Phenology explained by your certified Toronto arborists

It’s not reading the bumps on your head

Many of our clients inquire when we are going to be performing certain spraying operations for pest and disease control. The timing of specific applications is related to plant “phenology”. Briefly, phenology is the study of cyclic and seasonal natural phenomena. In relation to the control of insects and diseases on trees, plant health care practitioners use phenology to accurately predict the optimal times to implement pest control options.

The Swedish botanist Linnaeus first developed a system of plant watching in 1750 which was the forerunner for modern phenology. Linnaeus observed that flowering times varied from area to area in Sweden, and from year to year within the same area. By recording climatic conditions at the time of flowering, Linnaeus made a direct association between weather and flowering.

A similar association exists between weather and the life cycle of tree pests. As a result, we are able to associate the flowering of certain plants common in the GTA with the best time to apply a pesticide for the control of a particular tree pest. For example, we know that the optimal time to begin to apply insecticides for the control of eastern tent caterpillar is when magnolia flowers are pink but the petals have not yet opened. We know this because phenology has shown that the eastern tent caterpillar emerges to eat leaves at the same as the magnolia flower bud is pink.

Relying on phenology rather than specific dates

As weather patterns are variable, particularly in a country like Canada, phenology is a vastly more reliable timeline to rely on than specific dates. An application made in May of one year for the control of a pest may be too late or too early in another year.

Arborists are also using phenology to combine our knowledge of a changing climate with a reassessment of the various plant species suitable for our area. As the climate warms different species can be utilized within the GTA for urban greening that could not readily be planted before. In fact, Toronto has been upgraded in its hardiness zone recently, meaning more plants and trees from more southern climates will grow here that 30 years ago would not.

After a winter of -30 temperatures and feet of snow, it is difficult to believe that plants are flowering up to a month earlier than 100 years ago. But phenology reminds us that our man-made calendar will never be as accurate as Mother Nature!