Every winter we get our our annual reminder that North Florida is not actually tropical. Usually a number of cold fronts interrupt our mostly warm warm weather with a dose of north-temperate zone reality, bringing us a series of chilly days, and nights dipping below freezing. The latest such cold wave was January 3rd through 7th, 2018.
Despite what some in in the news media were saying about “record cold” in Florida, this latest stretch of several days of frosty weather was a completely typical of what happens in winter here. In my area (between Ocala and Gainesville), the temp briefly dipped down to about 25F(-4C), which is right about our average extreme winter low. During really nasty winters, I’ve seen the temperature drop as low as 15F (-9c) here, so this week was not bad at all. Sub-freezing temperatures penetrated down into the middle of the peninsula, with the Orlando and Lake Wales areas reaching about 30 F ( -1C), and coastal Sarasota stayed just above the freezing point. While these temperatures are lower than those areas got during the previous couple of extremely mild winters, they are pretty much exactly in line with the long-term average extreme lows for the region.
But since even a mild episode of sub-freezing temperatures has a profound impact on what plants can grow here, I figured I’d give a roundup of what happened in the latest cold snap in my area of north-central Florida , and how methods I and some other growers employed worked or didn’t work.
First, the fruit trees that normally handle frost sailed through this freeze fine: citrus, figs, cold-hardy avocados, mulberries, olives, persimmons, grapes, plums, pears, blueberries, and loquats – all had zero damage that I can see. It helped that this freeze came in early January, right when all these trees are at maximum winter dormancy, which helps them handle cold weather.
During some years, we get weeks of warm weather in January and February, causing some fruit trees to wake from dormancy, pushing out tender new growth. If a hard freeze then hits in late February or early March, it can cause them catastrophic damage damage to these actively growing trees. But in this latest freeze, we had some cool weather leading up to the cold snap, and the cold happened in early January, so this was a textbook example of a “good freeze”.
Even though the tangerine fruits came through totally undamaged by the cold, unfortunately my trees have a very small crop of tangerines this year. At least part of that is due to the spring drought we had – citrus trees here flower in March, and the fruits steadily develop for the rest of spring, summer, and fall, ripening in winter. An intense spring drought can make the trees drop those developing fruits, and that definitely happened this past year.
Papaya plants really thrive in the heat and humidity of our Florida summers, and can produce tremendous amounts of food. But the plants are quite cold-sensitive, and can easily get killed by single freezing night, so it’s worth a bit of effort to get these tropical plants through our winter cold spells.
We successfully protected a planting of young, 3-6 foot (1-2m) papayas in this way: first we wrapped Christmas lights around the papaya plants as a heat source, one 40-watt string of lights per plant. Then we put in a ‘T’ of PVC next to each plant as a support, and covered the plants with a layer of frost cloth, then a layer of clear plastic. This method worked beautifully to protect the plants. If we can get these papayas through the rest of the winter, they should reward us with bountiful production of fruit next summer and fall.
I tried giving some other papaya plants lighter protection, just draping sheets and blankets over them. This method works to protect plants in milder freezes, but in this case the papayas protected this way received substantial cold damage, and may have frozen down to the ground. (Not a major setback, as I’ve got potted papayas ready to plant out in spring.)
Potted plants are easier to protect than plants that are growing in the ground, using a simple technique: just lay the potted plants flat on the ground, and cover them with sheets of cloth and plastic. The warm earth produces heat, and the sheets trap it around the plants.
During this latest freeze, I used this method successfully in my nursery to protect potted tropical plants. Prior to the freeze, I had grouped all the cold-sensitive plants together. When the forecast showed sub-freezing weather was about to arrive, I laid all these plants all on their sides, piling them on top of one another, and covered them with a big sheet of plastic, then an old sheet of frost cloth on top (frost cloth is available from nursery suppliers.) I used no heat source other than the ground itself. I left them covered like that for the few days of the freeze, then when things warmed up I pulled the covers off and stood the plants back up. Everything was fine.
This method is labor-intensive and messy, with lots of spilled soil, and it can be kind of visually unattractive. If you only have a few plants, this is quick and easy. If you have a whole nursery full of plants, as I do, it’s a time-consuming pain in the butt, both to set up, and to take down after the freeze. But it works really, really well.
Another method I’ve been experimenting with is to simply plant tropical plants under the canopy of evergreen trees, especially our ubiquitous evergreen oaks. It’s quite remarkable how much protection that these trees can give to smaller plants underneath them. I’ve been very impressed seeing the results other Florida growers are having with this method, getting tropical fruit trees to survive winter far north of where they are “supposed” to grow. There is, of course, a catch-22 to this method: plants need sun to make fruit, so trees shaded by a heavy canopy will make much less fruit than ones in full sun. But in a region where the same fruit species would freeze out in the open, this may be a way to get more cold-sensitive species to make a few fruits, at least occasionally.
I’ve been employing this method in a super-simple way: when I eat mango, jackfruit, and non-cold-hardy strains of avocado, I walk out into areas shaded by oaks, and push the seeds into the ground. So now there are lots of little fruit tree seedlings sprouting up under the oaks. I also planted a seedling chocolate pudding fruit (aka black sapote) in a shaded spot. Surprisingly, it’s now come through a full winter (last year) and some of this winter, so far without apparent damage. I don’t know if these tree-canopy-protected tropical trees will ever make fruit – if we get a series of mild winters, it’s possible. But I’m a fruit-tree geek, so for me just out of knowing I’ve got a few little mango and chocolate-pudding fruit trees growing in the ground gives me a warm glow.
The way I protect my most important tropical fruit trees is by growing them planted in the ground inside a greenhouse. That way, I can protect a bunch of plants all in one place (instead of running all over the property the day before a freeze, throwing covers over scattered tropicals – which is what I’ve done in the past). Also, in the greenhouse environment, trees can get big enough to actually produce significant amounts of fruit.
The greenhouse is a pretty simple affair: a standard metal hoop-house, covered with UV-stabilized polyethylene. I take the plastic off for the summer, both to protect the plastic from the summer sun, and to allow the trees to get the full benefit of Florida summer sun and wind and rain. When I have to use something fossil-fuel-based like plastic, I try to get the absolute maximum use out of it I can. In the case of this greenhouse, I’ve been using the same sheet of plastic over the roof since 2006.
The last few years, I’ve been experimenting with setting up smaller greenhouse within the larger greenhouse, for protecting fruit trees that are especially cold-sensitive, or trees which develop fruit over winter, which could be impaired by exposure to near-freezing temperatures. Some of the trees in the outer greenhouse are carambola, jackfruit, casimiroa (aka white sapote), sabara jaboticaba, red jaboticaba, Australian beach cherry, and miracle fruit. In the inner greenhouse are canistel, sapodilla, and pineapple.
To keep the greenhouse warm during freezes, I rely on a combination of extra insulation, thermal mass, and active heating. Commercial nursery frost cloth comes in big rolls – I’ve found I can cut it into a series of strips, which go over the top of the greenhouse, overlapping each other slightly. It’s easy to scrunch them together in the morning to let the sun in, and then pull them back out at night.
For thermal mass, I keep a number of barrels of water in the greenhouse, so they can pick up solar heat during the day, and release it all night. It’s kind of a varied assortment of containers, pretty much anything that will hold water, including a number of plastic trash cans that I’ve painted black to absorb solar heat.
For active heating, I used to use a wood stove in the greenhouse. That provided quite a bit of heat while the wood was burning, but I found that even using large diameter wood pieces, the wood burned down to nothing in just three or four hours, allowing the greenhouse temperature to plunge during the cold early morning hours. So keeping the greenhouse warm on a freezing night meant getting up several times during the night to stoke the fire. Not good. What I’ve switched to the last few years is a wood-fired hot tub in the greenhouse. That way, I can heat the water up the night of the freeze, have a nice soak in the hot water while the cold winds blow outside, and then go to bed, knowing the hot water is at work, releasing its heat to the greenhouse all night. I can tell you, it is absolutely luxurious to be soaking in steaming hot water, surrounded by tropical fruit trees, on a cold winter night.
For really extreme cold, I can just keep stoking the fire to get the water all the way to approaching the boiling point, and I can stoke the fire under the tub with big fat chunks of wood before bed. That way, even though I know the wood itself will burn out in a few hours, the heat from that wood will outlast the fire, steadily releasing heat from the water.
After this last freeze, it became apparent there was a small amount of cold-damage foliage burn to the tops of the jackfruit and carambola trees in the outer greenhouse. It’s nothing major, but it does show my heating system could use some improvement. There are a few ways I could do temperature management better than I did this time. During this freeze, I only had two barrels of water in the greenhouse as thermal mass, with a total of less than 100 gallons (400 liters) of water, while in the past I’ve used many more. Also, the frost cloth I used this year was a thin grade, compared to a thicker type I’ve used in the past. And I didn’t cover the end walls of the greenhouse with frost cloth, which I’ve done in the past. I suspect that with better, more complete insulation, much more thermal mass, along with my existing hot tub heating system, the greenhouse could come through an even colder freeze than we just received, with little or no damage to frost-sensitive fruit trees.
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