When to Sow Hardy Annuals
Most annuals can be split into one of two groups: Hardy Annuals (lovers of cool weather) & Tender Annuals (lovers of warm weather).
Tender Annuals are your usual summer annuals - sown in spring, which need protection from frosts, they'll then flower in summer & through to autumn if you keep sowing & keep up with the deadheading.
Hardy Annuals on the other hand can be a little trickier with their timing as they need cool temperatures throughout their growing period to grow into their biggest & best selves. So for most Hardy Annuals, we start sowing/planting in Autumn, overwinter the plants, for them to then flower in Spring (much earlier than any spring sown annuals!).
Getting the autumn timing can be a little tricky and honestly can take a few years of tweaking to figure out the exact ins & outs of how different flowers perform in your very specific micro-climate. But you want to make sure you have decent sized plants in the ground, so they're not washed away by the winter misery, but at the same time you don't want to time it too early as they may then rush to flower in autumn instead of waiting for spring, or if they're too big & lush they may sulk all winter.
So as a generalisation I aim to plant out my plants around 6 weeks before my first frost date—which is so irregular I gamble it being around the end of May/start of June when frost or not things are slowing down in general due to shorter days anyways. And to make sure I have plants big enough I then work backwards and from that approx planting date then and aim to sow my hardy annuals around 5-8 weeks before the plant date, eg working backwards roughly:
First Frost = end of May
Planting Date = mid-April
Sowing Dates = end of February–start of March
These are just rough guides in reality I'm almost always later but they're what I'm aiming for roughly (your first frost dates will likely be different so you'll have slightly different dates to aim for).
I also aim to plant another succession or two of most Hardy Annuals, starting another round of sowing in July & September (approx). Later not-fully overwintered plants will be smaller plants, and slightly less productive but they're totally still worth it - in fact in my mild, wet climate, where nothing ever stops growing, I'm actually finding I want to hold off on more and more hardy annuals to only sow from July onwards, as overwintered they just keep growing and growing and growing that by the time they flower they're sooo big and unruly I just don't have the will to deal with all the netting/corralling and staking needed to keep them in all their oversized glory upright through all the wet & windy spring storms! But we've probably got a bit of an unique climate to most on that front :)
My favourite Hardy Annuals to autumn sow (now-ish)
Snapdragons (see our tricks for germinating tiny seeds like snaps)
Iceland Poppies (see our tricks for germinating tiny seeds like poppies)
Strawflower (but they're only slightly hardy - hard frosts will pinch them when they're young)
Foxglove (Dalmation series - as they're first year flowering)
Oversized Hardy Annuals I like to hold off till July (most of the time :)
Ammi / Choc Lace
Rudbeckia (but only because I want their autumnal colours as late in the season as possible)
Orach + Quinoa + Phlox (can be sown earlier but they always stall over winter for me so I just wait till July for my first sowing)
Summer Flowering Sweet Peas - I sow start of July - sown earlier they just grow too much foliage before the days are long enough to start flowering (*winter/spring flowering I sow around April/May)
Other Hardy Annuals I just sow whenever willy nilly