Freezing temperature is something we constantly deal with in a northern garden. There can always be another frost after the “last day of frost” and before the “first day of frost”. Plant protection from frost is crucial for growing a successful garden in cold climate. In this post, we will talk about the annual vegetable garden.
Plant protection from frost is much easier in spring when plants are small, which is one of the reasons we like to start things early and extend the growing season as much as we can from a get-go. Young plants are also more frost tolerant than mature ones.
But also in late summer/early fall, we want to be able to protect plants from frost to give them more time to mature.
Here are a few things to know for successful plant protection from frost.
Not all plants need protection from frost
Generally speaking, annual edible garden plants are divided into two categories, heat-loving, and cold weather plants. Gardeners often talk about winter and summer crops. Growers in the south grow summer crops in the summer and winter crops during the winter months.
Cold weather plants are frost tolerant and do not need protection from frost until about -2 °C (28 °F). Some can withstand even more. Crops like peas, the whole cabbage family, salad greens, radish, carrots, and beets do not mind a bit of frost. Kale is fine till about -10 °C (14°F), and spinach continues to grow even after -40!
Summer crops, on the other hand, do not tolerate any frost. The most sensitive are the Cucurbitaceae plants: cucumber, squash, pumpkin, zucchini, watermelon, and other melon varieties. These plants need protection before it even freezes because the slightest frost can severely damage or even kill the plants.
Nightshade family plants are summer crops as well. The most known are tomatoes, peppers, eggplants, and potatoes. A light frost will damage them, but might not kill the plant completely. If in doubt, it is always better to give them protection.
Beans, corn, and sweet potatoes are also summer crops.
Read more about how different plants react to frost here. With that knowledge, you can plant frost-resistant plants in times when there is danger of frost and keep plant protection work to a minimum.
See when to plant what to avoid losing tender plants, but still get an early harvest from plants that don’t need as much protection.
How to protect plants from frost
Frost usually either flows like a river or falls like rain. It most often occurs on calm, clear nights.
Having this in mind you always want to have something that covers the plant from above. Sometimes just putting the plant under a roof can help if they are in pots or containers.
Microclimates can also make a big difference, just having a plant in a raised bed can give it some protection. Plants in low spots in the garden will have to deal with cooler temperatures. Read more about microclimate here.
Snow is natural frost protection. It covers the plants gently and keeps the ground from freezing. However, snow will not protect heat lowing summer crops.
Common plant protection from frost are a greenhouse, plastic tunnels, straw, a cold frame, glass jars, milk jars, “Hot caps”, frost blanket, bed sheets, … the list goes on. It does not matter so much what you cover the plant with, just do not let the frost touch the plants.
When covering the plant, keep in mind that the plant has no heat in itself, the warmth comes from the ground. To cover as much ground as you can provide the plants with more warmth.
If frost protection needs to be done for a longer period, a few days or weeks, a greenhouse, frost blankets and glass jars are our favorites. Let us take a closer look at them.
Plant protection from frost with a blanket
A Plant Protection Blanket or also called Floating Row Cover is a non-woven garden fabric cover, ultra-light and resistant to exposure to the environment; it allows light, water, and air to pass through and is reusable.
A Floating Row Cover comes in different thickness and acts as frost blanket and some wind protection. It also can provide organic insect control, traps heat and moisture to produce bigger, better, and earlier crop yields. It is hands down one of our favorite products in a cold climate garden.
Frost blankets are lightweight and can just be put over the plants with or without any support wiring. You will have to weigh them down so the wind does not blow them away. The Floating Row Cover can be left on all day since it allows air circulation and light – plants will not cook under them. It needs way less work than plastic protection would be, and I am always for simple.
Frost blankets are a simple way to protect whole rows or grow beds.
It is just like with us, we are warmer if we wear layers, so cover plants in layers, too. A Floating Row Cover is so light weighted that two to three layers will still have less weight than a bed sheet. However, a bed sheet or blanket works too and is much better than no cover.
Remember, whatever cover you are going to use, the cover does not have any heat in itself. The heat comes from the ground that is still warm from daytime. So cover as much ground as possible to give the plants access to the warm ground. If the temperature drops really badly, string lights can be used under the blanket to give some heat.
Plant protection from frost with glass jars
To protect plants with jars or glass cloches is very old-fashioned. It is not very practical for commercial growth, and that might be the reason it is not very well known anymore. But for a home garden, it is great. Glass jars are thicker than plastic or frost blankets and give more protection. Also, glass jars are not so sensitive to wind, another advantage over the milk jars.
Under the glass jar, a micro climate is created, and if the weather gets warmer than expected it starts to build up condensation and rains in there – a fun thing to watch. However, if the weather gets really warm, the jar needs to be removed, otherwise, you might cook the plant.
Our last frost day is at the end of May, on May 9th I planted a zucchini seed and placed a jar over it. May 28th the zucchini plant is growing nicely. This works very well!
Choose a jar big enough for the plant to grow in. Leave the jar on all day, except for really warm days. If you remove the jar during the day, make sure to put it back on the plant before it is getting cold at night so warm air is trapped in the jar. On very cold nights it is easy to give the jars a second layer of frost protection with a blanket or even a bucket on top of the jar. Jars are my favorite way to protect single plants, like an early zucchini plant or a cucumber.
Glass jars are also great for protecting seedlings from a sudden, wet, spring snow. Spring snow is not unusual in cold climate. Seedlings that can break from the weight of the snow, like my Brussels sprouts, can be easily protected with a glass jar over them.
Any container can do the same job in a sudden frost or snow event. A bucket, pail, or rubber container can all be placed over plants for a short period. We explain more of this in our post blizzard conditions in the garden.
Plant protection from frost in a greenhouse
We are big fans of greenhouses. When it comes to frost protection, a simple greenhouse covered with plastic gives you about an extra 4 degrees more or less, depending on the thickness of the plastic.
If temperatures drop below freezing, a greenhouse might not protect the plants from frost.
A Plant Protection Blanket used in a greenhouse would double the protection. Since there is no wind in a greenhouse it makes for a perfect way to grow plants early, or in milder climates even all year long.
Thermal mass and a heat exchange also help to keep the greenhouse from freezing. In this post, we explain how it works.
After a cold night -8C (17.6F) there are beautiful frost paintings on the greenhouse covering. The plants are still holding on.
Note: This active heat exchange only works in combination with solar energy. On cloudy days there might be not enough heat to hold through a freezing night. A supplemental heater will be needed.
5 things to know about plant protection from frost
- Winter crops are more frost tolerant, summer crops need protection.
- Watered plants are more frost resistant. Water the plants before the frost or in the morning after before the sun comes out.
- Cover plants before the frost with whatever you have on hand, so the frost can not drop on them.
- When you cover the plant make sure to cover as much ground as possible to give the plant access to the ground heat.
- Cover in layers. It is just like with us, we are warmer if we wear layers.
Inspiring books for winter gardening: