by Sheila Dunning | Dec 4, 2020
Frost on Dune Sunflower
Picture by: Pam Brown
UF/IFAS Extension
December is finally here and that means consistent nights of watching for dropping temperatures. Tropical plants and newly installed shrubs are susceptible to cold injury. Those colorful, blooming plants that have added a tropical look to the landscape all summer may begin to suffer when the temperatures drop below 500 F. Leaves may turn yellow and flowering stops. These plants will need to be moved inside or have temporary greenhouse built around them. But, hardy plants that haven’t established a sufficient root system will also need additional attention when the temperatures drop dramatically.
The ability of plants to endure a freeze depends on the species and the weather leading up to the extra cold night. Gradual decreases in temperatures helps plants acclimate to winter. But, a sudden one day drop of 40-50 degrees results in a rapid freeze that causes ice to form inside the plant cells. Leaf and stem tissue expands so quickly that it splits, resulting in parts of the plant incapable of transporting water and nutrients, as well as, performing photosynthesis. However, it may be late the following spring before the damage is noticed, when that section of the plant has slowly staved to death.
Cracked bark from frost
Additionally, plants can experience desiccation or drying out. This happens when dry winds and solar radiation result in the loss of more water from the leaves than can be transported by a cold root system. The resulting symptom is marginal and/or tip burning of leaves that leads to totally brown leaves.
Freezes (when the temperature drops to 320 F) are characterized as radiational or advective. Radiational freezes occur on calm, clear nights when heat radiates from the surfaces of plants, making them colder than the air due to the rapid loss of heat. If there is moisture in the air, ice will also form on the surfaces. Under these conditions, ice forms between plant cells, rather than within the individual cells. Most hardy plants can tolerate these type of freezes if they are properly hydrated.
Advective freezes occur when cold northern winds move in rapidly, dropping temperatures quickly, and causing widespread foliage desiccation. Cultivation and maintenance practices can impact a plant’s ability to endure extended periods of low temperatures. Shade-tolerant species installed under the canopy of a tree typically display less injury from radiational freezes because the trapped heat from the ground and the overhead foliage creates a microclimate. Well-watered soil around a plant will absorb solar radiation during the day and re-radiate heat over night, raising the temperature around the plants. Shrubs that are not pruned in the late summer or fall have leaves that can withstand frost and wind. But, the removal of foliage late in the growing season triggers a flush of new growth, that is very sensitive to lower temperatures. The same response can result from fertilizer applications after plant’s have slowed down in growth.
Irrigation Frost Protection on Citrus
Row Cover
Picture by: Univ of Maryland Extension
So, what can you do to prepare for upcoming freezing temperatures? Begin by avoiding pruning and fertilizing at the end of the season and making sure that the plants have been watered within 24 hours of a cold night. Next, insulate against water loss and increase heat radiation by adding a three-inch of mulch, as well as, covering the trunks of sensitive trees with a commercial tree wrap. Then, consider what needs to be covered. Frost cloth or other breathable fabrics can trap heat for the night and provide a protective layer from frost settling on the leaves. It needs to be placed by mid-afternoon and removed the next day when temperatures are above 320 F. Plastic is not recommended unless the timing regime can be followed reliably and a structure is used under the material to keep the plastic off the foliage. Anchoring of the cover is critically important in the event of an advective freeze.
Finally, turn off the sprinkler system. Commercial agriculture often uses a running irrigation system to keep the leaf surface temperatures near, but not below, 320 F because sprinkling utilizes latent heat released when water changes from a liquid to a solid state. The thin layer of ice melts and re-freezes on the surface throughout the night, without ice forming within the plant tissues. For the technique to work, sprinkling must begin as freezing temperatures are reached and continue until thawing is complete. Landscape systems are not designed to deliver the amount of water over the length of time required to accomplish this type of frost protection.
When the cold nights have passed, don’t forget to check your plants for water. But, wait until winter has passed before pruning out the frost damaged stems.
by Matthew Orwat | Dec 2, 2016
In Northwest Florida gardeners are fortunate since it is possible to grow ample supplies of vegetables throughout the winter months. While the Florida Panhandle does receive the occasional hard freeze, many winter vegetables such as radish, onion, lettuce, carrot and the various cole crops, can easily withstand mild freezes.
When a hard freeze occurs, defined by temperatures that dip below 28°F for over five hours, it is important to be prepared with frost cloth, cotton sheets or other suitable material. Frost cloth is a good option since it protects plants from morning frosts while still allowing for a little air transfer. It is a synthetic fabric which offers 4-8 degrees of protection and is available through many online greenhouse and agricultural suppliers. Old cotton sheets, often found at thrift stores for very low cost, offer good protection from frost damage. With both of these products, ensure that the coverage extends completely to ground level with no gaps, to not allow cold air to infiltrate from the bottom.
While these methods will not provide enough protection to allow summer vegetables to be grown during winter months, it can be beneficial in reducing frost and freeze damage to winter vegetable crop.
Additional Tips:
Do not use plastic covers, since they radiate cold and heat more readily. Even if they protect the plants from the cold, they can quickly trap heat in when the sun rises and “cook” the plants.
Additionally, it is important to keep vegetables well-watered at all times, but this is especially true during frost events since moist soil holds more heat. Healthy, appropriately watered plants withstand cold stress at greater rates.
Happy gardening over the Holiday Season!
by Les Harrison | Jan 20, 2016
North Florida’s gardeners are facing a new set of challenges dealing with the effects of cold weather. However, a little planning and creativity can make plant protection in the landscape a relatively simple process.
Covering plants to protect from frost. UF/IFAS Photo: Sally Lanigan.
Many homeowners and landscape managers want to know when plants will need protection. The point of freezing is a good rule of thumb for most temperate zone plants.
It is worth noting there is a difference in the terms used for cold weather conditions. Frost, freeze and hard freeze all describe different circumstances.
- Frost is when water vapour freezes on surfaces. It usually happens on clear nights with still air and can happen when reported air temperatures are above freezing.
- Freezing is when cold air moves in and causes temperatures to drop below 32 degrees Fahrenheit. This condition commonly involves low humidity and wind, making drying out a big problem for plants.
- A hard freeze is when temperatures dip below 28 degrees Fahrenheit. Some tropical plants will survive a few degrees below freezing for very short periods, but extended periods of freezing or heavy frost may require lights or other heat used safely in combination with covering the plant.
Some plants can be moved indoors for the holidays and incorporated into the interior décor, rather than cramming them last-minute into a chaotic bundle when a freeze looms.
Get prepared by identifying old sheets, blankets and drop cloths which can be used as covers for tender or tropical zone plants which must remain outside. Test potential covers beforehand to assure all plants will be thoroughly covered.
It is best if the covers enclose the plant entirely without crushing it. Heavy blankets are great insulation, but only a good idea on the sturdiest of plants.
A tomato cage or other support structure can be used to keep the weight off the plant. Covers also need to be secured at the ground with pins or weights to assure cold air does not enter from below and collect under the cover.
Finally, keep storage bins handy and remove the covers in the daytime if temperatures are above 55 degrees Fahrenheit.
Monitor weather reports and react accordingly so tender and tropical plants see spring 2016.
by Matthew Orwat | Nov 20, 2015
Although it seems like summer outside, especially with such warm weather the week before Thanksgiving, winter temperatures will be coming. Possibly sooner rather than later. Perennials that are meant to die to the ground each winter look ugly and decayed after the first frost. Faced with such unattractive plants, most gardeners are inclined to cut them to the ground right away. That action might be a mistake.
Many perennials, such as hibiscus species, hydrangea, salvia species, firebush (Hamelia patens), rudbeckia, echinacea, lantana and others still have a large amount of starch stored in their branches, even after the first frost. Although it is very tempting to remove these unattractive reminders of the beautiful spring gardening season, it is better to wait to allow the starch to translocate (move) down to the root system. The time that this takes varies by plant species, but can be as late as February.
To determine if there is “life” left in those burned and singed stems, just scratch off the top layer of the stem. If any nutrients remain in the stem, it will be green or yellow where the scratch was made. If it is brown and dead, it is safe to prune the perennial back.
If you can stand to wait, allow those unsightly stalks to remain until February. If they contain any plant nutrients this will give the plant some energy during the winter months. This is not possible in every situation, but following this practice will bring stronger flowering perennials next season!
by Eddie Powell | Jan 22, 2015
Florida homeowners enjoy a wide range of landscape and citrus plants and often times desire a tropical or semitropical appearance to their landscapes. Many landscape plants are often planted past their northern limit such as here in Northwest Florida, although microclimates differ dramatically. Tropical and subtropical plants can be used in the landscape, but they must be protected or replaced if necessary during cold weather. A good variety of tender and hardy plants should be planted in order to prevent total devastation of the landscape by extremely cold weather.
The site selection for tender plants should be number one on your list when preparing for a freeze. Tender ornamental plants need a higher site with good air drainage, and not in a low area where cold air settles. Arranging tender plants along fences or other barriers to protect them from cold winds improves the plants’ cold protection, especially from very hard freezes.
Poorly drained soils result in weak shallow roots which are more susceptible to cold injury. Plants grown with the correctly applied rate of nutrients will tolerate colder temperatures better and recover from cold injury faster than plants grown with little or no added nutrients. Be aware that late fall fertilizing of nutrient deficient plants or fertilization before unseasonably warm periods can result in a late growth which is more susceptible to cold injury.
Ornamental plants that are planted under large tree canopies can have more cold protection and require less cold prevention for the owner.
Watering landscape plants before a freeze can help protect plants. A well watered soil will absorb more solar radiation than dry soil and will reradiate heat during the night by as much as 2°F . However, saturated soil conditions can damage the root systems of most plants over a few days, so make sure the ground is well drained.
Avoid late summer or early fall pruning which can alter the plant hormonal balance resulting a flush of new growth. This new growth is more susceptible to cold injury. Healthy plants are more resistant to cold than plants weakened by disease, insect damage, or nematode damage. Routine inspection for pests and implementation of necessary control measures are essential.
Here are a few methods for plant protection. Plants in pots or containers must be moved indoors where heat can be available to them. Containers that have to be left outdoors should be protected by mulches around the container or root ball of plants in the landscape to reduce heat loss from container sidewalls.
Cold Protection for Veggies!
Coverings can help protect plants more from frost than from extreme cold. Covers that extend to the ground and are not in contact with plant foliage can lessen cold injury to the plant. Foliage in contact with the cover is often cold burned or injured because of heat transfer from the foliage to the colder cover. Some examples of excellent plant covers are cloth sheets, quilts or black plastic. If plastic covering is used, it is extremely important to remove the covering during the day to provide ventilation of trapped heat. A light bulb under a covering also is a simple method of providing heat to ornamental plants in the landscape during the night hours.
Cold Damage on Fruit Tree
Pictures By Eddie Powell
Immediately after the frigid weather has passed, plants need to be checked for water loss. The foliage could be losing water vapor or transpiring on a sunny day after a freeze while water in the soil or container medium is frozen. Apply water to thaw the soil and provide available water for the plant. Soils or media with high soluble salts should not be allowed to dry out because salts would be concentrated into a small volume of water and can burn plant roots. Root burn can cause the roots to dry out and not take up water to the plant.
Pruning should be delayed until freezing temperatures have risen or until new growth appears to ensure that live wood is not removed. Dead, unsightly leaves may be removed as soon as they turn brown after a freeze if a high level of maintenance is desired. But it is best to wait until the threat of freezing weather is gone. Cold injury may appear as a lack of spring bud break on a portion or all of the plant, or as an overall weak appearance. Cold injury is usually found in the upper portion of the plant. Branch tips may be damaged while older wood underneath is free of injury. Cold injured wood can be identified by examining the cambium layer under the bark for black or brown coloration. Prune these branches back to a bud 2-3 inches behind the point of discoloration.
For more information on freeze protection of plants see:
Cold Protection of Landscape Plants
