Wanted to give you some info. I WS red yarrow at the beginning of the 2013 winter season. The seeds germinated Apr 2013. I left them all in the container til mid June, that's when I transplanted them into different garden spots. They continued to grow well last summer. They all wintered over and are putting out new growth now. They don't bloom their first year. I should see flowers on my mine this summer (their 2nd year).
You certainly can open your containers on warm days. Water them when they need water and they will do just fine. Give them a bit of time to grow and then plant them wherever you wish.