One of the things I love most about living in a temperate climate is that we get to experience four traditional seasons. You only need to look out the window at this time of year for a reminder – the leaves are turning – and while global warming will have an effect on our climate, it’s reassuring to know that unless it can tilt the earth’s axis, the timing of the seasons will remain constant for all the years to come.
Today, March 20, is the autumn equinox. It’s the official start of the autumn season (March 1 is just for convenience), and in scientific terms, it means that the earth’s poles are an equal distance from the sun. Today we experience approximately 12 hours of day and 12 hours of night. From today, the earth’s South Pole will gradually tilt away from the sun, shortening our days until the winter solstice arrives on June 21. Then the days will again grow longer until the summer solstice. Then the ancient cycle will repeat. And repeat. And repeat. They say nothing in life is certain except death and taxes. When Benjamin Franklin coined the phrase, he forgot about the seasons.
The role seasonal change has on plants is varied. Flora originating from tropical climates tend to be responsive to periods of wet and dry. Lots of Australian plants show these tendencies, and many of those indigenous to our area will flower and seed just prior to our wet season in summer. Other plants respond primarily to temperature, needing a period of cold weather to initiate flowering (vernalisation).
Plants growing closer to the poles, however, tend to be responsive to day length. The fancy term for this is “photoperiodism”. It’s all still a bit of a mystery to scientists, so let’s consider how it works in gardening terms: Photoperiodism basically means that some plants are long day length, forming flowers as the days grow longer, and some are short day length, forming flowers as the days grow shorter. Others are day length neutral.
Onions illustrate this response to day length perfectly. Short day varieties form bulbs after the summer solstice as the days are getting shorter (and night grows longer). The seed of these plants should be sown in summer, and the swollen bulbs harvested in early winter. Long day varieties form bulbs as the days grow longer after the winter solstice. They should be sown in late winter or early spring, and harvested in early summer.
Another member of the allium family that can be sensitive to day length is garlic. In most parts of Australia cloves are planted now, on or close to the autumn equinox. This allows the plant to grow foliage through the winter, and form bulbs as the days lengthen in spring. Harvest takes place around November.
For gardeners, this poses a potential problem because the closer you live to the equator, the less day length changes throughout the year. There are roughly 12 hours of day and 12 of night all year round. Day length sensitive varieties of garlic will either form very small cloves, or they may fail to form a bulb at all.
Toowoomba is located at a latitude of approximately 27 degrees south, which is roughly the same as South Africa and southern Brazil, or equivalent to California and southern China in the northern hemisphere. The wash up for garlic (and other light sensitive crops) is that day-neutral varieties may perform better than those that originated closer to either of the poles. If your garlic is failing to form bulbs, make sure you plant it at the correct time of year, or try varieties such as ‘Glen Large’ (day-neutral, bred in Gatton) or ‘Australian White’ (originally from California).
Everything is relative of course. Factors such as elevation, proximity to the ocean, and oceanic cycles all affect the way plants grow. The key point is that the rotation of the earth around the sun doesn’t change. The change in day length is reliable, and for that reason, the seasons are largely fixed. Why this knowledge gives me a sense of comfort I’m not really sure, but it’s probably got something to do with hope: When I stuff something up in the garden (or in life for that matter), I know that there’s always tomorrow, or next season, or next year. As long as I draw breath I can always start anew.
First published in the Toowoomba Chronicle 20th March 2010. Photo by Jennifer Dickert via flickr.
