Many typical garden plants – lots of long-living flowers, herbs, wildflowers from where you are, and even some vegetables – won’t start to grow as soon as they get wet and warm. They need a time of being cold and moist first, before something inside them that’s stopping growth will finally let them sprout. This is called cold stratification and it’s what happens when a seed drops to the ground in the fall, gets cold through the winter, and then starts to grow when spring comes. If these seeds don’t get that cold, they’ll just stay in the soil forever, and people often think they were bad seeds.
Usually, cold stratification means keeping seeds at a temperature of 33 to 40 degrees Fahrenheit for a certain length of time, often from two weeks to three months, depending on the type of seed. This happens on its own if you plant seeds outside in fall or very early winter, or you can do it yourself by putting the seeds into something damp in the fridge.
Perennial Flowers That Require Cold Stratification
Lots of garden favorites that come back year after year won’t start to grow unless they’ve been exposed to cold temperatures. Echinacea, or coneflower, rudbeckia (black-eyed Susan), lupine, delphinium, columbine, milkweed, lavender, yarrow, catmint, hellebore, primrose, foxglove, and penstemon all need this. Wild roses, native asters and joe-pye weed do too. In fact, most mixes of perennial wildflowers have many of these kinds of plants that need cold stratification. Which is a big reason why it’s good to plant those mixes in autumn. The cold of winter will do what they require, and you won’t have to do anything special at all.
Herbs and Vegetables With Cold Requirements
Cold stratification, or a period of cold, helps or is needed by quite a few herbs. Rosemary, thyme, sage, sweet cicely and angelica will all sprout more easily if they’ve been chilled first. When it comes to vegetables, asparagus seeds do much better at sprouting after being stratified and rhubarb simply needs some cold to wake them up from their rest. Even the little bulbils from walking onions (also known as Egyptian onions) are happier when given some cold before you plant them. And a short chilling of certain cold-hardy lettuce types will give you a better chance of them germinating.
Trees, Shrubs, and Fruiting Plants
Most tree and shrub seeds from places with moderate temperatures need a period of cold, and that includes apple, cherry, plum, maple, oak, birch, walnut. Even the seeds of berries such as blueberries, gooseberries, currants and elderberries won’t start to grow until they have been chilled. Trees usually need a longer cold spell for their seeds to get going, frequently 90 to 120 days of cold, and this is a good reason why it’s easiest for a gardener who is raising trees from seed to simply plant in the autumn.
How to Cold Stratify Seeds in a Refrigerator
If you didn’t get around to planting in the fall, or you’d rather start things off inside, giving seeds a “refrigerator stratification” is pretty easy. Simply combine the seeds with a little bit of vermiculite, perlite, or sand, and the material needs to be just slightly moist. Then put that mix into a sealed plastic bag or container. Don’t forget to write the type of seed and the date you put it in on the bag! Keep it in the fridge for as long as that type of seed needs. The mix should be damp, but not soggy, because too much water will cause mold. After the fridge time is over, plant the seeds inside or outside as you normally would; they should sprout in the usual amount of time for the plant.
Key Takeaway
Many flowers that come back year after year, herbs, vegetables and tree seeds need a period of cold to “wake up” from their resting state. They won’t start to grow unless they’ve been at a cold temperature for a certain length of time. The easiest thing to do is to plant them in the ground in autumn, and let the cold of winter do its work. If you are starting seeds inside, you can mimic winter, and get the seeds ready for a good start in the spring by putting them in a slightly wet substance in the fridge for anywhere from two to twelve weeks – the exact amount of time depends on the kind of seed.
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