Frost Damage To Tropical Plants

Photo The photo on the right shows a 7 foot mango tree two weeks after a killing frost. It was given no protection from the cold, and is therefore displaying pretty brown and gold fall colors which is unusual adornment for a mango tree. The tree has a 2.5 inch diameter base and is 4 years old, so it is starting to become fairly large. This same tree can be seen in a healthier state in the mango article on this site. Overview While freezing weather and frost are discouraging for anyone growing subtropical and tropical plants, they can also provide a valuable learning experience. With a clear understanding of how tolerant plants are to frost, one can avoid wasting time protecting plants unecessarily and can also know when to really get serious about helping them make it through the night. Knowledge also aids a gardner in deciding which plants they will attempt to grow. Of course, with enough technology and effort any plant can be grown anywhere in the world but deciding whether it is worth the effort is a personal decision. Described below is an account of an unusually cold winter night in Phoenix Arizona, followed by a breakdown of how selected plants handled a hard freeze. Lastly, techniques for protecting plants from freezing weather are outlined.

A killing frost visits Phoenix
On the night of Saturday January 13th 2007, the South Western US was visited by an extremely cold arctic air mass. In California, a great deal of frost damage was done to the citrus crop. Here in Phoenix, temperatures were the lowest they had been in 18 years, and all but the largest and most protected ficus nitida trees, a very common landscape plant, were killed. Even oleanders around town were frozen back, which is very unusual. Unfortunately, the weather forecast was only predicting lows at Sky Harbor Airport in Phoenix of 32 F, so many people were taking by surprise when the temperatures in the urban areas dropped even lower.

Hard Freeze Damage Analysis For Tender Plants As Of May 14th, 2007 (4 months later)

Adenium A small adenium close to the house and covered with a cardboard box on the two coldest nights was killed all the way to the ground. Its frost damage initially appeared to be just on its extremeties, but within 8 weeks it became mushy all the way to its base.

Avocado, Zutano No damage at all. It is located on the west side of a cinderblock wall.

Banana, Enano Gigante All of the larger leaves and stocks were frozen back. Tiny banana shoots growing at the base were not damaged and continued growing. Amazingly, even the larger stocks which looked completely dead started putting new leaves out of their centers a month after the frost. The bananas are planted against the east side of the house.

Bougainvillea, Barbara Karst All of its leaves were immediately killed by the frost. At 8 weeks new sprouts appeared close to the base of the plant.

Carissa, Natal Plum Larger carissas were severally frozen back except for parts of the bush closest to the ground or wall. Parts of the plant that did not freeze kept their leaves. At 8 weeks, even carissas that lost all of their leaves started to sprout at the base.

Cherry Of The Rio Grande Suffered some superficial damage on the smallest trigs and newest growth but was generally unaffected.

Citrus The most sensitive citrus trees, such as lemons and limes had damaged leaves and fruit on the tops. It took a while for this damage to show, and the overall health of the plants seems unaffected. All of the other citrus varieties were unaffected.

Fig, Black Mission The fig tree was already dormant having lost its leaves and had no frost damage.

Ficus Benjamina This ficus was killed completely in all but the most protected locations. It had been planted widely in court yards and next to houses all over the valley, but even being right up next to the house did not save most of them. Ficus Benjamina proved to be even more frost sensitive than Ficus Nitida.

Ficus Nitida Small, newly planted ficus trees in the neighborhood lost all of their leaves and after 8 weeks had not sprouted and appear to have been killed. The very large ficus trees retained some foliage in their centers. Some medium sized ficus trees lost all of their leaves but started sprouting at around 6 weeks. Proximity to buildings and other heat sources was quite significant.

Guava, Tropical The guava trees had superficial leaf damage in a light frost several weeks earlier. All of the leaves on the guava trees were immediately killed by the hard frost. 8 weeks later, both my pink and white guavas started sprouting new leaves on the main branches near the trunk. The pink guava started sprouting several days before the white, and recovered more quickly in the weeks following.

Hibiscus A small canary hibiscus out in the yard was killed all the way to the ground but maintained one tiny green leaf at ground level. At 10 weeks after the hard freeze it started showing vigorous growth. A larger hibiscus next to the east side of the house was frozen back, but right at ground level the leaves survived and continued growing. At 8 weeks, the larger hibiscus showed new growth on the larger branches even in the sections that had previously appeared completely dead.

Jaboticaba This tree had already lost all of its leaves before the killing freeze. At 12 weeks its main stem was still green and alive.

Jacaranda All of the Jacarandas around town lost all of their leaves, but after 3 months they started pushing out vigorous new growth.

Lantana All lantanas were completely defoliated, but sprouted new growth after 4 weeks from the stems closest to the base.

Loquat No frost damage.

Lychee, Brewster Damage appeared superficial immediately after the hard frost, but 2 weeks later 50% of its leaves had dropped while the remaining leaves still looked healthy. After 6 weeks only 2 leaves remained. There appeared to be buds on the stems but it was difficult to tell for sure. At 8 weeks, new growth appeared on one of the main branches but was very slow. At 10 weeks vigorous new growth began.

Madagascar Palm This plant was next to the wall of the house and suffered no frost damage. It was already completely dormant and leafless before the freeze and sprouted new growth in May as it usually does.

Mango My mango trees were completely unaffected by a light frost, at approximately 32 F., several weeks before the hard freeze, but took the subfreezing temperatures more poorly.

The largest mango, pictured at the top of the page, had only a thin sheet tied around its lower trunk on the two coldest nights, and was completely defoliated. Interestingly, it took a while for it to show damage and the leaves looked fairly healthy for almost a week afterwards. About 2 weeks after the freeze, it had green buds on its main trunk but they later turned brown, which led me to believe it would not recover. Also, rotting on the frost damaged branches seems to have started progressing into healthier tissue, so damaged areas were pruned an inch below the rot. This trimming seems to have stopped the progression.

At 8 weeks, after 6 weeks of no activity, some green buds formed on the trunk several inches from the ground. At 10 weeks these buds started to grow vigorously into new branches.

Several very small grafted mango trees I had growing in the ground were killed outright, even though they were covered. This definitely shows that smaller trees need a source of heat in addition to coverage if temperatures drop below 29 F. A heat source would obviously help a larger mango to sustain less damage as well.

A mangoes ability to handle cold temperatures seems almost identical to that of ficus nitida, leading me to believe that very large mango trees in urban areas easily survive this weather. Now, imagine if all of those ficus trees around town were mangoes. It makes me wonder why anyone would want to plant a ficus.

Oleander Suffered frost damage on the edges, which took a long time to show up. Other than that they are in good shape.

Orchid Tree, Hong Kong Lost all of its leaves within days of the hard freeze. At 6 weeks it had both brittle and more flexible branches, but no new growth. At 8 weeks, strong new growth appeared on the largest branches.

Papaya The most frost sensitive plant I have grown. Solo papayas are more sensitive than Maridols. The unprotected Solos had leaf damage even with a light frost, at approximately 32 F., several weeks before the hard freeze, but the stems were undamaged, and the emerging leaves shadowed by the larger leaves were also undamaged. Light frost did not affect the Maridols. During the hard freeze, the solos froze solid somewhere between 32 F. and 28 F. while the Maridols were fine down to 28 F. at which point they froze solid. Keeping a papaya alive below the freezing mark definitely requires coverage and a heat source.

Passion Fruit, Frederick Lost all of its leaves immediately after the hard freeze, but its main stem close to the ground remained green. At six weeks some new growth appeared on the main stem near the base. Growth was slow at first but by 12 weeks became stronger.

Pineapple Guava No frost damage.

Plumeria The second most frost sensitive plant I have grown. Unprotected plumerias had minor leaf damage even with a light frost, at approximately 32 F., several weeks before the hard freeze, but the growing tips were undamaged. The night of the hard freeze, I covered a small plumeria tree with frost cloth. Later in the night, when the temperature was 29 F., I added a 100 watt light bulb underneath the cloth. The tree's growing tips ended up being frozen back, but the main trunk survived.

At 2 weeks, rotting on the frost damaged branches seems to have started progressing into healthier tissue, so damaged areas were pruned an inch below the rot. This trimming seems to have stopped that progression.

At 8 weeks, the tree was looking worse than at 6 weeks and sections of the trunk appeared to be in decline. It definitely would have been better to put the light bulb in earlier in the night.

At 10 weeks the stem started to bud vigorously even though the trunk is a patchwork of living and dead tissue and very cracked. This tree is definitely on the mend.

Royal Poinciana A very large Royal Poinciana in the neighborhood, with a diameter of about 6 inches at its base, lost of all its leaves. Amazingly, after 4 months it started sprouting from its main trunk 2 feet above the ground.

Star Fruit A small star fruit which I had in the ground for years was completely killed. It was always an extremely slow grower and never flowered. Star fruits do not seem to be a good match for our climate or soil, even though they can be kept alive through summer if they get a lot of shade.

Tropical Bird Of Paradise No damage to the leaves or stems. A flower that was currently open was desiccated. It is planted next to the north side of the house.

White Sapote The same white sapote tree that is pictured in the article on this site was left unprotected. It was completely defoliated by the killing frost, but started to push out new buds 4 weeks later. By 6 weeks it was covered with new growth, except on the smallest most exterior twigs.

Yellow Oleander Almost completely defoliated, but survived with only peripheral branch damage. At 4 months most of these plants in the neighborhood have new growth but still look beat up.

A review of the physics of cold damage to a plant

Plant Hardiness Freezing Water freezes at 32 degrees Fahrenheit (0 degrees Celsius). Plant sap contains water and when it freezes and crystalizes it expands breaking and killing the plant cells that contain it. However, the sap of a plant is not just water, but is also a mixture of sugars and nutrients and other components which lower its freezing temperature below 32 F. Just how low temperatures have to go to freeze the sap of a plant is dependant on the type of plant. Plants that are more cold resistant are refered to as being more hardy. Tropical plants, being from climates that are warm year round, are the least hardy, but few if any will be damaged right at 32 degrees, because all plants have some sugar in their sap.

Any article stating that a plant cannot handle temperatures below 50 F. or 40F. is highly likely to be incorrect. It is possible that certain plants will become sick and eventually die if temperatures are consistently that low, day and night for weeks, but that is different than saying a plant will be damaged if temperatures drop that low at night. It is important to make this distinction when researching what will grow where you live.

Plant Hardiness Frost A plant can still be damaged at 32 F. even if its saps freezing temperature is lower. The reason this can happen is that frost forming on the the leaves of a plant can pull moisture out of the cells on the leaf surface, which dehydrates and kills those cells. Frost dehydration damage is typically superficial and only affects parts of the plant most exposed to the open sky. A plants ability to resist this kind of damage depends on the anatomy of the surface of its leaves, and once again tropicals are the most sensitive.

Interestingly, frost can actually occur when thermometers read temperatures above freezing. Here is an article that explains this apparent paradox. Frost above freezing?

Heat Transfer Heat flows from objects at higher temperatures to objects at lower temperatures. On a cold night the warmer objects are the ground, buildings and bodies of water and the colder objects are the air and the night sky. Warmer objects can be refered to as heat sources and colder objects as heat sinks. Plants are not large enough to be a heat source or sink so they are caught between the two, and heat will simply flow through them. The closer a plant is to a heat source the warmer it will be. So, the goal of keeping a plant warm is to actually have it physically close to a heat source and to insulate it from the heat sink. Insulation slows down the transfer of heat through the plant and makes its temperature closer to that of the heat source.

Techniques for protecting tropical and subtropical plants from frost and freezing weather

Covering Plants Unlike people, plants do not have an internal heat source, so they must be covered in a way that insulates them together with a source of heat. This source is most commonly the ground. Simply wrapping a plant in a blanket will do little to keep it warm, because one would just be insulating the plant itself against everything around it. Yes, the insulation will slow the transfer of heat to the surrounding environment from the plant itself, but there isn't a whole lot of heat in a plant compared to other sources like the ground. Instead, the insulation should be put over the top of the plant and attached to the ground around it, creating a tent, with the ground as a floor. The less that air can move in and out of this tent the better.

With particularly tender plants, an additional heat source can be added to the tent. Incandescent light bulbs give off a significant amount of heat. For safety, use grounded outdoor cords. For a socket, the light bulb cages that come with a hook and extension cord, for hanging under the hood of a car while working on it, are a good choice. The old style outdoor Christmas light strings, that have hot incandescent bulbs, are also a good choice because they will distribute the heat over a larger area.

One will also have to decide what to use for insulation. A bed sheet is a good start but a more insulating fabric will provide more protection. However, a very heavy blanket can create problems because the weight of it can break limbs, and it can be hard to keep on top of the plant. Also, an ideal cover can be left on the plant during the day, in the case that temperatures are going to drop down at night for several days in a row, which is usually the case. Frost cloth is a special fabric made to address all of these problems. It is light yet insulating, but it still allows some sunlight to come through during the day. For smaller plants or large areas one might also consider building a structure over the plant on which to mount the cloth. Using 1.5 inch pvc pipe is an inexpensive and easy way to create a temporary scaffold.

Watering Plants Water is a substance with a high heat capacity, meaning it stores a relatively large amount of heat compared to other substances at a given temperature. For this reason, it is a good idea to water ones plants the day before an especially chilly night. Not only will the plant have a little bit more heat stored for the cold night, but more significantly the ground around it will be wet and store more heat as well. It is not a good idea to this night after night because if the ground is kept too wet it can cause a plants roots to rot.

Spraying Plants With Water All Night When a glass of water is being frozen it will stay at 32 degrees Fahrenheit until all the water in the glass is solidified. This is an interesting concept from Physics called a phase change. Since most plants actually freeze below 32 degrees, they can be saved from freeze damage by keeping their temperature at waters freezing point. Spraying water on a plant when temperatures are below freezing will keep the plants temperature at 32, because the warmer liquid water is constantly being supplied. Another way to think of it is that the process of freezing water requires the colder surrounding air to suck heat out of the water. So, constantly supplying liquid water means constantly supplying heat. The downsides of this method are that ice will build up and its weight can crush the plant, one could waste a lot of water, it can create a muddy mess, and all of that soaking might not be good for the plants roots.

Microclimate Planting frost sensitive plants near heart sources to begin with can go a long way in protecting them. Significant sources of heat are buildings, concrete walls, and bodies of water. Well developed neighborhoods will always be warmer at night than outside of town. The eave of a house reduces heat loss from the space below it to the sky .

Links to more frost damage information

Frost Damaged Plants      Frost Protection      Cold Tolerance