False spring can do more harm than good

Most gardeners are in agreement, that they can’t remember when they experienced a late winter heat wave, like the one we recently came through.

Although we enjoyed the end-of-May temperatures in mid-March, plants will undoubtedly be set back when normal weather returns.

The horticultural term “false spring” refers to a period of weather in late winter or early spring, when temperatures are significantly above normal and extend for a period of time. A similar time of the year called Indian summer has a reprieve of warm summer days past.

During a false spring, vegetation is “tricked or fooled” out of dormancy. Warm temperatures are the trigger for plants to break the dormant cycle and begin growing.

When plants wake up early, new succulent tissues begin to grow and can be easily damaged by frost. Normally, this is a time when the plant should remain dormant.

False spring temperatures rarely affect native plants, but ornamentals are quite vulnerable. Most areas of the country went through a sudden change of temperature and in some regions, humidity.

During the heat wave temperatures broke record highs, climbing to 29.8C in the late afternoon, with a humidex of 35C (95F). The last time temperatures got that high (29.4C) was in 1965.

Unfortunately, the wrath of Mother Nature can be severe. The dilemma for gardeners will be to solve the harsh consequences. Cooler temperatures would keep any new growth on hold, whereas freezing will be detrimental.

The unseasonably warm weather provided homeowners with a head start on the spring cleanup. Some gardeners raked up the winter debris, while others pruned shrubs and trees. Still others planted cold sustaining varieties. Some traditional salad greens, snow peas and other cole crops would be able to withstand cold temperatures.

The biggest danger is the prematurely opening of tender young buds on trees and shrubs.  

Most fruit trees (apples, pears, cherries, plums and peaches) have a tenuous relationship with cold weather. Those trees require freezing temperatures to start the winter dormancy period.  Warm spring temperatures signal the tree to begin the spring blooming process.

If fruit trees begin the process of spring budding and the weather reverts to freezing temperatures, those buds could be damaged.

According to the Ontario Ministry of Agriculture, Food and Rural Affairs, tender crops, pears (blossoms and fruit), plums (blossom), cherries (blossom and fruit) are damaged at minus l to minus 20C.

Half-hardy crops like apples (fruit, buds) are harmed at minus 2 to minus 40C.

Growers and fruit farmers could see a year’s growth destroyed if killing frosts show up when trees are in full flower.

Environment Canada’s climatologist David Phillips, said, “It’s truly extraordinary in so many ways. The mild winter is responsible for the current conditions. Usually, air coming from the United States would be cooled down somewhat by Canada’s snow cover. This year, however, the warm ridges from Kansas and Tennessee stayed warm as they moved north.”

Record hot temperatures have certainly brought an early end to the maple syrup season this year. The trees require cold nights and warm days to keep the sap flowing.

Many fall bulbs and perennials are well under way, weeks ahead of schedule. Daffodils are in full bloom which makes them three to four weeks early.

Hellebores, scilla, and draba sibirica have been seen in neighbourhood gardens.

Ontario asparagus is a cool spring crop that usually comes to market in late April, early May. Unfortunately, it has begun growing due to the extraordinarily warm weather. That will eventually ruin the crop if the inevitable frost returns.

Although temperatures recently toppled 30°C during the day, lows could drop below freezing. The concern is that the earlier that asparagus comes up the more likely it is going to be at risk.

Unlike ourselves, plants have to go through a long process to get ready for warm temperatures. Many of us enjoy a tropical vacation during the winter, but have the ability to readjust to the cold on our return. Plants do not have the same adjustment tolerance.

Many plants have to get ready for the cold or they will suffer irretrievably. Their physical structures demand that they have internal defenses for dropping temperatures or even frost bite.

Essentially fresh new growth does not have any ability to protect itself against temperature changes, unlike full season vegetation.

Ron Stevenson is with the Fergus Horticultural Society