its growth.

Just make sure when you bring potted plants inside that they don't have any insects or pests on them and aren't currently suffering from any diseases. This will not only potentially exacerbate the problem, but it could also infect your other plants.

If you opt to cover a hanging basket rather than bring it inside, place it on the ground first, and then place the cover over the basket in order to take advantage of the ground's relative warmth.

10. Wrap Fruit Trees

If you grow fruit trees, be sure to wrap the trunks in the fall with burlap strips or tree wrap. Most fruit trees have thin barks that are susceptible to splitting when temperatures fluctuate dramatically. Tree wrap will prevent this splitting, which is known as frost crack.

It's often a good idea to use multiple layers of cloth or weatherproof paper, while still keeping the wrapping a bit loose. This provides more effective insulation. You should also extend the wrapping all the way to the ground and at least as high up as the lower limbs or branches. 
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If necessary, this wrapping can be left on for the majority of the winter season.