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Gardening Column
by Marjorie Harris
January 24, 1999

After an incredibly warm autumn, we're now having record snow falls in these parts. When I see snow I put away all the worries about how plants will survive. After all, a good blanket of snow is the very best mulch there is. Everything underneath is kept in a consistent state: no freeze-thaws to ravage the plants. But what are all these confused weather patterns doing to our plants?

I called Diana Beresford-Kroeger, a botanist who lives in Merrickville, just outside Ottawa, in Carrigliath Gardens. It was devastated by last year's ice storm. All of her trees (and this is one of her specialities) were badly damaged with the exception of a few native Carolinian species such as Catalpa speciosa and Liriodendron. She points out that the full extent of the storm's damage won't be known for at least another ten years. Millions of acres of trees were affected and the canopy from Ottawa to Montreal was almost removed. This reduces their ability to survive and the forest has to start all over again. This, in turn, affects global warming.

Just as our toes are about to freeze solid we don't wants to hear about global warming. But as gardeners, we must pay attention. These are the points that Ms. Beresford-Kroeger makes:

1. With global warming there is an increase in soil temperature which changes the balance between bacteria and fungi. This will give rise to an increase in plant diseases of all kinds and extends to the mites in bird's nests. If mites increase with higher temps and humidity, it affects the young of the next generation. In some instances fledglings die or their health is seriously impaired. Birds and bugs (good and bad) emerge at the same time. We need those birds to pick off the pests.

2. Swings from very cold to very warm weather means an increase in fungal diseases such as downy mildew. This steps-up the wasp and other insect populations like sawflies and thrips who love heat. In the winter when weather can go from zero one day to -30C the next, we're looking at sun burn. A rapid rise in temperature can also shatter bark in a straight line. An increase of ice in the bark will create a long open wound. Trees and shrubs have difficulty in living with this.

3. There is also a stepping-up of sun burn across the North America _ it fries the non-cuticular green coloured soft plants such as tomatoes and petunia which have no waxy cuticles to protect themselves from the sun's rays. Those most affected are in the Solanum family. This includes potatoes and tomatoes which are becoming more difficult to grow in southern regions because they get burnt up in summer. Plants such as petunias and calla lilies are also suffering, though most of the herbs aren't.

4. All over the continent, monocotyledons (grains and grasses and sedges) and some trees grown in the garden are being affected by increased UltravioletA and UvB irradiation. When fertilization takes place, the pollen goes on the female part of plant which is in a vulnerable position. The pollen tube is injured when it is exposed to ultraviolet light. Fertilization is affected and all seed production is lowered.

5. Anyone who uses pesticides should be very careful. Increased heat especially around houses make all pesticides are more toxic to people. They are more volatile and can get into lungs, and affect the bodies of small children and animals more quickly. Use them only with extreme care. Signs announcing pesticide use should go up well in advance and stay in place for at least five days especially in parks and public areas.

6. Using organic and inorganic fertilizers should be cut back as the heat goes up. The higher the temperature the greater the stress for plants. This summer, use at least twenty per cent less fertilizer and that includes those of us who are organic gardeners and use only compost. Step up the amount of watering.

7. Another indirect function of global warming is the lack of ozone which is leading to increased UvA and UvB particularly in Canada. It disables the plant's immune system so they can't fight off diseases as they normally would. We see it on the onions, carrots and potatoes. This is the first year that Phythoptera infestans (potato blight) has raised its ugly head. This is what created the Great Irish famine of 1848.

8. Crazy weather patterns are leading to more and more lightening strikes thus additional nitrogen in the air. This means the potential for germination increases with an increased nitrogen load in summer rains. And so we get more weeds.

9. Winter: last autumn trees did not all go dormant. If you were to prune right now, you'd see that they are still bleeding. The abscisson layers (or bud scars) are also bleeding.

10. It's not all bad news. Many North American native species are increasing: native orchids, agrostemma, club mosses and ferns are on the rise. There are more butterflies who thrive with the heat and drink the sap spilled from broken trees; many animals like the fisher and the wild turkey are also doing better.

Diana Beresford-Kruger's book The North Temperate Garden: Bioplanning for the next millenium, Quarry Press, $35 will be published this spring.

Marjorie Harris is Editor-in-Chief of Gardening Life; her most recent book is Pocket Gardening (HarperCollins). Visit her web site:

199 Albany Avenue, Toronto, Ont. M5R 3C7 fax: 416-531-3774 phone: 531-3774

Marjorie Harris is working on a social history of native plants of Canada and the United States. this is the end

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