Updated: Nov 9, 2022
You can feel it in the air as soon as the sun goes down, that light nip urging you to give up on the tank tops of summer and opt for sleeves. Fall. It’s one of the most glorious of seasons, the leaves are changing and harvest is in full swing. But for those of us that live in colder climates, it means that the threat of frost is imminent. Nothing will bring a successful harvest to an end faster than an early frost. Luckily, it’s easy to protect your plants if you understand which ones need protection and have the right tools at the ready. Let’s dive in and discuss 10 ways to protect your plants from an early frost.
What is a Frost?
A frost is a thin layer of ice that forms when water vapor changes from a gas to a solid and freezes forming crystals. A light frost happens between 28-32°F, and a hard frost happens when the temperature drops below 28 degrees.
What Temperatures Cause Frost Damage?
Frost damage occurs in plants when the water in their cells turns into ice crystals, stopping the movement of fluids and damaging the plant’s tissue. A "killing frost" happens when the temperature drops below 28 degrees, for a longer time, but this is not necessarily universal. Some crops taste better after a light frost.
For instance, root crops develop more sugars in colder temperatures making carrots and beets just a little sweeter. In the case of brassicas like broccoli and Brussel sprouts, the flavor is actually enhanced by frost. So, how low can you go? Below is a graph that shows what temperatures will cause frost in some of the most common garden varieties.
What Plants Need to Be Protected from Frost?
As discussed above, frost doesn’t affect all plants equally so it’s important for gardeners to understand the cold tolerance of their crops. Plants are characterized by the degree of cold they can tolerate and are classified as either “hardy” or “tender”. When selecting your plants in the spring be sure to be mindful of the notation on the seed package for hardy or tender.
Tender, or cold-sensitive, plants are easily damaged or killed by frost unless you offer protection. All summer or warm-season crops (see chart above) fall into the tender category. Along with the plants listed above other tender plants include citrus trees, many annual flowers, and common houseplants. Additionally, young seedlings of any variety even perennial and cold-hardy plants are considered tender.
Hardy plants handle frost and moderate freezing temperatures well (again, see chart above), particularly the brassica family such as broccoli, Brussel sprouts, and kale. You will also find that most root crops such as turnips, carrots, and beets are cold-hardy as well. Once established, cold hardy crops should survive a frost in the high twenties with little-to-no protection.
You can find a great resource on cold tolerance in vegetables over at Sustainable Market Farming.
Know Your Frost Dates
If you don’t already know the average first/last frost dates for your planting zone, you can find them by zip code here. Keep in mind that average first/last frost dates are typically calculated on a 50% chance of frost and based on 30 years of historical temperature data for your location, but your actual first/late frost dates may vary.
Also, note that local weather conditions can have an effect on the likelihood of frost. Heavy cloud cover will offer insulation from temperature swings that could cause a sudden frost, but clear skies allow heat to escape into the atmosphere and increase frost chances. Another factor that increases the likelihood of a frost is calm conditions without any wind. A lack of air movement means that warmer air currents don’t move over the ground to protect it from frost.
As your average first frost date approaches, it’s time to start checking your local forecast daily.
How to Protect Plants from an Early Frost
Now that we’ve covered the basics, of what frost is and when it occurs let’s talk about some practical ways that you can protect plants from frost an early frost.
Bring Potted Plants Inside
Potted plants lack the earth insulation that in-ground plants benefit from so, first things first, bring any potted plants indoors. Tomatoes are sometimes difficult to grow to maturity in the cool Colorado climate so I actually planted all of my tomatoes in pots this year so that they can be moved inside if they need protecting.
When moving your plants, remember that you don’t want to shock them by moving them into a hot or heated room. Instead, consider moving them to a garage, shed, or basement for the nights and place them back in the sun during the day.
Mulch, Mulch, Mulch
One of the first lines of frost defense for your garden is mulching. As temperatures cool mulch delicate plants 3” to 6” deep to protect the roots and soil from freezing. Think of mulch as your garden's winter sweater. The thicker the layer, the more protection your crops have against freezing. You can mulch with any of the following:
Create a Cloche for Each Plant
Cloches are great for protecting plants and the good news is you don’t need to go out and buy fancy ones. The recommended first planting date for my region (zone 5b) is June 5th (or if you follow old-time advice, once the snow is off the mountains) but I moved all of my starts out a month ahead of schedule. I stocked up on black nursery pots in advance, and each night that frost was expected I placed an individual post over each seedling before dark and I didn’t lose a single plant to frost.
If you get creative, a lot of things around your house can become DIY garden cloches. You can use nursery pots like I did, cut the bottoms off of milk jugs, use Rubbermaid totes or tubs, or use a 5-gallon bucket. Pretty much any bowl bucket or container that is open on the bottom will do!
Get Under Cover
If your plants are too large to conveniently cover with a single cloche, the next step is to move up to a row cover. There’s no need to buy fancy things here either. You can use lightweight, flexible plastic, sheets, blankets, towels, fabric, or even a tarp or drop cloth in a pinch.
The important thing here is to make sure the covering doesn’t touch the plants. One option is to use an irrigation hose or flexible poles to create hoops over your rows, alternatively, you can raise them up with stakes or rocks but you want to keep the cover elevated. At the same time, you’ll need to make sure that you have complete coverage so that no cold air can leak in over the night.
It may seem counter-intuitive but if a freeze is expected in your area you should water your garden soil thoroughly during the day. The reason for this is that damp soil conducts heat as the soil releases moisture and it can hold up to four times more heat than dry soil. Therefore, watering your garden during the day when temperatures are still above freezing will help hold in some thermal heat when the temperatures dip overnight.
This is another interesting way to protect plants from frost: On the morning of the day a frost is expected, fill plastic bottles or milk jugs with water and set them around your plants. During the day, they will heat in the sun; when the temperatures drop, the water will emit enough warmth to keep your plants from freezing. If the expected temperature drop will be dramatic you can combine this with a row cover to ensure that there is no frost damage. I keep a selection of filled bottles that I painted black out in my garden ready to go for this purpose.
What to Do if You Discover Frost Damaged Plants
Even the most cautious gardener can get caught off guard by an unexpected frost. So what’s to be done if you wake one morning to find your plants have been kissed by a light frost? The first thing to do is check for damage, plants that have experienced frost will have blackened foliage and “soggy” or wilted stems and leaves. Allow the plants time to thaw and warm naturally before pruning away any damaged areas. Once the day has warmed, you may discover that only a few leaves at the top of the plant were killed by frost. Simply prune off the damaged foliage and your plant should recover. Unfortunately, plants that have black foliage and stems are not likely to recover so you’ll need to cut your losses there and remove the plant.
Think Long Term
Now we’ve gone over all the various ways you can protect this year's crops from an early frost, I want to touch on some of the ways that you can extend your season right from the start.
Pick Appropriate Plants for Your Zone
An important step in successful gardening and the reduction of plant loss due to frost is to pick appropriate plants for your growing zone. For instance, it would be somewhat unrealistic for me to think that I can grow avocados or citrus here in zone 5b – unless I plan to keep them in a greenhouse. If you’re not sure what your growing zone is you can find out here.
But also remember that there are always ways to stretch the zone limits. Take for example the fact that specific varieties of kale, spinach, or cabbage may be more frost-tolerant than others. If you have harsh winters, selecting the hardiest varieties will help extend your growing season. And on that note, let’s look at other season extenders.
A High Tunnel or Greenhouse
High tunnels or greenhouses are the perfect solutions to an extended growing season, and they don’t have to cost a fortune. Greenhouses can be made from recycled materials and windows if you are a crafty DIY type of person. We are hoping to get a greenhouse up before next season and I’m planning to build it out of pallets and cattle panels with a thermal heat system made from irrigation tubing. (Here’s hoping I’m as crafty as I think I am!) As for high tunnels, All you need is an inexpensive frame and greenhouse plastic to make one.
Cold frames are akin to a tiny greenhouse, or you can think of them as raised beds with hinged or removable covers. They’re a great way to begin your growing season early because you can plant directly in them in the early spring when it is still cold out and the cover will keep plants protected. We build ours from old pallet wood, plastic sheeting, and some old window we found on a curbside one day and they have been great. I planted all of our melons in them and thanks to being able to plant early and protect them into the end of the season we’ll be getting a nice harvest of melons that wouldn’t normally grow in Colorado. You can find some great examples of cheap and easy DIY greenhouses and cold frames here.
These were game changers for me with my tomatoes this season! You can purchase ready-made ones here (not an affiliate link) or do what I did. I saved up a large collection of tall water bottles, filled them with water, painted half of them black, and wired them to tomato cages around my tomato starts.
Bonus Tip - Protect Soil in Winter
As our seasons come to an end and we put our gardens to bed for the winter, it’s easy to forget about the soil. This can be a big mistake, if you’ve spent the season nurturing your soil it should be teaming with life in the form of earthworms and fungi. Before it gets too cold, add a thick layer of organic matter such as straw, leaves, or grass clippings to the surface to keep soil life fed and protect the soil itself from erosion.
If an unexpected early frost hits, losing all of your hard work can be a real disappointment. Successful fall gardening demands careful attention to tender plants and shifting weather. Hopefully, these tips can help you keep your harvest going for as long as possible this season.
If you have any questions about protecting your garden from early frost or extending your growing season, please leave them in the comments! And as always, until next time: