The winter cold has not really arrived yet this year! As a northern transplant, I love the cold weather because it always puts me in the holiday spirit. It makes my work a lot easier as well and this is always a welcome break for people in the landscaping business after a long, hot grueling summer of backbreaking work. Unfortunately, we will have to keep mowing and trimming a bit longer while we wait for the lower temperatures to arrive in.
Our landscapes have gone into a slight dormancy as the recent lower temperatures have been low enough to cause a slight slowing of growth for tropical and subtropical plants. Tropical plants and annuals do not normally adapt to withstand temperatures below freezing and can be injured when temperatures reach below 50 degrees F. Fortunately, freezing temperatures are rare for South Florida and even more rare on coastal islands like Marco.
However, subtropical plants can, under the right conditions, acclimate to withstand freezing temperatures. The ability of plants to withstand freezing temperatures is affected by temperature fluctuations and day lengths before a freeze. A gradual decrease in temperatures helps plants to harden and withstand cold temperatures. These recent periods of lower temperatures can help to hardened our plants. However these were followed by periods of warmer temperatures which can de-acclimate plants resulting in budding or flowering making these plants more prone to freeze injury.
Freezes are either radiation or advective. Advective freezes occur when cold air systems move into Florida from the north causing a sudden drop in temperature along with windy conditions. Although radiant heat loss occurs during this type of freeze the conditions are very different from a radiation freeze and plant protection is much more difficult.
Radiation freezes or frosts happen on calm, clear nights when heat radiates from the surfaces of plants into the air. These surfaces can become colder than the air above them because of this rapid heat loss. When there is moisture in the air, a radiant freeze results in deposits of ice or frost on surfaces. Dry, radiation freezes leave no ice or frost but can still cause damage. To minimize damage from a radiation freeze you must reduce radiant heat loss from plant and soil surfaces. Covering annuals and other sensitive plants traps this heat to help warm them.
Cold injury can occur to the entire plant or just some parts of the plant such as fruit, flowers, buds, leaves, trunks, stems or roots. The fruit and roots are most vulnerable to cold damage. Especially roots of container plants. This damage won’t usually become evident until the plant is stressed by higher temperatures in the spring.
Another type of cold injury is plant desiccation or drying out. This causes leaf-tips to burn in mild cases and totally brown leaves in severe cases. Desiccation occurs when dry winds and solar radiation cause the loss of more water from the leaves than can be absorbed and transported by a cold or frozen root system. Leaves on a lot of shrubs and trees will begin turning yellow and dropping in the next few weeks. Hibiscus, gardenias, gumbo limbo trees, and most tropical and subtropical plants in our landscapes.
Steps, from selection of proper planting sites to good cultural practices, can help acclimate plants to cold temperatures and protect them from temperature extremes.
Site selection. Temperatures can fluctuate from one location to another in most yards. Thus, microclimates should be considered when choosing the planting site for cold sensitive plants. Tender plants should be planted in areas with good air flow never in low areas where cold air settles. Arranging plants, fences and other barriers to help protect plants from the cold winds of advective freezes can help. Avoid poorly drained soils which will result in shallow, weak root systems making plants more susceptible to cold damage.
Plant nutrition. Properly fertilized plants will tolerate cold temperatures better and recover from injury faster than weak, underfed plants.
Shading. Tree canopy covers can reduce cold injury caused by radiation freezes. Canopies can raise night temperatures under them by reducing heat loss into the atmosphere. Early morning sun after a freeze can cause bark splitting on plants and the shade can help prevent this problem. However, be sure shrubs planted in the shade are shade tolerant or you will end up with sickly plants which will also be more vulnerable to cold injury.
Watering your landscape before a freeze can help because a wet soil will absorb more solar radiation than dry soil and will re-radiate this heat during the night. However, it is not good to saturate soil for prolonged periods as this could cause root damage.
Avoid late fall pruning which can result in budding and a new flush of growth. This tender, new growth is more susceptible to cold injury.
Keep your plants healthy. Plants weakened by insects or diseases are not as resistant to the cold. Inspect your plants regularly and treat them as necessary to avoid serious problems.
Potted plants that can be moved indoors for a freeze should be. Larger containers that must be left out should be pushed together and mulched to reduce heat loss from the tops and sides of the containers. This will help protect the roots. The soil should be moist not dry. Be careful not to leave canopies of plants together for extended periods as this could cause leaf and stem damage.
Radiant heat from the soil will protect low growing plants on cold, calm nights. Taller plants do not enjoy this benefit as much because of their distance from the soil. Mulches help to reduce heat loss from the soil helping to protect root systems.
Covering your plants should be done carefully. Covers that extend to the ground and are not in contact with the foliage can trap radiant heat from the soil and plant. Foliage in contact with the cover can be injured because the heat transfers from the foliage to the colder cover and is lost. You can purchase special “frost blankets” from the local garden center or use old sheets or blankets. Plastic is not recommended but if it is all you have, be sure to remove it promptly the next day to avoid burning your plants with trapped solar radiation.
Sprinkling plants with water helps keep leaf surface temperatures near 32 degrees F by utilizing latent heat released when water changes from a liquid to a solid state. This is a very technical form of cold protection and should also be done properly to avoid damaging your plants. Sprinkling must begin as freezing temperatures are reached and continue until thawing is completed. Water must be evenly distributed in ample quantity to maintain a film of liquid water on the foliage surfaces. This method would not be advisable for an extended period of freezing weather as the soil would become water soaked causing root damage and plant breakage due to ice buildup.
After a freeze the plant foliage could be transpiring on a sunny day while the water in the soil is frozen resulting in desiccation. Apply water to thaw the soil and provide available water for the plant.
Avoid severe pruning of damaged plants until new growth appears to avoid removing live wood. Cold injury can show up as a lack of budding in the spring or an overall weak appearance of the plant. Tips of branches may be injured while the older wood of the same branch is still healthy. You can identify cold damage by inspecting the cambium layer under the bark for a black or brown color. Prune these branches behind the point of discoloration.
Keep your plants healthy, know the cold tolerance of the plants you choose for your landscape and place them accordingly because even Marco Island temperatures can occasionally go below freezing. This is where native plants out perform all others as they are acclimated to our climate changes.
Annual flower beds may show some moderate to severe damage farther inland. If the damage is severe, the entire plant will become very wilted as though in need of water. It would be best to remove flowers this badly damaged and replant the beds. Moderate damage will show some wilted tips here and there. Try to trim the flowers back to healthy stems and fertilize to encourage new growth. Applying a fungicide will help keep disease at bay.
Marco Island is usually spared the severe damage of the inland communities due to our close proximity to the water. Just remember to be prepared as it does happen and when it does we lose a lot of our more tropical plants. Remember last year!
Eileen Ward and her husband Peter have owned and operated Greensward of Marco, Inc., a lawn maintenance and landscaping company since 1981. Watch Eileen’s gardening videos on MarcoIsland-TV.com. Eileen is an FNGLA Certified Horticultural Professional, has a Commercial Pesticide license in Natural Areas Management and is a registered Dealer in Agricultural products in Florida.