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How Hardy is Hardy?

Martin Gibbons, with everything you need to know about winter protection of palms, but were afraid to ask.
Martin Gibbons
Chamaerops No. 4, published online 23-11-2002

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"How hardy is 'hardy'?" is a question I am often asked by customers. "Is this palm hardy?" they enquire, hoping for a straight yes or no answer. When I reply by asking them, "How cold is the winter going to be?" they begin to see the edge of the problem and perhaps understand the inadvisability of an unqualified answer.

Sometimes I hear people claim that such-and-such a palm is hardy down to -3c or - lOc or whatever, and I smile a wry smile. It probably is, I think, if it's a mature tree and it's growing in Florida or New South Wales, and for half an hour one night during their normally mild winter the temperature briefly dips down to -3c or -lOc and then the following day it's back up to its normal 15c and sunny with it. But that's very different from -3c or -lOc in the English Midlands, in February, when it's been wet and foul for all of January and stays at that low temperature for 6 or 7 hours or 2 or 3 days.

And it's very different from -3c or - lOc on the exposed east coast when it's accompanied by a biting northeast wind. Or if the temperature suddenly and unexpectedly drops down to that level, during a previously mild December.

So, what are the factors that determine how dangerous a particular temperature can be?

They are as follows:

1. Is the plant in active growth, or has the temperature cooled down gradually? This makes a big difference.

2. Is the cold accompanied by wind? Wind chill can make an apparent difference of several degrees.

3. Is the plant and the soil in which it is growing wet or dry? Agaves can take -20c if bone dry, only -2c if wet.

4. How long is it cold for? An hour or three days? A brief drop in temperature will be tolerated by many plants that would certainly die if exposed for several hours.

5. How warm is it the day following the cold? If the plant has the chance to warm up during sunny days between cold nights, it can make all the difference in the world.

6. Is there snow cover? The temperature under snow is never less than zero degrees.

7. How mature is the palm? A mature tree may take -15c; its seedling may die at -3c.

8. Provenance: Did your Trachycarpus come from generations of Trachycarpus grown in the south of Spain, which had never experienced frost? Or was it from a seed from a tree grown in Scotland which itself had experienced many bitter winters?

9. Finally, luck plays a part too. Plant 6 identical palms in a row, and you might find that three survive the winter while the other three die of cold.

Having digested the above, it will be seen that there are no absolutes in the world of palm hardiness. However there is much that we can do to improve the palm's chances of survival, and the first concerns the choice of subject. There are palms that are incredibly hardy but that grow so slowly in the temperate summer that it hardly seems worth the effort. This group includes the Sabals, Brahea, and the Needle Palm, Rhapidophyllum hystrix, supposedly hardy down to -20c (there I go). Unfortunately they really need baking summer heat to grow well, so do best in pots in greenhouse or conservatory. If they are grown in the ground outside, then for preference choose a sunny, protected, south-facing, well drained corner between two brick walls, and pray for hot weather. But at least they won't need protecting during the winter.

The next group is of palms that are the reverse of the above, that is, they are not particularly hardy, but do grow well in our summers. It includes Washingtonia, Rhopalostylis sapida, Arecastrum (I just can't get used to its new name: Syagrus), Parajubaea, Ceroxylon, and Phoenix. They need wrapping up well (Rhopalostylis will die at -6c) during the cold months, but will reward the effort by often fast growth, even in a mild summer. Washingtonia can lose all its leaves, and make a whole new crown the following summer, even in England.

Finally, there are those palms with the best characteristics of both the above groups: Trachycarpus, Chamaerops, Butia and others are not only extremely hardy to cold, but in varying degrees, grow well or at least reasonably well during an average summer.

An amusing footnote to this is that there is at least one palm (Ceroxylon utile) and quite possibly several more that would probably find London TOO HOT! In its habitat in the Andes mountains of South America it apparently has a maximum temperature of 1 5c, a regular nightly temperature of just 2c, and an average temperature of 4c. Reykjavik here we come!

Now let's turn our attention to the actual business of protecting our palms during the winter. Those in the first and third groups above need no protection, or very little, but those in the second group will surely die without it. Except of course in the mildest of winters, or areas.

Protection can be simple or complicated, cheap or expensive, effective or useless. The most simple is just a sheet of plastic (clear) thrown over the plant and secured against the wind. Even this basic cover can make an incredible difference to the palm's chances of survival. It will keep the wind off for one thing, and will act as a mini greenhouse, trapping and concentrating any sunshine during the day. Advantages: cheap and easy. Disadvantages: moisture is trapped inside and can condense on the plant. This in turn can freeze and kill it, or cause fungus and rot. Remedy: partially uncover it every day and have Benlate or other fungicide at the ready. Remove the cover entirely when the temperature rises.

As simple as this is the method used for Cordylines: tie up the leaves into a bundle and secure with string (if you're really 'green' you can use an old leaf!). This will protect the growing point, the most vulnerable part of the plant. Again, untie in mild weather.

As an alternative, the palm can be wrapped using that brown fibrous blankety stuff that plumbers use for lagging pipes. It comes in rolls and is about 10cm wide and maybe 2cm thick. Wrap it around the plant, starting at the bottom, after netting the leaves together. Secure it when the whole plant is covered, and then put a plastic bag or sleeve over the whole thing to keep out the rain (leave it open at the bottom so the air can circulate). This is a good method of protection, much better than bubble polythene, which can cause the plant to sweat, whereas the brown lagging will absorb any moisture.

Advantages: not expensive, very effective. Disadvantages: allows no light to reach the plant so cannot be left on for too long.

Next: Mulch. Don't you just love that word? Plants that are apparently killed by cold may sprout again in the spring if their roots are undamaged. To protect them, put a good thick layer of MULCH on the ground around the plant. Insulate the soil, and the roots will stay undamaged. It can be of forest bark, dead leaves, lawn clippings, bubble plastic, or anything else that will provide an insulating layer. Snow, that most marvellous insulator, is just as good. Shovel lots of the lovely stuff around your plants if and when it falls.

Guests on "Desert Island Discs" are always asked the question: Could you build a shelter? I could, I've been building them for years, to house Washingtonia, Shaving Brush Palms (Rhopalostylis), Phoenix etc. Loosely bundle the palm's leaves together with open-mesh netting. Bang four lengths of 2" X 2" timber (somehow 5.08cm X 5.08cm doesn't have the same ring to it) into the ground around the plant. Staple thick (120 gauge) polythene to the inside of them, and again to the outside. The top should be sloped to shed the rain. Leave generous flaps of plastic at the bottom, and shovel soil over them. And mulch over the soil. Your palm will be as snug as a bug in a rug in its double glazed shelter.

Could you manage an opening lid, too? It's a good idea; see the note about condensation above. Good variations on this are to plant your palm, and thus build its shelter, against the wall of your house, where, even on the best-insulated homes, enough warmth will filter through the brick to keep frost away.

Advantages: very effective and not too expensive. Disadvantages: can become very hot on sunny days without an opening lid. This could be dangerous for the plant.

Now we come to the Rolls Royce of palm protection with which, theoretically, you could protect a coconut palm in Siberia. This involves a structure like the one above, but which has the benefit of a soil warming cable inside. Install the 4 posts as before, then clip a soil warming cable round the inside with cable clips. (Warning: electricity in the garden can be dangerous. If in doubt, consult a qualified electrician). Work up in a loose spiral, closer at the bottom, making sure the cable doesn't touch itself. Exit at the top, and for the GT version attach to a thermostat, which is fitted inside the shelter. Then fit the plastic as before. If the thermostat has a light, which comes on with the power, make sure you can see it from the house. It'll give you such a nice feeling to see it glowing through the snow. Set the thermostat at perhaps 5c.

Remember, you're not trying to keep the plant warm; you actually don't want to stimulate it into growth. And if you're alarmed by the idea of running live cables down to the end of your garden, there are other ways of introducing a little warmth into the structure. A vacuum flask filled with boiling water in the shelter will lose some heat slowly and constantly throughout the night. Or one of those big soft drink bottles filled with hot (not boiling) water will do the same thing. But wrap it well in some insulating material, or it will cool down too quickly.

If all this sounds like a lot of trouble to go to, well, it is. But for me it is worth it. In our average winters (when we used to get average winters that is) we only get a few days of very cold weather and even this last February, protection would only have been needed for 10 or 14 days.

Protect your plants for those two weeks and they will be there for you to enjoy for the other 50.

Readers Comments:

Im new to the palm scene but i do love them and have planted various palms in the garden. I have been thinking about protection for this winter. My occupation is a carpenter/joiner, and recentley observed roofers using a membrane to the roofing timbers which lets condensation out so the roof can breathe but wont let water in. May be worth trying out?? anyone used it before??
By deano (5th May 2004)

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