Most Louisiana landscapes can fool us into believing we live in the tropics. It’s true that some winters are mild and tropical plants survive, but severe freezes do occur and can be devastating to those tropicals in the landscape. All it takes is one night in the low 20s or teens to severely damage or kill many of the bright and beautiful tropicals.
There’s no need stop planting tropicals in landscapes, but the best idea is to mix them with hardy plants so in the event of a severe freeze, the entire landscape is not wiped out. Tropicals will always be the most popular flowers and dramatic foliage for our intense summers. They just need to be protected when the temperatures drop.
What are Hardy vs Tender plants?
These two terms are used to describe a plant’s ability to tolerate the cold. If a plant will survive with no damage when the temperatures are 32 degrees and below, it’s a Hardy Plant. The level of hardiness varies by plant. Some plants can tolerate 15 degrees while a less hardy one may be damaged at 25 degrees. Commonly used Louisiana landscape plants (trees, shrubs, ground covers, vines & lawns) are generally hardy to 10-15 degrees, and won’t typically die in our winters. These hardy plants do not need much protection other than perhaps some mulch.
Tender plants are those that are severely damaged or killed when temperatures are below 32 degrees. When these plants are left out without protection, even on nights when there is a brief temperature drop, there is a risk for damage.
Many tender plants may survive by coming back from their lower trunk, roots, crown, or bulbs. Plant parts below the soil surface typically survive since our ground does not freeze.
To assist both hardy and tender plants in the event of an oncoming freeze, make sure good care is given during the summer. Pruning and fertilizing trees, shrubs and ground covers should be avoided after September because it can stimulate late growth.
How much damage occurs to a tender plant during a freeze can be a factor of where the plant is in the landscape. Place them in a more sheltered area that blocks the north wind and traps the sun’s heat. Planting in areas covered with overhang and tree canopies will also help.
What are the types of freezes?
Freezes can be characterized as radiational or advective. Radiational freezes occur on clear, calm nights. These freezes are considered generally light and mainly damage the foliage of tender plants, like tropicals. Covering plants during this type of freeze will significantly reduce damage.
In an advective freeze, a drastic temperature drop occurs and windy conditions are normal. The temperatures tend to be very low, and will last longer, thus making protecting tender plants more difficult. Freezes that last 8 hours or more are particularly damaging.
Frost- 30-32 degrees Plants will have little or no damage
Light Freeze – 28-30 degrees Plants will have light damage, mostly to foliage
Hard Freeze- mid 20s temps This will kill most tropical plants, and damage other
Severe Freeze- low 20s temps This will most likely bring major damage or kill all
Catastrophic Freeze-temps in teens This will most likely kill most tropical plants
What to do before a freeze?
Water- If the soil is dry, water landscape plants thoroughly to help them deal better with the cold. Cold weather is often side-by-side with strong, dry wind. That wind may dry a plant out, causing damage. Watering helps prevent that.
Cover- Smaller plants can be covered with cardboard boxes, and larger plants can be covered with fabric or another breathable material. The cover should extend to the ground and be sealed with stones or bricks. Any covering should be vented or removed on sunny days. During advective freezes, covering will need to be paired with a warm source, such as wrapping small Christmas lights around the base of plants. For plants that are too large to cover completely, wrap at least the trunk with an insulating material such as foam rubber or blankets. Even if the top of the plant dies, the plant may regrow from the surviving trunk. For palm trees, the trunk must be wrapped from the ground level to the base of the leaves.
Mulch- Use a dry, loose material like pine straw or leaves. Note that mulch only protects what it covers. For example, mulch at the base of a bird-of-paradise will help protect the roots, but not the foliage. Mulch can be left at the plant’s base all winter.
Move Inside- Move all tender plants in containers/hanging baskets into a warmer (above freezing) area, such as patio corner or garage. If plants remain outside, cover. If taken inside, provide as much light as possible.
What to do after a freeze?
Move container plants back outside unless they will be inside for the winter.
Remove any cover from plants to prevent excessive heat buildup if the next day is sunny. If a freeze occurs two or more nights in a row, the cover does not have to be completely removed, but air and light should be received by the plants.
Do not prune anything for a week or more after a freeze.
After a week, damaged growth or dead foliage on herbaceous plants may be pruned. If any tissue is oozy, mushy, slimy or stinks it should be removed.
Make sure to prune prior to the spring, so any new growth is able to appear.
Try scratching the bark of any plant with your fingernail. If the tissue underneath is green, it’s still alive. If it is brown, the branch is dead. Start at the top and work your way down to see how far the plant was killed. Remove all dead branches.