Lettuce And Frost: Does Lettuce Need To Be Protected From Frost

Lettuce And Frost: Does Lettuce Need To Be Protected From Frost

By: Amy Grant

Lettuce is a veggie that does best when grown in cooler, moist conditions; temperatures between 45-65 F. (7-18 C.) are ideal. How cool is cool, though? Will frost damage lettuce plants? Read on to learn more.

Does Lettuce Need to be protected from Frost?

Growing your own lettuce is a beautiful thing. Not only is it rewarding to pick your own fresh produce, but once picked, lettuce will continue to grow, giving you successive harvests of fresh greens. But what happens when temperatures dip toward the freezing mark? Does your lettuce need to be protected from frost?

Lettuce seedlings will generally tolerate a light frost and, unlike most vegetables, continue to grow through the fall when the possibility is a probability in some regions. That said, cold, clear nights may create frost damage in lettuce, especially if the duration of the cold snap is lengthy.

Lettuce and Frost Resulting Symptoms

Frost damage in lettuce causes a variety of symptoms relating to the severity and length of the freezing period. A common symptom is when the outer cuticle of the leaf separates from the underlying tissue, causing a bronzed color due to the death of those epidermal cells. Severe damage causes necrotic lesions of the leaf veins and spotting of the leaf, similar to pesticide burn or heat damage.

On occasion, the tips of young leaves are killed outright or frost damages the edges, resulting in the thickening of the leaf tissue. Any damage to lettuce due to frost should be removed or the plants will begin to decay and become inedible.

Lettuce and Frost Protection

Lettuce is tolerant of cold temperatures for short periods of time, although growth will slow down. To protect lettuce in frost prone areas, plant romaine or butterhead lettuce, which are the most cold-tolerant.

When frost is predicted, cover the garden with sheets or towels to provide some protection. This will help in the short term, but if prolonged frost is due, your lettuce is likely in jeopardy.

Finally, outdoor freezes may not be the only concern for lettuce and frost. Frosty conditions in your refrigerator will definitely damage the tender lettuce greens, leaving you with a slimy mess. Obviously, don’t put lettuce in the freezer. Adjust the setting of your fridge if it is prone to frosting up.

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Frost and Freeze Protection for Vegetables

The clear skies and calm winds predicted for this weekend will likely lead to freeze or frost conditions in North Carolina. Sunday morning, May 10, 2020, the temperature is predicted to drop into the 30s across the state and even lower in low-lying rural areas. Not only are daily low temperature records likely to be broken, but this will tie the latest spring freeze for Raleigh, May 10th (31 ºF) and set a new record for Greensboro where the current record is May 8 th (32 ºF).

If water within or between the plant cells freeze, this can result in damage to plant tissue. Cold damage results from the actual temperature and the duration of that temperature. Some plants are more resistant to freeze damage.

Frost occurs at temperatures from 31-33 ºF and will result in damage or destruction of the foliage of warm season plants like beans, corn, cantaloupe, cucumbers, eggplant, okra, southern peas, peppers, potatoes, sweet corn, sweetpotatoes, squash, tomatoes, and watermelon.

Temperatures below 26-31 ºF, cause a hard frost or freeze. Some cool season crops will tolerate a temperature dip to these temperatures for a limited period of time. These include broccoli, cabbage, cauliflower, chard, English peas, lettuce, mustard, onion, radishes and turnips. The plant may survive, however, the foliage may be damaged resulting in a lower yield.

Some cool season crops like Brussels sprouts, beets, collards, kale, parsley, and spinach will survive even If the temperature drops below 26 ºF for an extended period of time.

  • Ensure they are well watered. Drought stress are more vulnerable to cold damage. In addition, moist soil retains heat longer and releases it slowly during the cold event.
  • Do notcultivate the soil just prior to a frost or freeze since cultivation can damage plant roots increasing plant stress, result in loss of soil moisture, and allow cool air to penetrate deeper into the ground.
  • Cover plants
      • Frost cloth – rated by the degrees of protection it provides
      • Waxed paper cups – for overnight protection of small transplants
      • Carefully monitor the temperatures under the covers and remove before temperatures rise too high.
  • In extreme cases, newly transplanted tomatoes and peppers, can be carefully dug up and brought inside to avoid the freeze and then replanted when the danger is past.

Select a strategy and prepare ahead of time to minimize frost damage.

This article originally written for vegetable growers, has been edited by Lucy Bradley for home gardeners.

If you're a North Carolina resident with a question about a topic on this site, your local N.C. Cooperative Extension office can help.


MSU Extension Vegetables

Recent frost advisories are relevant for late-season vegetables.

A "water-soaked" appearance is a common identifier of freeze damage on fruiting vegetables. These decorative gourds will still harden off, but the discoloration is permanent. Photo by Ben Phillips, MSU Extension.

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 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's Enviroweather.

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 has typically had 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.

Plant hardiness

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 area of origin. 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).

When freezing occurs, water expands and can burst cell walls. However, leaf and root vegetables are generally more capable of withstanding hard frosts (less than 28 F), and have more room to spare in their tissues for water expansion and internal ice-formation.

Hard frost hardy (less than 28 F)

  • Collards
  • Endive/escarole
  • Kale
  • Kohlrabi
  • Lettuce
  • Mustard
  • Onion (sets and seeds)
  • Pea
  • Potato
  • Rhubarb
  • Rutabaga
  • Spinach
  • Turnip

Light frost hardy (28–32 F)

  • Beet
  • Broccoli
  • Cabbage
  • Carrot
  • Cauliflower
  • Celeriac
  • Celery
  • Chard
  • Onion (plants)
  • Parsnip
  • Radish

Light frost susceptible (28–32 F)

  • Cucumber
  • Edible beans
  • Eggplant
  • Muskmelon
  • Okra
  • Pepper
  • Pumpkin
  • Squash, summer/winter
  • Sweet corn
  • Sweet potato
  • Tomato
  • Watermelon

How to tell if you have frost-damaged vegetables

Freeze-killed leaves will at first turn brown and look somewhat transparent as they thaw, a term generally referred to as “water-soaked.” Once dry, they may curl up and become brittle. The marketable part of the plant may also show signs of damage.

The list below is adapted from Purdue Extension Bulletin HO-203, “Effects of Cold Weather on Horticultural Plants in Indiana,” and describes what to look for in freeze-damaged vegetables. For positive identification of suspected freeze damage, contact your regional Michigan State University Extension educator.

  • Beet: External and internal water-soaking sometimes blackening of conducting tissue.
  • Broccoli: The youngest florets in the center of the curd are most sensitive to freezing injury. They turn brown and give off strong odors upon thawing.
  • Cabbage: Leaves become water-soaked, translucent and limp upon thawing epidermis separates.
  • Carrot: Blistered appearance, jagged, length-wise cracks. Interior becomes water-soaked and darkened upon thawing.
  • Cauliflower: Curds turn brown and have a strong off-odor when cooked.
  • Celery: Leaves and petioles appear wilted and water-soaked upon thawing. Petioles freeze more readily than leaves.
  • Cucumber: Transparent, water-soaked appearance in cross section, just under the skin.
  • Garlic: Thawed cloves appear grayish-yellow and water-soaked.
  • Lettuce: Blistering dead cells of the separated epidermis on outer leaves become tan increased susceptibility to physical damage and decay.
  • Onion: Thawed bulbs are soft, grayish-yellow and water-soaked in cross section often limited to individual scales.
  • Pepper: Dead, water-soaked tissue in part or all of pericarp surface pitting, shriveling and decay follow thawing.
  • Potato: Freezing injury may not be externally evident, but shows as gray or bluish-gray patches beneath the skin. Thawed tubers become soft and watery.
  • Pumpkin: Water-soaked spots on upper surface of fruit which soften the rind. Badly damaged fruit will eventually collapse in on itself.
  • Radish: Thawed tissues appear translucent roots soften and shrivel.
  • Squash: Water-soaked spots on upper surface of fruit. Ornamental and winter squashes may still harden, but others will soften and rot.
  • Sweet corn: Reduced ear size and weight with shriveled kernels. Ears can take a “bar-bell” shape if they are still developing.
  • Sweet potato: A yellowish-brown discoloration of the vascular ring and a yellowish-green, water-soaked appearance of other tissues. Roots soften and become very susceptible to decay.
  • Tomato: Water-soaked and soft upon thawing. In partially frozen fruits, the margin between healthy and dead tissue is distinct, especially in green fruits.
  • Turnip: Small, water-soaked spots or pitting on the surface. Injured tissues appear tan or gray and give off an objectionable odor.

Methods for protecting frost-sensitive crops

Depending on what materials are available, as well as what crops are being protected, there are several options growers can use to extend the productive season. Commercial growers often rely on passive or heated high tunnels, greenhouses, hoop houses or cold frames to offer several degrees of protection for light-frost susceptible crops in the fall. These structures can also be used to protect hard and light frost hardy crops deeper into the winter months, long after the internal temperatures have dropped below what is appropriate for light frost-susceptible crops.

Commercial growers and home gardeners also rely on floating row covers and other protective covers as a low-cost way to protect sensitive crops from frost. These covers are supported above the crop using wire or metal hoops, or bent PVC hoops. Material edges are commonly weighted with sand bags or simply buried with soil to prevent loss due to wind. Lightweight covers come in varying sizes and weights, providing different levels of frost protection.

The table below provides a few examples of row cover options that provide frost protection. It must be noted that as covers grow heavier, the light transmission drops, meaning less photosynthetic activity will occur unless covers are removed. A notable exception is that of greenhouse film (plastic), which provides significant frost protection while still allowing substantial light transmission. The primary drawback, however, is that this material is not self-venting, meaning growers must remove the cover on sunny days to prevent overheating.

Row covers that provide frost protection
ProductFrost protection (degrees)Light transmission (%)
Floating Row Cover 0.55 ounces per square yard 4 85
Floating Row Cover 0.9 ounces per square yard 6 70
Floating Row Cover 1.5 ounces per square yard 8 50
Floating Row Cover 2.0 ounces per square yard 10 30
Typar Row Cover 1.25 ounces per square yard 6 70
Greenhouse Film 6 mil 10 95

Hoop house and greenhouse structures can be more effective when used in conjunction with interior floating row covers. This double layer of protection creates a microclimate at plant level that can be significantly warmer than exterior temperatures. In areas with relatively mild winters, a lightweight row cover can be effective and does not need to be removed for ventilation or to allow solar exposure. In colder climates, multiple layers of lightweight covers or heavier covers can be used to protect cold-hardy crops throughout the winter months. These covers are typically removed on sunny days to warm the soil, allow plants to thaw or photosynthesize, as well as ventilate and exchange air to discourage disease.

For further reading on low-cost season extension options for commercial growers and home gardeners, visit the MSU North Farm’s Resources page, including the Low-Cost Season Extension Skill-Seeker workshop presentation.

The same row covers used in early season production can also protect some vegetables now. Fall plantings can be double-insulated under low tunnels inside of high tunnels or greenhouses as well, but will need ventilation on sunny days. Photos by Ben Phillips, MSU Extension.

References

  • Effects of Cold Weather on Horticultural Plants in Indiana, Purdue University Cooperative Extension Service
  • Understanding Frost, Cornell Cooperative Extension
  • Frost protection: fundamentals, practice and economics, Food and Agriculture Organization of the United Nations
  • Irrigation Method and Rowcover Use for Strawberry, Journal of the American Society for Horticultural Science
  • Row Covers for Commercial Vegetable Culture in Florida, University of Florida Extension

This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).

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6 Ways to Protect Plants from Unexpected Frost

Sudden cold snaps and shifting temperatures can take their toll on frost sensitive vegetables. Here are several methods to protect your plants from unexpected overnight frost:

Water the Plants During the Day

Water your garden soil thoroughly during the day before evening frost is predicted. Damp soil can hold four times more heat than dry soil. It will also conduct heat as the soil releases moisture and warm the air near ground level.

Watering your garden when temperatures are still above freezing will help hold in some thermal heat when the temperatures dip overnight.

Cover Plants with Old Blankets and Sheets

Old blankets and sheets are a great way to protect plants from frost. The blanket will help insulate tender plants and keep them alive during a frosty night.

Use stakes or hoops to hold the material up and away from the foliage and drape the blanket over the plants until it touches the ground. Secure the edges with boards, stones, or bricks to hold it in place to prevent cold air from seeping in.

Slip old pillowcases over the tomato cages creating an insulating air pocket around the plants. You can cover these with additional blankets for extra protection.

Cover in early evening just before the sun goes down. Remove the covers in the morning so the plants do not overheat.

Protect Your Garden with Hoops and Plastic

Use a grow tunnel mini-greenhouse to protect your plants from overnight frost. Or use 10-foot pieces of PVC to make hoops over a raised bed and drape with 5 mil painters plastic for some frost protection Secure the edges to keep heat in and cold out.

Wait to cover until the sun begins to set so your plants don’t overheat. Remove the plastic in the morning to allow your plants to breathe and warm up naturally.

Place Containers Over Frost Tender Seedlings

Place buckets, pots, storage totes, garbage cans, cloches or any large container over frost tender seedlings. Weigh down with rocks or bricks if it is breezy.

The containers will create an insulating pocket around your plants. Cold air is prevented from seeping in and residual heat from soil is held near plants. Uncover in the morning once the temperature warms up above freezing.

Use a Garden Frost Blanket or Row Cover

Frost blankets, floating row covers, and garden quilts are made from a lightweight woven material made specifically for protecting plants. The fabric allows some light to penetrate and is breathable, so it can be left on during the day if extended protection is needed.

Frost blankets come in varying thicknesses. Thicker quilts protect plants to a greater degree than thinner row covers. Thinner row covers can protect plants down to 28˚F, while thicker frost quilts protect plants down to 24˚F.

Like other coverings, frost blankets work by excluding cold air and creating an insulating air pocket around plants. Heat from soil, which is warmer than air, is trapped beneath the blanket and held near plants.

Use hoops or fencing to create an arch over your plants, drape the frost blankets, and anchor the edges to keep heat in and cold air out. You can remove the frost blanket once the threat of frost has passed. Or, since the fabric is breathable, you can leave it on for additional protection. Just be sure to check your plants to make sure they are not overheating.

Add Heat

If the predicted overnight frost is unusually cold, extra heat may help keep your plants from freezing. In addition to covering frost tender plants, you can add heat beneath the cover.

Fill milk jugs and containers with hot water and place under the protective covering to serve as a source of extra heat around the plants.

Use a string of older, non-LED lights to help keep plants warmer. String the lights along the arch supports beneath the coverings. Keep the lights from touching your plants and fabric.


Frost and Freeze Protection for Vegetable Crops

Frost and Freeze Protection for Vegetables

Chris Gunter, Extension Vegetable Production Specialist

Jonathan Schultheis, Extension Vegetable Production Specialist

Horticultural Science, NC State University

Weather predictions for this weekend include clear skies with minimal or calm winds, which are conducive to cold temperatures and could lead to freeze or frost conditions in North Carolina. Sunday morning, May 10, low temperatures are predicted to drop into the 30s with some temperatures possibly below freezing in low lying rural areas. Record low temperatures for May 10 are in the lower 30s at the Greensboro and Raleigh-Durham locations, and 39 ºF at Fayetteville. Not only are daily low-temperature records threatened, but the latest spring freeze on record is May 10 for Raleigh (31 ºF) and May 8 for Greensboro (32 ºF).

As cold temperatures are threatening, agents and specialists are getting calls from concerned vegetable growers worried about potential damage on their crops. A frost occurs when temperatures dip to 32 ºF and water starts to freeze on low lying surfaces. If water within the plant cell or between the plant cells freeze, this can result in damage to plant tissue. The following vegetable crops can be grouped according to their cold tolerance based on their physiology. It is important to remember that cold damage results from the actual temperature and the duration of that temperature.

We usually think of frost as occurring at temperatures from 31-33 ºF and this will result in damage or killing the foliage of warm-season plants like beans, corn, cantaloupe, cucumbers, eggplant, okra, southern peas, peppers, potatoes, sweet corn, sweetpotatoes, squash, tomatoes, and watermelon.

When temperatures dip below 26-31 ºF, this is a hard frost or freeze. There are cool-season crops that will tolerate a temperature dip to these temperatures for a limited period of time. These include broccoli, cabbage, cauliflower, chard, English peas, lettuce, mustard, onion, radishes, and turnips. They will likely show foliage damage due to the cold and this may result in a yield reduction later in the season, but the plant will survive.

In the event that the temperature drops below 26 ºF during a hard freeze for a longer period of time, there are some vegetables that will survive these temperatures. Cold season crops like Brussels sprouts, beets, collards, kale, parsley, and spinach should survive.

Charlotte Glen wrote a great article about seeds and seedlings surviving cold temperatures. We encourage you to check it out here:

Protecting from these cold temperatures can be a little more difficult to answer. Growers who have solid set irrigation in place and have experience with frost protection using overhead water applications, may be planning to use this method. This requires large volumes of water and continuous applications throughout the cold event. For most vegetable growers however, overhead solid set sprinkler irrigation is less common than drip irrigation or travelling guns and pivot irrigation systems.

Usually maintaining good soil moisture prior to the event can be helpful, as the water in the soil retains heat longer and releases it slowly during the cold event. For this reason, it is also recommended not to cultivate just prior to a frost or freeze, so that as much water can be retained in the soil as possible. The cultivation can damage plant roots and increase stress on the plants. In addition, cultivation opens additional spaces in the soil, allowing cool air to penetrate deeper into the soil profile.

Other methods for frost protection include using row covers, which come in various lengths and thicknesses depending upon the level of protection needed. Growers can also use waxed paper cups, to cover the transplants in the field, during an overnight cold period. These are labor-intensive methods, but may be an option if areas are small enough and the farm has sufficient labor to put on and remove the covers as temperatures warm back up. If coverings are used, it is important to monitor temperatures under the covers. Be prepared to remove the covers before temperatures under the cover rise too high and result in heat stress.

Growers can wait to transplant those crops that are most frost sensitive. In extreme cases, with recently transplanted Solanaceous crops like tomatoes and peppers, newly transplanted seedlings could be removed from the field until the cold event has passed and then retransplanted. Care must be taken during this process to minimize damage to the young roots of these plants. Place seedlings in a protected area under cover to prevent damage to those seedlings collected and placed back in trays.

Though much less common in vegetable crops, there are air fans that are used to move cold air away from low spots or frost pockets. In these low spots, cool air sinks and cannot drain away from the crop. These large air fans can either be permanently mounted in a field, or movable on a trailer or tractor mounted. They are designed to mix the air above which is warmer and prevent cool air from pooling around the crop. These methods are used more commonly in high-value orchard and vineyard crops. Care should be taken when evaluating the economics of any frost protection strategy.

Perhaps the conditions forecast will not be met. However, the ideas conveyed in this communication can be contemplated and potentially prepare, and act upon depending on your situation.


Now you know which plants can survive frost and which ones will succumb to it. You also know how to protect your plants from cold to extend the growing season and harvest more vegetables.

I hope you found this article helpful. If so, please share it with someone who can use the information.

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