Four Strategies to Protect Your Plants from Frost
Eek!! The weather report predicts frost and overnight lows near or below freezing! What to do?!?
Don’t worry, keep in mind that a frost is different than a freeze, and there are several easy strategies you can do to help protect your plants from frosty temperatures. And in the fall, some crops, like carrots, parsnips, and beets, only become sweeter in flavor after a frost.
Know Your Frost Dates
First things first! Always keep in mind your local average frost dates - first frost and last frost.
In the fall as temperatures start to cool, the first day of the year that a frost occurs is considered the first frost date. As the temperatures continue to cool, the first freeze date of the year will occur (this is what kills most annual plants). Missoula’s *average* first frost date is September 22nd, and the *average* first freeze day is in mid October.
In the spring, as temperatures begin to warm back up, the last day of the year we can expect a frost is the average last frost date. Missoula’s *average* last frost date is May 18th. For more on what to plant during this time, read this blog: Waiting for the Last Frost.
These frost dates are based on historical weather data collected over a 30 year period, so they are usually accurate but by no means exact. Be diligent and check the weather report regularly or set up a weather app alert to keep an eye on the overnight lows. When temperatures are expected to dip near or below freezing (generally, I fret over anything in the mid-30 degree Fahrenheit range which could nip my most tender annuals), utilize these four strategies below to temporarily protect your plants.
Strategies to Protect Your Plants
Frost or Freeze? Assess: How bad it is going to be
It’s not just the temperature but the length of time that temperatures are at or below freezing that damages plants. Just as a freeze is harder on plants than a light frost, cold temperatures that last several hours are much harder on plants than an hour or less of freezing temps. Keep this in mind along with several key definitions listed below when evaluating the severity of the weather report.
Frost Advisory - This is when the temperature is expected to fall to 36 degrees to 32 degrees Fahrenheit.
Freeze Warning - This is issued when there is at least an 80% chance that the temperature will hit 32 degrees Fahrenheit or lower.
Light freeze - 29° to 32° Fahrenheit will kill tender plants.
Moderate freeze - 25° to 28° Fahrenheit is widely destructive to most vegetation.
Severe or hard freeze - 25° Fahrenheit and colder causes heavy damage to most plants.
2. Prioritize Your Plants
Do you know which plants in your garden are considered “hardy” and can handle a light frost or considered “tender” and will be injured or die by freezing temperatures? Check out this handy growing guide from Montana State University Extension that lists common garden veggies and their frost tolerance.
Hardy vegetables will do just fine without any extra protection from the frost. But, take a good look at your tender crops and prioritize what to protect based on what has a good harvest that could ripen in the next couple weeks. Prioritize your time, energy and money on the plants and anticipated harvests that are most valuable to you. Is it worth protecting your lettuce or beans that aren’t producing a lot anymore? Probably not. Do your tomato plants have a lot of fruit that can still ripen in the next month? Yes? Great, let’s take action.
3. How to Take Action
1. Water – Water acts as an insulator. Plant cells that are plump with water will be stronger against cold damage. Likewise, moist soil will tend to stay warmer than dry soil, so a good soaking right before freezing temperatures can help protect plants.
2. Cover - The soil also acts as a great insulator and thermal regulator (which is one reason why root vegetables nestled in the soil can handle a couple frosts). Cover tender plants with commercial frost cloths or row cover (found at most garden and hardware stores). Old bed sheets, burlap, tarps, or even plastic buckets placed over small plants in early spring can protect plants from frost. Make sure to stake the material down so wind doesn’t blow it off and that the cover goes all the way to the ground in order to maximize insulation and keep the heat from the soil close to the plants.
Be careful! If you do use a plastic sheet, keep it from touching any foliage or fruit. The cold will transfer through the plastic and nip the plant. Remember to remove any sheets or plastic cover when temperatures rise during the day, so the plant can get the air and light it needs. Commercial frost cloths actually allow some light and air to pass through so you can leave those on the plants during the day if needed.
Students and staff at the PEAS Farm lay commercial row cover over the pepper plants. You can do this too in your garden to protect tender plants from a frost!
3. Plant Later - Some of the most devastating frosts can happen in the spring. Before planting your frost sensitive crops (e.g. tomatoes, peppers, squash), check the weather forecast and plan to plant after any frosty weather. Don’t worry, even if you have to wait a week or two for the weather to warm up, your warm weather crops will catch up quickly with growth and vigor. Cold air and soil temperatures stress warm weather crops and can make them more susceptible to pests or disease later down the road. As difficult as it is, it’s best to wait. Your tomatoes will thank you!
4. Harvest Early - If it’s fall and nearing the end of harvest season, keep in mind that many vegetables and fruit will ripen at your home after being harvested. Tomatoes, tomatillos, apples, peaches, plums and pears will continue to ripen off the plant. Place in a paper bag in a dark, cool part of your house and check on it a couple times a week. Eat as they ripen and always remove anything that is moldy or rotting.
5. Mulch - As we move into late fall and consistently cold nights, mulch hardy root vegetables to bolster their frost tolerance.
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Pick your tomatoes a little early if you're worried about a frost or freeze. Most will ripen at home stored in a brown paper bag or cardboard box.
Updated by Genevieve Jessop Marsh 9/8/2023