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Day length – gardening by the solstices and equinoxes

All over the world, wherever people depend on growing food crops to live, major festivals are traditionally held on pretty close to the same dates. These dates have historically been so important that even in cultures like ours we still hold many of the festivals, though we may not remember the original reason for them.

The winter solstice is the longest night and the shortest day of the year. It comes near the end of June for us, but in the Northern hemisphere it’s late December, and its traditional name is Yule. The spring equinox is the date when the day and the night are of exactly equal length. For us it happens near the end of September, but in the northern hemisphere it’s late March, its old name is Oestre, and traditionally the new spring fertility is celebrated with painted eggs. Recognise it?

Once upon a time our ancestors depended for the success of their food crops, and so for their lives on remembering these dates. Now we live in the wrong hemisphere for the traditional associations, we’ve bred some plant varieties that are a bit less sensitive, and our food often comes by ship or plane from another hemisphere entirely.

But for gardeners these turning points in the length of the days and nights are still so important they’re worth designing your gardening calendar around, because they’re how plants tell the time.

For most plants spring is not spring because it is getting warmer. Air temperature is way too unreliable if your very survival as a species depends on predicting, accurately, whether it is heading towards snow or not. The ancestors of our food plants that tried that strategy only had to mistake a warm autumn day for spring, once, over the space of millennia, and they got all their seeds covered in snow which was the end of the road for those genes. Evolution favoured those that worked out how to use day length to trigger storing food or setting seed.

The scientific name is Photoperiodism, and most of our common food plants that evolved in climates where it snows use it to some degree. It has been bred out a bit in modern cultivars, but we’ve only been at it for a few thousand years, which is nothing in the scheme of evolutionary time.

Photoperiodism affects every part of the plant life cycle. Some winter-tender plants have seed that will go dormant and survive to germinate in spring. They know not to break their dormancy when the nights are too long. As the days shorten and the nights lengthen, they rush to produce seed before the winter kills them. If you plant them before the winter solstice, they can develop small, spindly shapes and bolt to seed before they are big enough to set a worthwhile crop.

Other plants have a root or bulb store of food and plan to survive a winter as mature plants. Their strategy is to go to seed as early in spring as possible, to give their babies as long as possible to become mature and save a store of food before winter. Shorter nights signal time to turn the food store into seed, and the longer nights after the equinox signal that it is time to stop growing and start storing food. Onions planted after the winter solstice for example, tend to forget about storing food as a bulb and just bolt straight to seed.

For gardeners, remembering and reinvoking the traditional plant based reasons for seasonal festivals can be very useful. You can have more success growing the feast, and a good excuse for sharing it with friends and family.