Giving seeds the cold shoulder
Stratification can assist strong growth for autumn seedling germination
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.