I’m a zone lower than you, but I think it’s still pretty early. The benefit of winter sowing is stronger plants, it doesn't give you a big head start like starting seeds indoors.

Trust the plants. They know what they’re doing. If they sprouted, they’re fine. Keep the soil moist and remove the container tops, if warm weather (not freezing) is predicted.

I agree with docmom - trust the plants. Besides, there's not much you can do about it anyway hahaha! But seriously, keep an eye on them, make sure they don't dry out (or get waterlogged). I wouldn't take the tops off just yet, (but you know your zone better than I do!) but definitely keep an eye out.

With the crazy way the climate is going, (i.e it was 60 here on Saturday (which was crazy warm!) and then three inches of snow later that night into the next day, with temps in the single digits) the hardest thing about wintersowing these days, imo, is the unpredictability of the weather. So keep an eye on your weather forecast. Honestly I think the bigger danger at this point is warm weather as opposed to cold. The last two years we've had 90-degree days in April - as crazy as 60 degree days in February! - so you have to pay more attention to the temps these days.

:)

Dee

P.S. I'm not too familiar with the asclepias, but echinacea is a tough perennial, and I think these two plants in particular should be fine for you