Example Article for Broadwater County Extension Agent
The Science Of Frost And Plants
Author: Allison Kosto, MSU Broadwater County Extension Agent
The Science of Frost and Plants
Allison Kosto
MSU Broadwater County Extension Agent
According to the Old Farmer's Almanac, the average first frost date for Townsend is September 14th, which means we can expect it to come anytime. While frost is a natural phase each fall here in Montana, it is also a fascinating lesson on science and the importance of frost in the cycle of plants.
Even before temperatures reach freezing, plants are affected by reducing their metabolism and growth. When the temperature reaches 32 degrees F or below, frost can injure the plant by forming ice crystals inside the rigid cell walls, which causes them to burst or kill the tissue. Additionally, the ice disrupts the movement of fluids and nutrients within the plant, further leading to plant death.
However, there is a wide range of cold tolerance of plants. Plants are categorized as very tender, tender, half-hardy and very hardy. Very tender plants cannot tolerate cold temperatures and will show signs of damage when temperatures drop below 40 degrees. Examples of this include tropical plants but also common garden varieties, including cucumbers, eggplant, melons, peppers, pumpkins and squash. Very tender vegetables require warm soil for germination and good growth and should be planted one to two weeks after the average last frost date in the spring. These plants will be the first to succumb to cold in the fall and will quickly slow their growth when temperatures drop.
Tender plants cannot tolerate temperatures at or below 32 degrees. They die very easily from just a light frost. Examples of tender plants are beans, corn and tomatoes. Additionally, their seeds do not germinate well in cold soil, so they should be planted on or after the average last frost date.
Half-hardy plants are able to survive a light frost. The actual temperature they will tolerate is difficult to pinpoint because duration is also important for half-hardy plants. For example, they may be able to handle an hour or two of frost but cannot handle it for longer periods of time. Typically, half-hardy plants can be planted 2 to 3 weeks before the average last frost date. Examples include beets, broccoli, cabbage, carrots, cauliflower, chard, onions and radishes.
Hardy vegetables are able to withstand freezing temperatures for short periods of time and/or lower temperatures. Examples are asparagus, kale, lettuce, onion (sets or seeds), peas, potatoes, rhubarb and spinach. They can often be planted 4 to 6 weeks before the average last frost date.
There obviously is a wide difference in plant handiness, but why is that?! There are a few reasons, but the most commonly accepted rationale is that plants have different levels of sugars in their leaves. Sugar water freezes at a lower temperature than plain water and research shows that plants with higher sugar content are more resistant to freezing temperatures. It also makes plants more resilient to the cellular damage caused by freezing.
Woody plants and perennials go through a slightly different process during low and cold temperatures. Low temperatures and shorter days will trigger plants to begin hardening off. This is a process when plants stop growing, overwintering buds mature, leaves drop, and a plant’s metabolism slows down to allow the plant to go into dormancy. If temperatures drop too quickly in the fall to allow this to happen, it can result in frost damage on trees and perennials. Sometimes frost damage is not visible until the next spring.
If you aren’t ready to be done gardening yet, there are some strategies you can use to extend the fall garden season and protect plants. This includes covering the plants with sheets, blankets, tarps or row materials. Make sure the edges reach the ground to prevent cold air seeping. If possible, you can bring them indoors. Even an unheated garage or shop can extend the growing season. A longer-term solution is to use cold frames, hoop tunnels or greenhouses.
Personally, I’m often ready for the first frost. It allows me to slow down, enjoy the last harvest and put the garden to bed for next year. The average last frost for Townsend is May 25th and it’s never too soon to start planning! If you have any questions about garden care, you can reach the MSU Extension Office in Broadwater County at 406-266-9242 or allison.kosto@montana.edu.
