Seedlings and Cold

Florida Indian Plantain (Arnoglossum floridanum) seedlings

Water dropwort (Tiedemannia filiformis) seedlings

I am often asked about the best times to sow seeds. To me, it should be a relatively simple answer. I sow them when they are ripe. It might be different if I were growing plants in a greenhouse, but I sow them outside. In this situation, they are put into the same growing conditions as if they were sown in nature. Plants "know" when to let their seeds go. They germinate when the conditions are right for them to emerge. Anything else makes little ecological sense.
To germinate, seeds require stratification. For some, it is a simple requirement to be put in a moist environment. Others, especially those from colder environments, require a period of cold followed by warmth. Seeds of these species will not germinate without this cold stratification. You can do this by putting them in a refrigerator first, but I grow my plants in flats outdoors before I pot them up and so I simply plant them and wait.
Ecologically, it makes perfect sense for seeds to not germinate too soon. My purple coneflowers (Echinacea purpurea), for example, rarely sprout before late February even here in Florida where our cold temperatures average in the low 40's F. Right now, I have three seedlings even though I sowed about 100 seeds two months ago, right after my move. They are programmed to be cautious of winter freezes after thousands of years of evolution. Further north, they would freeze to death if they sprouted too soon. The vast majority of my Florida wildflower seed, however, germinated within a week or two.
The question of when to plant seed is not so much about waiting for the right conditions, but in giving them the conditions they would experience in nature. It is not natural to keep seed for long periods of time in containers waiting for spring. In nature, they would be scattered by their parents and they would remain on or just below the soil - kept moist by rain events (or snow). Such conditions are mandatory for the embryos inside to maintain their viability. Just because they are not sprouting, does not mean that they are "dormant."
Embryos inside the seed are living/breathing baby plants. There is a very complex system in place that maintains their viability. They are just waiting for the trigger that tells them that it is finally time to safely germinate. We risk throwing that system off by keeping them indoors until spring. We also risk causing their death.
Viability of the embryo is not an infinite time period. Some, seeds from plants that live in very harsh environments like deserts, can remain alive (viable) for years, but most embryos have a limited amount of time to germinate before they die. The blazing stars (Liatris spp.) that are some of my very favorites here in Florida, have about three months before their viability declines dramatically. After about six months, I am fortunate to get 10% of them to sprout.
Over the years, I have found that seedlings are far more resistant to cold/freezing temperatures than their parents. Nights in the 20's may cause their parents to die back to the ground, but they will weather the cold with virtually no damage. The seedlings I'm growing this year for both my wildflower garden and for sale in my Hawthorn Hill nursery show signs of the unusually cool weather we've been having. Their leaves are blushing red because of it, but they have continued to grow. These are species adapted to this part of Florida and they'd be doing this exact same thing if they had sprouted in "the wild" as opposed to my seedling flats with potting soil. If I had "protected" them from this weather by waiting to sow them when colder weather was less likely, I suspect most would have failed to germinate at all.
When the weather warms up here, I will get more seedlings to emerge. Some are just below the soil surface waiting for that time, but all of my seed needs this weather that we are getting and they will thrive despite any of the cold we've experienced - or will yet experience.
A lot of things can go wrong before I can add these wildflowers to my landscape this spring, but it will not be related to the fact that I sowed them as soon as the seed was ripe. I have the place ready where I intend to plant them. Now is just a time for patience.

Seedling of the southern Florida ecotype of black-eyed susan (Rudbeckia hirta).  There are many forms of this widespread wildflower. Use the one most adapted to where you live. The north Florida ecotype does not do very well here in south-central Florida, though it is the type most often sold here by the native plant nurseries.