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. http://www.plantmaps.com/
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
loving4
-----------------------------------------------------
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
Turnip
-----------------------------------------------------
*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
References
•Effects
of Cold Weather on Horticultural Plants in Indiana, Purdue University
Cooperative Extension Service
•Understanding
Frost (pdf), Cornell Cooperative Extension
(This article was published by Michigan
State University Extension. Adapted by James Quinn for University of Missouri,
October 2013)
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