When you’re just getting into seed-starting from seed, it’s natural to want to keep things simple, and you can do that with common annuals and edibles, like zinnias and beans. They’re easy to handle, quick to sprout in indoor or outdoor warmth in spring, and don’t need any special treatment.
When you get into less-common seeds, there are many more variables to take into consideration. Some annuals need to be sown in summer or fall to sprout in fall and flower the next year, for example, while others are best sown outdoors in fall to early spring so they can germinate in cool conditions. Some perennials need one or more cycles of alternating warm-then-cold or cold-then-warm conditions (around 3 months at each temperature), or alternating day and night temperatures. Other seeds may greatly benefit from a special treatment, like having a hole nicked in the seedcoat and/or soaking in water before sowing, or even more advanced techniques, such as exposure to gibberellic acid or smoke. Some may germinate quickly if stored a particular way and slowly, or not at all, if treated otherwise. Little wonder these unusual seeds are unusual. If they were easy, everyone would be growing them!
Taking the time to find and implement the right protocols is the secret to success with challenging seeds. Sometimes, their needs may seem ridiculously slow or esoteric, but once you start learning about seeds in detail, it begins to make sense, and when you start having success with seeds that other people describe as difficult, you’ll feel a real sense of accomplishment.
I have been keeping seed-sowing records for years now, and whenever possible, I include the details I have learned about each seed I sell right in the listings on this site. You can find many other sources of information with a web search, but be careful: there is plenty of misinformation out there too. Tom Clothier’s Garden Walk and Talk is one source that I find to be reliable, as is Rob Broekhuis’ site, Rob’s Plants, which contains first-hand germination information on many unusual seeds (list here). For eastern native plants, I find that the book Growing and Propagating Wild Flowers by Harry R. Phillips (The University of North Carolina Press, 1985) is invaluable (Amazon is one source). And definitely, if you want to learn the whats, whens, and whys behind the process, The Science of Seed Germination by Dr. Norman Deno (available as a large PDF here for free) is a must-read reference.
Now, as far as when to sow…
There really isn’t “a” time of year for sowing seeds. When you really get into it, it’s pretty much a year-round process. I plan to cover that in more detail in a separate post about “seeds through the year.” To greatly simplify it, think about indoor seed-starting in late winter through mid-spring and outdoor sowing from mid spring through mid winter.
Deciding exactly when to start indoor seed-sowing is a real challenge. Once the winter holidays are over, we gardeners start wishing for spring, and it’s very tempting to start sowing in early February, or even in January. I have done that many times and nearly always have regretted that decision. It’s an extra month or more of electricity used for plant lights and heat mats, for one thing, and for the most part, the extra few weeks of growth wasn’t worth the effort and expense in the long run. And often, the jump start is a distinct disadvantage, as I’d end up with large seedlings that would take up a lot of indoor space and sometimes need to be moved to larger pots so as not to get rootbound, requiring additional potting soil. I’d spend a lot of time fussing about the weather, moving the seedlings outdoors during mild days and then back indoors in the evening, praying for frosty weather to end early and then worrying about the possibility (or reality) of sudden, late frosts. And, on the whole, the heat-loving annuals and tropicals really didn’t benefit, because the exposure to cool spring conditions could stunt their growth and set them back for several weeks, so they ultimately had no advantage over later-started seedlings.
These days, I try to make myself wait until around March 1 to start indoor sowing, beginning with tropicals, which tend to take a long time to sprout. The bulk of my sowing—most annuals and edibles—is in mid- to late March, with the last few fast-and-easy seeds, like zinnias and marigolds, in early to mid April. That usually works out about right to have things ready for planting out around Mother’s Day here in southeastern Pennsylvania. Depending on where you live, the ideal dates could be weeks or even a month or more earlier or later. I encourage you to connect with other gardeners in your area, either in person or online, to get more specific recommendations for your particular conditions. The regional gardening forums at Garden.org are a great place to start. (Their whole list of forums is available here; scroll down to find the regional ones.)