Protect Plants From Spring Frost

Before anything else, odds are, your spring-flowering bulbs will be fine, despite what nasty surprises Mother Nature decides to throw at them. You may experience a little anxiety over the welfare of your beloved bulbs, but remind yourself that many of the daffodils have been blooming reliably for years. Other bulbs, including hyacinths, muscari, scilla, tulips and lilies are frost-resistant. That said, let's examine a few factors that may influence the impact a late winter freeze may have on your garden!

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 32°F, 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.

Plant laced with frost 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	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.

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.

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.

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.

Check the ground temperature. Temperatures higher up may vary from those lower to the ground. In other words, just because an elevated thermometer reads above 32°F doesn't necessarily mean it isn't below that at ground level.

Planting a seedling 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 45°F, place the seedlings outside during the day in a warm, shady spot that's protected from the wind. At night, bring them indoors.

After 2 weeks, the seedlings will be stronger, sturdier plants, ready for transplanting.

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.

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. Make sure to place your covers completely over your plants and let them drape all the way to the ground—don't tie the cover to the trunk, otherwise the heat radiating up out of the soil won't be able to warm up your plant.

Garden cloches 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.

Note: Since cloches used for cold protection are temporary measures, you may opt to create your own makeshift versions. Flower pots, Mason jars, baskets, and milk jugs with the bottoms removed can all be placed over plants to shield them from freeze and frost.

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.

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.

Protective wrapping of fruit tree 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. If necessary, this wrapping can be left on for the majority of the winter season.

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.

One of the best preventative measures you can employ, well in advance, is to mulch the beds in late fall, after the ground is frozen. If you mulch while the ground is still warm, it may keep the beds from freezing when the rest of the soil freezes, and encourage the bulbs to sprout too early.

If you would like to learn more about applying mulch, read:
Mulch Basics

Mulching after the soil freezes helps maintain a constant soil temperature, and reduce frequent freezing and thawing as temperatures fluctuate. This also has the added benefit of reducing bulb "heaving," where the freezing and thawing cycles cause the bulb to come to the surface, where they are not protected by the insulating soil, and it helps the soil to retain moisture, which is critical for healthy bulb development in the spring.

You want to avoid mulches that form a thick, impervious mat, such as grass clippings, as they won't allow water to penetrate to the soil, and may encourage the growth of fungal diseases.

If you did not mulch in the fall, and have tulips and other bulbs emerging and setting buds, it is not too late to add some protection if an extreme freeze is predicted. A loose, light mulch, such as straw, can be added around the tulip plants and buds, covering them by at least an inch, to protect them from extreme temperatures and hard frosts.

This can be labor-intensive if you have extensive beds, as the mulch has to be removed once temperatures warm up, but it can definitely preserve your blooms! Just be sure to use "clean" straw without weeds or seeds, or your bulbs will be competing with grass and weeds for water and nutrients when the seeds sprout.