(Note: If you can't see the Index panel on the left, please click here to reload my site!)
(Note: If you can't see the Index panel on the
left, please click here to reload my site!)
|
For those of us who live in colder climates but want to bring a touch of the tropics to our gardens, the search for truly ‘cold hardy’ palms and other exotic plants is the horticultural equivalent of the quest for the Holy Grail. The trouble is, palms and exotics that are reliably cold hardy in one garden often wither and die in another garden with an apparently similar climate... |
...Here I’ll try to provide a few clues as to why some gardens are more palm-unfriendly than others. With luck it may help your palms and exotics to get through the long, cold days of winter.
The most widely used system of categorising climate is the United States Department of Agriculture (USDA) Plant Hardiness Zone System. This defines eleven Climatic Zones based on the average minimum Winter Temperatures, as follows:
| Zone | Centigrade Minimum | Fahrenheit Minimum |
| 1 | Below –45 | -50 |
| 2 | -45 to -39 | -50 to -40 |
| 3 | -39 to -35 | -35 to -30 |
| 4 | -35 to -29 | -30 to -20 |
| 5 | -29 to -23 | -20 to -10 |
| 6 | -23 to -18 | -10 to 0 |
| 7 | -18 to -12 | 0 to 10 |
| 8 | -12 to -6 | 10 to 20 |
| 9 | -6 to -1 | 20 to 30 |
| 10 | -1 to 4 | 30 to 40 |
| 11 | above 4 | above 40 |
These Zones, however, should be taken with a large pinch of salt. After all, according to this system, my garden in North Devon (South-West England) shares the same Zone (9) as much of coastal California and northern Florida, it is slightly warmer than Madrid (8), considerably warmer than Dallas (7 to 8) and almost tropical by comparison to Santa Fe in New Mexico (6). Let me tell you, from personal experience, North Devon is not quite the sub-tropical paradise its Zone might suggest!
The Zoning System does not tell the whole story. Over the months of winter, the temperature in North Devon varies over a fairly small range above and below freezing. In desert regions of Nevada or New Mexico, on the other hand, the daytime temperatures may be very high while night-time temperatures fall dramatically. When you take the average of the temperatures, North Devon surprisingly gets the higher zone classification. And yet, it is the 'colder' zones of Nevada and New Mexico which are getting all the heat and sun while here in North Devon we are getting months on end of unremitting cold and rain.
Rain… now, that's another factor. North Devon in winter is very wet. Many palms and exotic plants can survive desert-dry freezes easily whereas they will keel over in dank, wet European winters..
In short, don't be misled into thinking that you can just match the Zone number of your region with the Zone number of a plant and expect it to flourish. Life isn't that simple!
Incidentally, just to confuse matters further, you may also see plants categorised according to other, incompatible, zoning systems. Some people divide the USDA Zones into sub-categories (Zone 7a, 7b and so on). Then there is an older Zoning system called the Sunset Zone which is apparently still used by some nurseries in the USA.
In Europe, a completely different Zone system is sometimes quoted in horticultural books. The European Zones are shown below. If you see a Zone quoted in a European book, be sure to check which system it is using!
| Zone | Winter Minimum C | Winter Minimum F |
| 1 | about 0 | about 32 |
| 2 | -2 | 28 |
| 3 | -5 | 23 |
| 4 | -8 | 17 |
| 5 | -11 | 12 |
| 6 | -14 | 7 |
| 7 | -17 | 1 |
| 8 | -20 or less | -4 |
So what can you do, to improve your chances of growing sub-tropical and 'borderline' plants in your area? It turns out that there are several ways in which you can help exotic plants survive and thrive in cold climates.
First, check the requirements of the plants and try to match them with your climate. If the plant comes from a dry, desert region, it is unlikely to thrive in an area with high rainfall. If it comes from sub-tropical wetlands, it won’t like cold, drying winds.
Most palms and many other exotic plants need very good drainage. Prepare the planting hole thoroughly. Dig in lots of well-rotted manure, compost or peat, plus lots of coarse gravel or grit. In very heavy soil, it is also a good idea to plant them on raised ground or a slope if one is available.
It is generally best to plant palms in a fairly sunny position facing toward the sun - i.e. facing South (in the Northern Hemisphere, facing North in the Southern Hemisphere.
Mulch, mulch and mulch again. In other words, put a good, thick 'top-dressing' on the soil around your plants. Shredded bark or coarse grit make good mulches. A mulch has many benefits. Any mulch will help suppress weeds. A coarse gravel (I use 1 inch chunks, delivered from a quarry) also has the benefit of keeping the base of the plant well drained. It's a good idea to have the base (the point at which the trunk joins the roots) of exotic plants just on the surface of the soil, then heap the mulch around it. This can help prevent rotting at the base in cold, wet winters. A two inch-layer of mulch is a reasonable minimum.
Often people get so obsessed about protecting plants from the cold that they forget that wind can be just as damaging. A cold wind can dehydrate a plant, producing an effect called 'wind burn'. Some people protect plants by placing a layer of horticultural fleece over them or by tying up the leaf crown of palms and Cordylines thereby making them look like old-fashioned brooms. Both these solutions may be effective but they are also ugly. I prefer to place tender plants in the shelter of hardier plants. Bamboos are my favourite natural wind-breaks. There are plenty of tall, fast-growing, very hardy bamboos which look great in an exotic garden. I suggest you take advice on the appropriate species from a specialist bamboo nursery, as some (but not all) varieties can be quite invasive.
In winter, protect hardy bananas by covering them with second-hand chimney pots (the ones about 3 to 4 foot long) with a cover over the top.
Protect cactuses and agaves from the rain by placing them under an open-sided frame covered with transparent plastic.
If growing from seed, harden off the young plants by growing them in an unheated room or greenhouse for one or two winters prior to planting out.
If you want to find out more about Climate Zones, you might find these links useful:
The Virtual Garden's North American Zone Locator
http://www.pathfinder.com/vg/timelife/zonefinder/locatezone.html
The Swedish Fuschia Society's USA and European Zones
http://www.dicom.se/fuchsias/usdazones.html
I Can Garden's Canadian Zone Map
http://www.icangarden.com/zone.htm
Plant Hardiness Zones for North America
Zones of New Mexico