We are expecting our second frost this fall season and earlier this spring we had an incredibly late frost that devastated even some of our local nurseries. Covering vulnerable vegetation can protect plants from frost, but using the right methods and materials is essential.
While frost in the spring tends to be the most worrisome time because typically gardens have tender seedlings, that can't handle the cold at all, out frost in the autumn can be just as devastating.
Will one frost kill plants?
This depends. While more mature and established plants can typically handle and recover from a light frost of 28°F to 32°F. More tender, younger seedlings generally can't. Which means that frosts in the spring are typically a lot more worrisome than frosts in the autumn.
The hardiness level of your plant also comes into the equation. The hardier the variety, the more likely your plant will survive a light frost. This information can be found on the back of your seed packet or on the plant card if you purchased at a nursery. In fact, some vegetables actually taste better when they've been exposed to a frost before they're harvested.
What happens is the frost forms ice crystals on the plant. These crystals infiltrate the plants cells which actually disrupts the movement of water and damages plant tissues.
You'll know you have a frost damaged plant when you see the leaves looking water logged and brown or even black. Like my tomatoes did this past spring....
What temperature do you need to cover plants?
If conditions, or the forecast, are favorable for frost, it's time to start planning accordingly. A light frost of 28°F to 32°F will require most plants to be covered. When the mercury dips below 28°F this is considered a hard freeze and will devastate most gardener's crops.
Some plants like kale, cauliflower, carrots, cabbage, peas, and potatoes can withstand temps of 28°F and a bit below. In fact, some of them even taste better when they've had a little frost on them.
Other, more tender and tropical plants, like watermelon, tomatoes, corn, and sweet potatoes will feel the freeze at 32°F or even a bit higher around 33°F.
So, the rule of thumb is, if you're expecting a frost... most plants need to be covered with very, very few exceptions.
How to Protect Plants from Frost
Frost can quickly devastate a garden, but there are some measures we can take to protect plants from the detrimental effects of the cold.
Cover them with cloth
Fabric coverings are best for plant protection. These allow moisture to escape as opposed to trapping it inside a plastic can and often does.
The cloth can be secured to the ground to trap the heat radiating from the ground while also keeping the cold moisture from directly contacting the plants.
Things like bed sheets, blankets, and drop cloths make fantastic insulators.
Place some tall stakes throughout the area you're planning to cover so that the cloth can be draped over the plants without weighing them down or touching them. Then, secure the cloth by either staking it down or placing heavy bricks along the edges.
Don't cinch the fabric up close to the plant stalks or tree trunk as this can prohibit the surface heat from radiating up the plant.
Water the soil in the afternoon
Well before the sun goes down, while the temperatures are still warm, give your plants a drink.
While this may seem a little counterproductive (water freezes, after all), it's actually not.
By wetting down the soil it is able to better hold heat which protects roots and keeps the air near the soil warmer. Just make sure as you're watering that you only water the soil around the base of the plant. Do your best not to get the leaves or stem wet.
Cover with mulch
Mulch is a fantastic insulator. Mulching your garden not only helps control weeds but also keeps soil temperatures more even reducing the incidence of sudden temperature swings.
You should provide a thick layer of mulch for adequate frost protection. A layer of no less than 3 inches should be used, and 6 inches is even more adequate. Leave just a small inch or two opening around the stalk when mulching for frost protection.
While many different mediums can be used as mulch some of the best are spent hay, straw, wood chips, or leaves.
Once the weather warms, pull that mulch back from the stalk a little further and leave about a 6-inch diameter around your plants.
Cover plants with a low tunnel
Low tunnels can work for more than just extending your garden season. They can also be utilized for protecting plants from frost.
An easy, inexpensive, short-term frost protection solution would be to utilize some 9 gauge wire to make hoops about 18 inches tall. Cover those hoops with some fabric and fasten it down. That's all there is to it.
Alternatively, you can utilize plastic. Just make sure that the plastic does not touch the plants directly.
Cover small seedlings with cloches
If you have tender seedlings in the spring that need to survive a late frost, you can cover them with individual cloches to keep them insulated.
Cloches can be purchased and made out of glass or you can simply cut the bottom off of a milk jug or water bottle to create your own cloche.
If you choose to try this, cover your plants just before the sun goes down and uncover them as soon as it warms past freezing.
Insulate plants with a barrier
Following the idea above, you can create individual barriers for larger plants. To do this, gently tie the plant up. Then, place stakes that are as tall as the plant around the base of the plant.
Once you have the stakes placed, place milk jugs or water bottles filled with hot water. Next, wrap the stakes in burlap. Then, stuff the burlap with leaves or straw to insulate the plant.
Once the weather warms above freezing, remove the burlap and let the hay or leaves fall away and use them as mulch.
How long can you cover plants from frost?
You can keep stakes or hoops in place indefinitely while the threat of frost looms. But, each day you should remove the protective barriers as soon as the temperatures rise above freezing.
By doing this, you allow the sunlight to warm the surrounding ground as well as give the plants adequate airflow and sunlight.
Make sure you completely dry any covering before you place it back over the plants the following evening.
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