Last week I posted about winter protection for your container/indoor plants.  Now I am posting thoughts and suggestions regarding protecting your outdoor plants from the cold.

Here in Central Florida, we average usually less than 200 hours (less than 9 days) of temperatures below freezing – and when we do get cold temperatures, they usually occur during full moon and a High Pressure zone exists over our area.   Nights are also clear with little or no wind.   These cold spells, which can start anytime from late November (Thanksgiving) thru mid March, last usually only 4-5 days.  This accounts for us being able to grow so many tropical and subtropical plants with freezing temperatures lasting usually no more than 3-4 hours on any given night.

We experience 2 types of freezes here:

1. Radiational – This occurs on calm clear nights when heat radiates from the surfaces of objects into the environment.  These surfaces can become colder than the air around them resulting in freeze damage.  Plant damage can be minimized by reducing the heat loss.
2. Advective – This occurs when cold air masses (and wind) from the north come upon us causing a sudden drop in temperature resulting in freeze damage.  Plant protection during this type of freeze is more difficult and more severe on our plants.

During our past winter, temperatures dropped into the low 20’s in some areas, and even into the upper teens in low spots.  When this occurred, 2 things happened to plants.

1. They were damaged and/or killed by cold temperatures which caused fluids within plant cells to freeze, which later ruptured cell walls, and resulted in the plant’s inability to transport liquids.
2. Frost burnt plant parts.
3. Because we didn’t have a gradual cooling off last fall, many plants weren’t able to harden off and were injured.
4. Cold weather slows down plant growth, it reduces the activity of roots, and often weakens a plant to the point where disease(s) can become active and kill the plant.

Some plants fare better than others because all plants contain a chemical much like antifreeze, (cyroprotectants).  Those plants that can take the cold have higher levels of this chemical.  The University of Massachusetts has been doing research on this topic and they have isolated this antifreeze chemical.  Hopefully somewhere in the near future this material may become commercially available and we can “spray” our plants with this antifreeze like chemical in the fall and reduce/eliminate cold damage.

Other research has also shown that there are also bacteria within plants that may induce/delay ice formation.

Cold injury can occur to an entire plant or just to plant parts, and temperatures do not have to be freezing.  Some of the symptoms of cold injury are:

Obvious death of plant – especially herbaceous types.

Blackening (or browning) of foliage or spotting/streaking.  Later leaves turn brown and die.

Limb/twig die back (brittle).

Splitting bark and/or oozing of sap (this may not show up until much later), especially in azaleas and citrus.

Flower bud drop.

You might even see damage to your lawn.  Our southern turf varieties can’t take cold temperatures (in the teens) for very long and can “freeze out”.

Check color of cambium wood beneath bark.  If white or green, plant is alive.  If brown or black, that portion has died.  Suggest using finger nail or small pocket knife to check this out.  Start at tip of branch and work backwards.

NOTE:  These symptoms may not show up till next spring.

The following are some suggestions to prevent or reduce cold damage:

• Maintain good culture, especially proper watering, pest control, pruning, fertilizing, etc. at the proper time(s).
• Avoid heavy pruning after September 15th as this may induce new growth, which can later be cold damaged.
• Don’t fertilize after September 15th, and avoid using high N analysis fertilizer which is used to generate plant growth.  Instead use one with higher K.
• Know your plants, especially their particular needs and hardiness.
• Avoid planting cold sensitive plants (instead use hardy varieties) such as croton, bottle brush, copper plant, Mexican heather, poinsettias, bouganvilla, hibiscus, citrus (especially lemon), camphor trees, ixora, oleander, mango, avacado, papaya, jasmine, bananas, orchid and lime tree, mandevilla vine, any herbaceous plants, powder puff, alamanda, and queen palm.  If you must grow these, grow in containers, which can easily be transported and moved into garage when cold weather is forecast.  (Don’t leave outside as they will usually be damaged before similar plants in the ground.)
• Use native plants – though somewhat hard to find in nurseries, and abit more costly, they are extremely hardy and require little or no cold protection.
• Keep a close eye on the weather, especially for south Alabama.  Whatever weather they receive, we can usually expect similar weather within the next 24-48 hours – which gives you adequate time to make preparations for the pending cold.
• Thoroughly water your plants (helps keep in heat in soil and reduces desiccation of plant drying out) before cold arrives, and if weather has been dry.  Remember to turn off irrigation system.
• Assemble your materials for covering your plants.  Also remember that these should be put on early in the day if possible, to help trap heat, not the night of the freeze.  Also make sure that whatever covering you use on your plants, is going to be heavy enough to really do some good.  No sheets as they are too thin.  They won’t warm you, so how do you expect them to protect your plants?  Use blankets, heavy beach towels, shipping boxes (painted black if possible), or frost cloth (supposed to get up to 8⁰ F extra protection).  These help trap in heat around plant, so don’t just lay it on top of plants.  Show Sample   Also consider wrapping plants in old carpet and covering with black plastic  bag.  Show Sample  These can remain on your plants without injury until the cold spell passes.
• Add extra soil/mulch around base of plants; however, to avoid possible later damage, remove excess after freeze has passed.
• Use of heat lamps.  They use a lot of electricity (costly) and is a potential safety hazard and can also damage the plant if placed too close to your plants.
• Covering with plastic film (if plastic comes in contact with plant, it will burn the parts it comes in contact with).
• Covering your plants with water (this works with some plants, however, it uses a lot of water, which when it turns to ice, can often break branches, etc.   To be effective, you must begin applying it before the temperature drops below freezing and continue until the temperature goes back up above freezing.  Water must cover plants a minimum of 3x/minute.  If wind increases, you may also need to increase amount of water.
• White wash tree trunk.  (Keeps bark it from splitting.)
• Avoid planting cold sensitive plants in low areas where cold air settles.
• Spray plants with an antitransparent (example: VaporGard or WiltPruf) to reduce water loss.
• Keep night lights off if possible during fall and winter months.  Why?  Often they will delay a plant going dormant and makes it more vulnerable to freezing.
• If you have no other choice and a freeze is approaching, wrap lower portion of trunk to save plant especially if grafted.  It may be damaged, but if graft is protected, plant should come back.

If freeze is expected, turn off irrigation system.

NOTE:  With regards to the above recommendations, ask yourself is that plant worth all the fuss and costs you’re giving it?

If your plants are damaged after a hard freeze:

Don’t panic.  Examine your plants to check the extent of damage.  You may have lost some of your plants, however, the prospects are brighter than you may think.

Don’t prune.  Why?  If done too soon, could force out new growth which could be later killed or seriously injured by future cold periods.

Wait and see what happens.  Plant recovery is often very slow and consider what mother nature does.  She leaves bad foliage on, even though it is dead, it does provide some protection and delays new growth.  However, if a cold damaged plant retains its foliage over a long period of time, usually that plant has suffered some serious damage and its future is doubtful.

Maintain good cultural practices, especially watering during winter months.

• You may be able to write off your plant losses under casualty and theft loss.  Contact IRS or your tax accountant for particulars.