Have you ever planted some seeds, but they didn't grow? Of course. Every gardener has seen that happen. Maybe it was because the seeds were old, and couldn't sprout anymore. But maybe they were seeds that needed special conditions to germinate. Many perennials, wildflowers, tree seeds, and some herbs are like that - they go into a dormant state and won't sprout until they get a special signal. Most of the time, that signal is a long period of cold temperature.
Perennial seeds need some kind of safety signal to know when winter is over. If they were to sprout right away in the fall, the tender seedlings would suffer too much, so they have to wait until spring. Mostly, they do this by going into a deep sleep (called dormancy) which is broken only by repeated freezing and thawing. Seeds that are only stored indoors never feel the chill of winter. Without freezing and thawing to break their dormancy, they still think it 's the fall, so they wait, dormant.
Why don't annuals need a cold treatment to stimulate germination?
The reason is simple: most of the garden plants that we grow as annuals (like tomatoes) are actually perennials somewhere else in the world where it doesn't freeze hard in the winter. Tropical plants such as beans, peppers, petunias, and impatiens don't need a safety switch to tell them when winter has passed, so they are mostly able to germinate right away, with no special treatment. Only the plants that originate in colder latitudes need their seeds to break winter dormancy, and as it happens, many of those plants naturally survive cold winters so we think of them as perennials.
That's why many of our garden perennials, wildflowers, and native trees are difficult to raise from seed. Essentially, all you have to do is simulate winter conditions, so the seeds think it is spring and "wake up". The technical term is "stratification" but you can also call it "chilling", or "dormancy breaking". Many seed companies pre-stratify their perennial seeds, but not all, and certainly your home-saved perennial and wildflower seeds will need the treatment.
Methods of Stratification
The most common method of stratification is to moisten the seeds, then chill them for a few weeks. Some gardeners have found that it works to simply put the dry seeds in a refrigerator for 2-3 weeks, but dry seeds don't normally break dormancy when you chill them - that's how we store seeds in seed banks (dry and cold), to keep them dormant for a long time.
First, soak the seeds at room temperature for a full day. This will ensure that even the toughest seeds will absorb moisture through their seed coats. Then do either of two things:
1) Place the soaked seeds in a plastic bag of good household potting soil, and keep it sealed in a refrigerator for several weeks. The exact length of time depends on the species, but count on about a month to get good results.
2) Sow the seeds in garden containers, and put them outside during late winter or early spring, where they will experience about a month of temperatures near the freezing/thawing point. March or April is usually the right time.
After the seeds have experienced winter (indoors or outdoors) raise them like you would do with any seeds, and watch them grow!
TIP: Don't put moist seeds in the freezer!
Considering that the seeds in method 2 above (a month outdoors) will sometimes freeze at night, you might think that it would be okay to just pop them in your freezer instead. Actually there's a big difference between a block of soil freezing outdoors, and in your home freezer. As temperatures fall overnight outside, the ground freezes slowly, little by little, and causes minimal damage to seeds in the soil. However, plunging a moistened seed suddenly into a subzero freezer will cause it to flash-freeze, rending its little tissues into mush and killing it very quickly.