Monday, October 21, 2013

Frost and Freezing in Fall Vegetable Crops – Part 1

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

With frost imminent for Missouri it is good to review how the different vegetable crops respond when it occurs. 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 roads in winter cold spells. 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. A “First Frost” map shows ranges when frost first occurs on average in a state; look for frost advisories from the National Weather Service and your local news services. In general for Missouri frost begins in mid-October and by mid-November all the state has had frost. North Missouri and higher areas of the Ozarks are hit first. On a very local level, cold air will flow down and ‘puddle’ in low areas, thus low lying fields will often be affected earlier. Lastly, small cities and metro areas will often stay a few degrees warmer (sometimes called a ‘heat island’) and thereby not be frosted as quickly as the surrounding countryside. The PlantMaps website 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).

Table 2. Frost resistance of vegetables.*

very hardy1  Frost tolerant2   Tender3      Warm

Asparagus    Beet              Snap bean    Lima bean

Collards     Broccoli          Sweet corn   Cucumber

Endive       Brussels sprout   Tomato       Eggplant

Kale         Cabbage                        Muskmelon

Kohlrabi     Carrot                         Okra

Lettuce      Cauliflower                    Pepper

Mustard      Celeriac                       Pumpkin

Onion (sets  Celery                         Squash,

 and seeds)  Chard                           summer

Pea          Chinese cabbage                Squash,

Potato       Jerusalem                       winter

Rhubarb       artichoke                     Sweet

Rutabaga     Onion (plants)                  potato

Salsify      Parsnip                        Watermelon

Spinach      Radish


*Based upon information from university of Illinois publication VC 14 a2, Vegetable Planting Guide.

1very hardy vegetables can withstand freezing temperatures and hard frosts for short periods without injury. They may be planted as soon as the ground can be prepared. usually 4 to 6 weeks before the average frost-free date.

2Frost tolerant vegetables can withstand light frosts and can be planted 2 to 3 weeks before the average frost-free date.

3Tender vegetables are injured or killed by frost, and their seeds do not germinate well in cold soil. They are usually planted on or after the average frost-free date.

4Warm loving vegetables cannot tolerate cold. They require warm soils for germination and good growth, and should be planted 1 to 2 weeks after the average frost-free date.

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


•Effects of Cold Weather on Horticultural Plants in Indiana, Purdue University Cooperative Extension Service

•Understanding Frost (pdf), Cornell Cooperative Extension
•Frost protection: fundamentals, practice and economics (pdf), Food and Agriculture Organization of the United Nations

(This article was published by Michigan State University Extension. Adapted by James Quinn for University of Missouri, October 2013)


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