Frost is Here! Never Fear!
It’s that time of year again in Southwest Montana: the days are still warm, the kids are back in school, the tomatoes are starting to ripen, and, suddenly, someone says, “I heard it’s supposed to freeze on Friday!”
Did I hear that right? How!? Why!?
And what about my tomatoes!?
Yes– you probably heard it right. The average first frost date for Bozeman is 9/16, so it’s not unlikely that freezing temperatures will hit Gallatin County gardens sometime during the month of September.
Source: Better Homes and Gardens
The “How!?” and “Why!?” of it all is a bit more complicated, but a lot of it has to do with how far north, and how high up, Bozeman is.
Part is our latitude: this far north, Bozeman and the surrounding areas are quickly impacted as the sun starts to move farther south, taking with it the direct, warm rays that fueled the heat and abundance of summer.
The mountains make a big difference, too. Sitting at over 4000 feet, Bozeman has a lot less atmosphere to hold in heat than an equivalent place at sea level. Without that extra insulation, we can experience massive temperature swings, even from 90-plus degree days, especially on cloud-free nights.
Add to that one more *cool* mountain phenomenon: cold air sinks. As the sun sets, air far up mountain slopes chills and sinks, sometimes flowing down the slopes and into the valley like a river of cold air, causing a killing frost to occur in patchy local places.
So, yes: frost happens early in Bozeman. But never fear! Here are some tactics you can try to extend your season and protect your plants:
Tactic 1: Cover ‘em
One tried and true method to protect plants from a mild frost is covering them, which provides some extra protection from cold air and also helps trap warm heat from the soil near the plant.
You can use a lot of different materials to tuck in your plants, depending on how much you want to spend and how long you plan to keep them covered. Quick protection from a cold night can be done with anything from sheets and towels to tarps and buckets, depending on what you have available and what fits over your plants. Just be sure to remove anything that doesn’t let in light and air before too long the next day– you don’t want to save your plant from frost only to end up smothering it!
You can also buy fabric that’s specifically designed to protect plants from frost, and that can even be left over top of plants during the day to keep them warm and help protect from insect damage. If you’re planning to try to extend your season in the spring and fall, and are willing to care for the fabric so it lasts for a few years, this can be a great investment– but it’s not necessary for home gardeners, either.
Tactic 2: Soak ‘em
Water is a really good insulator: it takes a lot of energy to turn it into ice, and once frozen, it doesn’t tend to get colder than 32 degrees. Gardeners can use this to our advantage by turning on sprinklers when the frost is expected. The water spraying in the garden will help keep the air from getting too cold, and even if it does freeze, the ice that develops on your crops creates an insulating layer that remains at 32 degrees– which many (but not all) plants can actually tolerate.
Ice forms on berries, protecting the crop from frost (Source: Senniniger Irrigation)
The trick is to not just run your sprinkler all night, which wastes water and makes a muddy mess. Usually the coldest frost actually happens right before dawn, so ideally you want to wait until then to turn on the sprinklers.
Early risers can excel at this task. Us mere mortals who like a normal amount of sleep may not find this to be a sustainable solution– but if it means saving my tomatoes, I can do one or two mornings of a 4am sprinkler run.
Tactic 3: Bring ‘em Indoors
The first two tactics can be done pretty much anywhere, but this one takes a little planning. If your plants are mobile, you can just bring them indoors. (Container gardeners, this is your time to shine!) If you don’t want a mess, usually a shed or a garage will do as an overnight storage solution, unless it’s really cold.
To make it easier on yourself, consider using wheeled pots, a wagon, or furniture rollers to help assist moving the plants. Again, this is something that usually has to be set up in advance, but there’s always next year!
Tactic 3.1: Bring ‘em Indoors (Harvested)
If you can’t cover, soak, or move your plants, and a frost is coming, all is not lost– those green tomatoes will still ripen! Harvest any frost-sensitive plants before the frost to avoid damage to the edible parts. If your tomatoes are still green, you can put them in a cardboard box to ripen– check on them every few days and use the red ones, and you’ll be enjoying fresh tomatoes for weeks!
Tactic 4: Do Nothing. You Planted Frost-Hardy Plants!
If you are reading this and your garden consists of kale, carrots, spinach, radish, lettuce, or other frost-hardy plants, *congratulations!* Your plants will most likely be fine through these first few mild frosts with little extra effort from you. They may even start to taste better!
Before we know it, frosty mornings will be back. Good luck as you navigate Northern Rockies Mountain weather this fall in your garden! If you have a method that works (or doesn’t work) really well for you, please consider sharing it with us at lunchbox@gvfarmtoschool.org so we can tell other schools and families about what gardening tips work in Gallatin County!
