Giv­ing seeds the cold shoul­der

Strat­i­fi­ca­tion can as­sist strong growth for au­tumn seedling ger­mi­na­tion

Central Otago Mirror - - GARDENING -

Spring sow­ing is com­mon prac­tice but when sow­ing cer­tain peren­ni­als, it’s worth not­ing whether or not the seeds need strat­i­fy­ing first. While many seeds need no spe­cial care, some need ex­po­sure to cold and mois­ture be­fore they will ger­mi­nate. That’s Mother Na­ture’s way of pro­tect­ing her plants. For seedlings to thrive in na­ture at the op­ti­mum time, cer­tain seeds must ger­mi­nate in spring as op­posed to late sum­mer or au­tumn when many plants self­sow. If in­stead they started to grow dur­ing win­ter they might even­tu­ally be bowled over by an icy blast. There­fore, cer­tain seeds must un­dergo a pe­riod of strat­i­fi­ca­tion – mois­ture and an ex­tended pe­riod of cold tem­per­a­tures – be­fore they ger­mi­nate. In the home gar­den, we can trick seeds into think­ing they’re ex­pe­ri­enc­ing win­ter by giv­ing them a con­trolled pe­riod of cold moist strat­i­fi­ca­tion. There are a cou­ple of ways to do this. The first is to mix seed with damp (not wet) ver­mi­culite or coarse sand (3-4 times the vol­ume of the seed) and place the seed and mix in a plas­tic bag in the fridge for 4-12 weeks. The bags should be folded or tied loosely, not tightly sealed, as aer­o­bic con­di­tions must be main­tained. La­bel the bag with the name of the seed and the date. Check ev­ery cou­ple of weeks to en­sure the mix doesn’t dry out. Also check the bags reg­u­larly for ger­mi­na­tion and re­move from the fridge when the seeds be­gin to sprout. You can then treat them as you would any seedling. Plant them in trays or small pots to grow on. The sec­ond method is to place your seeds on moist pa­per tow­els and place th­ese into bags that are also loosely folded. One Nor­man Deno, who was Pro­fes­sor Emer­i­tus of Chem­istry at Penn State Univer­sity, is a mas­ter of the moist-pa­per method and out­lines its val­ues in his book, Seed Ger­mi­na­tion The­ory and Prac­tice (1993). ‘‘About twenty pads can be stacked in each bag­gie and about five hun­dred ex­per­i­ments can be con­ducted in one cu­bic foot.’’ An in­di­ca­tor of plants that need a pe­riod of cold strat­i­fi­ca­tion are those that are typ­i­cally sown in au­tumn. But where seeds are ar­ti­fi­cially strat­i­fied, do not plant them out in au­tumn. That will give them a dou­ble whammy. If you plant them out­doors in au­tumn af­ter chill­ing in the fridge, ger­mi­na­tion will kick-start the grow­ing pe­riod and seedlings may not sur­vive win­ter. At the same time, once seeds have been cold strat­i­fied for a spring sow­ing, plant them out straight away. Once strat­i­fi­ca­tion be­gins, a kind of bi­o­log­i­cal clock starts tick­ing, en­ergy re­serves slowly de­plete and shelf life is re­duced. Get them in the gar­den quickly and you’ll be able to en­joy the fruits of your labour.

Pretty: Pop­pies can ben­e­fit from cold strat­i­fi­ca­tion.

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