Frost and freezing in fall vegetable crops – Part 1

Late season vegetable plantings are up against frost and freeze events.

Vegetable crops planted for fall harvest can be susceptible to early overnight cold snaps, and delayed summer plantings may not fully mature before cold temperatures put the brakes on growth. Preventative actions can be taken, but once severe freeze injury occurs, it is irreversible.

Frost versus freezing

A frost occurs when air temperatures dip to 32 degrees Fahrenheit or lower at ground level. With a frost, the water within plant tissue may or may not actually freeze, depending on other conditions. A frost becomes a freeze event when ice actually forms within and between the cell walls of plant tissue. When this occurs, water expands and can burst cell walls like cracks in Michigan roads in January. However, some plants have more room to spare in their tissues and can withstand a certain amount and duration of internal ice-formation without serious injury. However, when freeze damage occurs, it is irreversible.

Climate and topographical conditions

Frost and freezing conditions can be combated in early fall by keeping up-to-date on weather forecasts and taking appropriate action. This “First Frost” map shows ranges when frost first occurs on average in Michigan, but you can also check for the up-to-date overnight hours below freezing at Michigan State University Extension’s Enviro-weather website. Michigan has a large range of first-frost zones dictated by the macroclimate of the Great Lakes region and the microclimates of local topography and land-use. For example, the Saginaw Valley region typically has its first freeze events of the year later (Oct. 21-31) than the upland areas directly to the east (Oct. 1-20) and west (Oct. 11-20).

The PlantMaps website also compiles and displays interactive climatological data showing last frost ranges, heat-zones, drought conditions and plant-hardiness zones that can be useful for planning a season for a new crop.

Crop tolerance

Depending on crop tolerance, a killing frost can result from canopy temperatures dropping 2-5 degrees below freezing for 5-10 minutes, or from a sustained temperature 31.5-32 F lasting 3-5 hours. Fall vegetables have a range of temperature tolerances, reflecting their origin of domestication. Vegetables that come from flowers, such as vine and solanaceous crops, okra, sweet corn and beans have largely been cultivated and bred from tropical and subtropical plants and are easily damaged by a light frost (28-32 F). However, leaf and root vegetables are generally more capable of withstanding hard frosts (less than 28 F).

Fall vegetable freeze tolerances

Hard frost hardy
Less than 28 F

Light frost hardy
28–32 F

Light frost susceptible
28–32 F

Collards

Beet

Cucumber

Endive/escarole

Broccoli

Edible beans

Kale

Cabbage

Eggplant

Kohlrabi

Carrot

Muskmelon

Lettuce

Cauliflower

Okra

Mustard

Celeriac

Pepper

Onion (sets and seeds)

Celery

Pumpkin

Pea

Chard

Squash, summer/winter

Potato

Onion (plants)

Sweet corn

Rhubarb

Parsnip

Sweet potato

Rutabaga

Radish

Tomato

Spinach

 

Watermelon

Turnip

 

 

Table of tolerance adapted from Purdue Extension Bulletin HO-203 and Snyder et al. 2005

Read Part 2 in this series, “Identifying and preventing freeze damage in fall vegetables – Part 2.”

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