Many annual varieties, like Zinnias, and Sunflowers, have soft shells and can simply be sprinkled on bare soil in the spring. But some perennials, especially native wildflowers, have a hard coating that helps protect the outer shell from breaking and sprouting too early. We’ve all experienced an unseasonably-warm spell in in the middle of January or February — this mechanism helps prevent the seeds from being tricked into coming out of dormancy until it’s just the right time.

The good news for gardeners is that the natural cold stratification needed for germination can be forced with just a few materials, water, a refrigerator, and patience.