Four Strategies to Protect Your Plants from Frost

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Eek!! The weather report predicts our first frost Friday and Saturday with 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 freezing temperatures. Also, several crops, like beets and carrots, actually weather frosts well and only become sweeter in flavor.

In the fall, as temperatures start to cool, the first day of the year that a frost day occurs is considered the first frost date for that year. As the temperatures continue to cool, usually about a week or two later, the first freeze date of the year will occur (this is what kills most annual plants). Missoula’s first frost date is September 27th, and the first freeze day is mid October. These 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, 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, utilize these four strategies below to temporarily protect your plants and extend your harvest for the next couple weeks until our first hard freeze.

Assess: How Bad Is It?

It’s not just the temperature but the length of time that temperatures are at or below freezing that damages plants. Just as a lower temperature is harder on a plant than a temperature at or near freezing, very cold temperatures that last several hours is much harder on a plant 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.

Assess & Prioritize

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 covering your bitter lettuce? 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.

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 or even plastic sheets to protect tender shoots and fruit from frost nip. This will also keep the heat from the soil close to the plants. 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. 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 burn the plant. For small plants, you can cover them with an inverted bucket or flower pot. Remember to remove the cover when temperatures rise during the day.

Students 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!

Students 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.       Mulch - As we move into late fall and consistently cold nights, mulch the hardy vegetables to bolster their frost tolerance.

4.      Harvest Early - Finally, don’t forget 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.

Good Luck!

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. 

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.