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Epic Gardener
Table of Contents
Issue 11
Summer 2005


Running Hot and Cold

Recent Developments
in Echinacea Breeding

The Monochromatic Colour Trend

No-Loss Planting

Epic Plants 2005,
Part 2

Hardy Cyclamen


Running Hot and Cold -
The Issue of Plant Hardiness

by Tony Avent
Plant Delights
Nursery, Inc.

Having returned from a couple of visits to Chicago several years ago, including a jaunt through the Morton Arboretum, I was struck with a burning desire to talk about winter temperatures, and in particular plant hardiness. It probably has something to do with the fact that during my entire trip, the temperatures ranged from –6°F (-21°C) for a low, to 9 above (-13°C) for a high. Back in the triangle area of North Carolina, our winter temperatures paled in comparison, with a normal winter low of 0-10°F (-18 to–12°C) ...but then, who has seen a "normal" winter in a while?

Most folks, who have ever read a gardening book or plant catalogue, are familiar with the plant hardiness zone maps. The first map, compiled in 1927 by Dr. Alfred Rehder of the Arnold Arboretum in Boston was based on a survey of plants and their survivability around different regions of the country. In 1960, the US Department of Agriculture got into the act, publishing its first map, based on the weather data from 450 weather stations around the country.

After a few more years passed, in 1967 to be exact, Dr. Donald Wyman of the Arnold Arboretum, revised the Arnold Arboretum map, and published it in the popular Wyman's Garden Encyclopaedia. The most recent chapter was completed in 1990, when the US Department of Agriculture, in cooperation with the US National Arboretum updated the USDA hardiness map, using data from 14,500 weather stations...are you confused yet?

With this mass of information in hand, gardeners traced the zone lines around the country to find out which plants they could possible grow. Let's see here...Raleigh NC is in the same zone as Reno Nevada and Dallas Texas...could this be right? You are beginning to see a few of the problems with the map.

Each of the USDA 11 climatic zones is based on the average minimum winter temperatures. In zone 7, which includes Raleigh, our average winter minimum temperatures should be from 0-10°F (-18 to–12°C). What the map doesn't tell us, is how many times the temperatures dropped that low, and how long these low temperatures lasted. There are a number of plants that can survive 0°F (-18°C) for a couple of hours, that could not survive these temperatures for a longer period, or more than once during a winter.

Another factor not taken into account is winter acclimation. Temperatures in the 20's (-7 to –2°C) can easily kill a plant growing in our gardens in mid summer. The same plant, if properly acclimated, can withstand temperatures of –20°F (-29°C).

Plants have developed several ways of shutting down their growth and preparing themselves for the winter season. Many plants shut down based on light. A good example is hostas. When the days begin to get shorter, a point is reached, usually in early October when the shorter day lengths trigger the plant to begin to go dormant. No matter how much heat is applied after this trigger has been pulled, the plants cannot be kept from going dormant. There are a number of plants that are triggered by day length, which can be a tremendous advantage, if fall temperatures are abnormally warm.

If we have an abnormally warm fall, many plants that rely on cool temperatures to trigger dormancy can be killed when the temperatures suddenly drop. Several years ago, we went from 70°F (21°C) to 4°F (-15°C), suffering losses on many "hardy" woody plants that simply had not prepared well for winter.

Another factor in hardiness is precipitation. I trade plants with friends in New Mexico, which are in the same hardiness zone. In New Mexico, the plants receive less than ten inches of rain per year, while we receive more than 6 times that amount. We have discovered that there are a number of plants that can survive our cold temperatures, but cannot tolerate winter moisture. Good examples are many of the barrel type cacti, which are naturally found in very cold mountainous regions, but regions that receive no winter rainfall.

The opposite effect is equally dramatic. Snow, which blankets many areas in "snow belts" helps to insulate many "non hardy" plants. Gardeners whose gardens are covered in snow most of the winter are often able to grow plants, especially perennials, nearly two zones outside their normal range, due to the insulating effect of the snow. On woody plants, a snow layer will often protect the roots and lower branches of a plant, while the top growth is killed back to the snow line.

Ice is an entirely different matter. Ice doesn't have the same insulating effect as snow since there are no buffering air spaces. A plant under an ice layer will actually "supercool" and become colder than the ambient outdoor temperature. Many growers use ice to protect crops during freezes, but this only works at a very narrow range of temperatures (not below 24°F/-4°C), and only if water is constantly applied (and at the proper rate). As the water freezes, it releases heat. As soon as the water application ceases, the protection disappears also.

There are a few factors that we as gardeners can control. Some plants tend to continue growing, despite cold temperatures and reduced day length. For example, eucalyptus, which are used to growing on the other side of the world, have to be slowed down by withholding fertilizer and water...in other words the growing conditions must be used to slow down the plant.

The Canadian Version
(text from Agriculture and Agri-Food Canada)

In 1967, Agriculture Canada scientists created a plant hardiness map using Canadian plant survival data and a wider range of climatic variables, including minimum winter temperatures, length of the frost-free period, summer rainfall, maximum temperatures, snow cover, January rainfall and maximum wind speed. It is therefore somewhat different than the USDA map. In 2000, Natural Resources Canada's Canadian Forest Service scientists have now updated the plant hardiness zones using the same variables and more recent climate data (1961-90). Natural Resources Canada, Canadian Forest Service is now "going beyond the zone" and trying to develop potential range maps for individual species of trees, shrubs and perennial flowers by collecting species-specific information. Check out more about this project here and see how you can get involved.

We have all heard about not pruning some shrubs in late summer and fall. This is because some plants respond to pruning with new growth, which is quite tender, and is easily killed since it has not become acclimated to the cold temperatures.

Did we mention anything about provenance? Provenance, in lay terms, means where did the parents come from? Just like parents and their kids, offspring bear some resemblance to the parents. Plants are similar, in that seed taken from a tree in Minnesota will be more cold-hardy than seed taken from the same type of tree in Florida. Conversely the plants from the Minnesota seed source might never break dormancy in Florida, due to the lack of cold.

Many reference books may say that Red Maples are hardy from zone 2-9. Granted, there may be red maples growing in both areas, but to interchange seed from each area would most likely prove disastrous. This problem is particularly dramatic in woody plants that are grown from seed. It is also usually the most important at the extremes of the zone for each plant. In the case that we mentioned, the gardeners in zone 2,3 and 8,9 would need to be the most cautious of the provenance in that particular case.

Cultivars or vegetatively propagated identical plants (clones) keep the same hardiness regardless of where they are produced commercially. In other words Hemerocallis 'Stella de Oro' has the same hardiness whether it is produced in Florida or Chicago. The hardiness of a plant is based on the origin of the original genes, not where we as humans move the plants.

One of the most frustrating problems for gardeners in the south is summer hardiness. Reference books and most plant catalogues have completely neglected the effects of heat on plants. Many plants from the north are not able to withstand our hot summers.

The American Horticulture Society has recently devised a "heat map", but while a good idea, it confuses the issue with two sets of numbers...one for cold and one for warmth. Heat hardiness is an issue of night temperatures and humidity as well as actual heat tolerance.

A good example of heat hardiness is Lady's Mantle (Alchemilla vulgaris), a perennial featured in English gardening books. Due to our summers, it is virtually impossible to grow this plant in the south. Another example is the beautiful Mountain Ash (Sorbus sp.) with the bright red berries in the fall. Try as we might, the mountain ash will not tolerate our summers.

But wait, gardeners in zone 7 gardens on the west coast can grow these plants successfully...what gives? Another wrench enters the picture when we talk about night temperatures as compared with day temperatures. In many cases, the culprit is not only the high day temperatures that cause plants problems, but also the high night temperatures. During the day, plants store up energy produced through photosynthesis. If the nights are cool, the energy goes into growth of the plant. If the nights are too warm, the plant burns up the energy. Many plants, due to their with metabolism which was derived in a cool night climate, are not hardy in certain areas simply due to the warm night temperatures.

The Epic Position on Hardiness Zones

We generally like to suggest that hardiness zones are best used as general guidelines only, and not as‘hard and fast’ rules. As Tony Avent explained very well, there are many variables affecting a plant’s ability to survive any given winter.

At the Epic Plant Company, we use the USDA zones on our information tools (i.e. tags, catalogues, etc.), as most maps, literature and trials are done using this data.

There is still another factor in heat hardiness that we have overlooked...one of heat dormancy. A Hosta, for example will not grow well in parts of Florida, parts of zone 9, and 10. The problem here is that temperatures do not drop low enough in the wintertime for the plant to go completely dormant. Many plants, both herbaceous and woody must have a specific dormant period in order to start growth again in the spring. A Hosta must be exposed to at least one month of temperatures below 40°F (4°C). If this temperature requirement is not met, the plant will begin to decline in the spring, or in the case of some trees, will never re-sprout in the spring until the dormancy requirement has been satisfied.

If I have completely frightened anyone from ever gardening again, I apologize. What I hope you will realize is that growing plants can be very complex. Don't be frustrated when a new plant dies, and certainly don't give up. There are many factors at work in determining whether a plant survives or dies, and you play only a very small part. After all, remember that gardeners are like doctors...they bury their mistakes.

Tony is a plantsman/plant explorer and the founder of : Plant Delights Nursery and Juniper Level Botanic Gardens in
Raleigh, North Carolina: 9241 Sauls Road
Raleigh, NC 27603 USA
Phone : (919) 772-4794
Website : http://www.plantdelights.com
His first book ‘So You Want to Start a Nursery’ was published in 2003.

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