Why plants need cold.

By | February 6, 2012

Of course  annual plants don’t.

However for perennial and especially biennial plants it’s often a necessary pre condition for flowering – we’ve just been hearing from the pundits in the fruit world that the recent  cold snap  will improve the prospects for bud formation for top fruit (that’s apples to you) and stone fruit (plums, cherries etc.)

As gardeners, we often lose sight of the  way that we are messing with the plant’s natural cycle. If we want leaves (lettuce, spinach etc) we are trying to hold them back from flowering – in fact we get rude about it and call it “bolting” if they manage to have their own way. And if we want them to fruit – beans, tomatoes, squash, we want to push them on as soon as their leaf structure is big enough to give them the push they need.

Cold plays its role in this by acting as a signal to get on with flowering. The technical term for this is vernalisation ( to distinguish it from hibernation I suppose).

It’s kind of like a board game where the aim is to accumulate degree days.  So one degree day is one day spent below a given temperature which varies from plant to plant. 1 day at 6 degrees below is worth 3 days at 2 degrees below. When the critical number is reached then the plants will move onto the next phase in the cycle.  This tends to be a feature of long day plants.

So if you are growing leeks and onions you can do without it because you want the plants to stay vegetative but if you are growing cauliflowers and broccoli you want the cold to get the plants to move on.

So subtle is this science that breeders have produced varieties of cauliflower with different degree day requirements to allow commercial growers to produce a continuous supply from autumn (where they produce florets at the drop of a hat to spring varieties that need a long sojourn in the deep freeze. One trick you might consider for getting uniform curd production is to put the seedlings into the refrigerator to saturate their cold requirement so that they’ve already got their quota. Then you can predict when they are going to flower. Generally about 14 days is enough according to Bleasdale. However for a gardener you might want to stagger the length of time you give them in the fridge.

Spring  Cabbage is generally on a knife edge here. The trick is to get just enough growth. Too little and the plants will be puny. Too much and they’ll bolt as soon as they get enough daylight if they have had a good freezing. Commercial growers manage this by careful control of the sowing date. About 20th July in the English Midlands is a good bet. And if we have mild winters and autumns – look to your cabbage and autumn sowed onions to bolt.

You can keep track of  degree days by using a thermometer – our soil test and max min thermometers work well for this purpose.

However because it depends on the precise variety and your growing conditions you will need to keep your own records for things that important to you.

Of course degree days matter on the other side by having an effect on how fast plants grow. We’ll look at that shortly.

One thought on “Why plants need cold.

  1. Richard

    Interesting points – I wonder sometimes how many miles of unnecessary horticultural fleece the garden centres sell every year?

    Plants themselves also seem to ‘adapt’ sometimes too. Laurus Nobilis (Bay) used to fail in UK gardens regularly when it first became an attractive garden feature plant. These days – mine are as healthy and invasive under frost and snow as rhododendrons (not that I’m complaining – I love the leaves both as a herb and to look at).

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