Brrrrr….it’s cold! We’ve had some chilly temperatures here in the North Bay, and regular frosts at night: a sure sign that winter is officially here! Most of us here in California are lucky to have a year-round growing season, but even then, many of us regularly get frosts at night. Colder temperatures make it necessary to protect tender plants from frost. Keep reading to learn more and how to protect your plants from frost by taking these simple steps.
Why You Need Protect Plants from Frost
Some plants can survive frost just fine. Others can not. When exposed to the freezing temperatures, the plant’s cells in the leaves and bark are damaged or even killed. You can identify frost damaged by blackened, wilted or shriveled leaves. Frost is particularly damaging to new growth or blooms, which are more sensitive parts of the plant.
Depending on the plant and its size, some (such as an established citrus tree) might be able to bounce back from getting exposed to frost. Others, such as tender annuals like basil or nasturtiums, can not. Sometimes just a section of the plant will be killed. This often happens to my jasmine vines, and just the top vines will get damaged. In this case, leave the damaged or dead areas until AFTER winter, as its protecting the rest of the plant, then prune off in the spring.
When Frost Occurs
Frost forms when water vapors condense and it freezes, cause a layer of ice crystals. Temperatures from 38℉ below can lead to frost. There are many conditions that can contribute to if you will get frost or not. Clear skies, winds, and local topography are all factors that might make your garden more or less susceptible to frost. Cold air settles in valleys, making frost damage more likely for plants rather than those on sheltered or hilly areas.
What Plants You Need to Protect from Frost
Any summer annuals that you still have growing such as basil, beans or tomatoes will get killed by frost. Harvest them up now, and call it a loss on anything unripe.
Fall vegetables like beets, broccoli, cabbage, celery, lettuce, parsnips, swiss chard, kale, leafy greens, and arugula can take light frost (those in the 30℉ range). Carrots, garlic, horseradish, leeks, parsnips, radishes, and turnips can usually survive hard frosts (those in the 20℉ range).
The best way to know if you have a tree or shrub that needs protection is to be mindful of its temperature requirements when you buy it. Look at the Sunset or USDA zones that the plant needs, and if you are outside of that, be prepared to protect. Some companies, such as Monrovia, list the temperature hardiness of plants. In my garden, perennial plants that I need to protect are jasmines, citrus,and geraniums (Pelargonium sp.).
Ways to Protect Plants from Frost
There are several things you can do at home to prevent damage from frost in your home, some are more long term solutions, but others a quick way to implement this afternoon.
Move plants. If your plant is in a container, move it either inside the house, the garage or into a shed. Or, bring under the eves, right up against the house.
Cover plants. For those too large to move, or planted in the ground, cover them up! You can use old sheets or blankets, burlap, plastic tarps, or even paper bags. I’ve covered up just transplanted artichokes with 5-gallon buckets. My favorite thing to use to cover plants with is a frost blanket. This is a special material that the sun can penetrate, making it so you don’t have to remove it every morning. A name brand, often used to apply to any frost blanket, is Agrobond. Any reputable nursery should have this or a similar product for sale.
When covering your plants, try to avoid touching the leaves with your cover. I use electrical conduit pipes, bent in a curve, to support frost blanket over my raised beds. For my citrus, I use bamboo poles stuck in the ground that extend taller than the tree, to drape the cloth over. Ideally, you would reach all the way to the ground and cover your entire plant, but anything is better than nothing. If you’re using something other than a frost blanket, make sure to remove them in the morning so the plant can get light.
Keep your covers loose, or anchored to the ground with rocks or stakes. Don’t gather and bundle up against the trunk. You want the heat from the soil to move up into the canopy of the plant, and get trapped under the cover.
Utilize Christmas Lights. I have not actually done this, but I have heard of people using old-school Christmas lights for frost protection. You need real lights, not LEDS, that put off heat. The bulbs warm the air around the plant, thus preventing frost from settling. I have several strands of vintage C7 lights that I pulled from a free pile several years ago, with the intention of using them to protect my non-existant large citrus trees.
Keep plants well watered, or spray them down. If you live in Sonoma County, you are probably well aware of the discussions/debates of wineries using water for frost protection. It seems counter-intuitive, but if you spray your plant with water, it can actually protect them. A thin layer of water on them freezes, and the layer of ice, being a good insulator, will then help keep the plant warmer.
Watering your plant a few days before anticipated frost can also prevent frost damage, as wet soil will hold more heat than soil that is dry. Damage can occur when the plant transpire moisture from their leaves, but can’t replace the lost moisture because the soil is dry. Light watering in the evening hours, before it gets cold will help raise humidity levels and reduce frost damage.
Use frost protection spray. I have never used this myself, but they do make products that you spray on your plants, and it forms a coating to protect from frost. Here is one example.
Plant in microclimates. In everyone’s yards, there are warmer and cooler spots. The north wall of your house is usually the coldest, and lower spots of the yard will be cooler than higher spots. Plant frost-tender varieties in areas that are warmer: on the south or west walls, and on hills. Avoid open windy spots, which bring cold air and can dry the plant out. Placing tender varieties under other plants, like a tree, can add protection.
Areas around rain barrels are also warmer, as the body of water creates a microclimate. I have a salvia (Salvia dombeyi) that is only hardy to USDA zone 10a, or a temperature of 35℉. I regularly get 30℉ or lower in my garden, but it’s growing next to my Blue Barrel rain system and it seems OK so far. I intentionally planted my lemon and a lime next to my larger rain tank, to get the residual heat. I haven’t yet covered them this winter, and they seem to be doing fine. I will likely be more concerned when we start to drop into the 20s℉.
Choose the right plant. If you live in the snow, you can’t grow bananas. At least, not easily. Bananas are from a tropical zone, and you live in an arctic zone. But part of the fun of gardening is pushing boundaries, and almost all of us have plants that aren’t really meant for our area. Growing these types of plants means we must coddle them: with special soils, water, or in this case, temperature control. Feel free to try to grow plants outside your zone, but just be prepared to give them extra care. Otherwise, choose plants that can grow in your temperatures!
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Any frost tips I missed? Share your story in the comments!