Why a big chill is cold com­fort for dor­mant seeds

There are a num­ber of mis­con­cep­tions about dor­mancy, which is why it helps to un­der­stand the science be­hind it

The Daily Telegraph - Saturday - - GARDENING -

Look on­line for ad­vice on ger­mi­nat­ing seeds that need a pe­riod of win­ter chill­ing to break dor­mancy, and it won’t be long be­fore you come across the fol­low­ing, or some­thing like it: “Sow your seeds in a pot and leave out­side to let frost break down the seed coat.” This is rea­son­able enough ad­vice, and quite likely to work, but I’m a firm be­liever that you’re a bet­ter gar­dener if you know not just what you’re do­ing, but why you’re do­ing it, and that isn’t quite right.

If a seed needs cold, it’s gen­er­ally to do one of two things, and some­times both. Some seeds have an im­ma­ture em­bryo that will only grow to full size at low tem­per­a­tures.

Other seeds (the ma­jor­ity) sim­ply have a chem­i­cal switch that is turned from “off ” to “on” by a cold pe­riod. In nei­ther case does this have much to do with the seed coat.

Nor does it have any­thing at all to do with frost. If a seed needs cold to break dor­mancy, the ex­act tem­per­a­ture re­quired varies from species to species, but any­thing be­low 10C is usu­ally OK. Men­tion­ing frost could lead you to be­lieve your seeds need to be frozen, which they don’t; frozen seeds will just stay dor­mant. Some­thing else that I’ve been guilty of as­sum­ing is ob­vi­ous, but per­haps isn’t, is that for the cold to work, seeds need to be wet, or “im­bibed”, tech­ni­cally speak­ing. Low tem­per­a­tures will not break the dor­mancy of dry seeds.

GREEN SHOOTSThe se­quence of ger­mi­na­tion, main; alpines, be­low, have few species with any seed dor­mancy. Bot­tom: chill­ing seeds, known as strat­i­fi­ca­tion

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