Any hardy perennial can be planted out as soon as you have two sets of leaves. Those are the first true leaves. I have two containers ready to be planted out using that guideline. I'm just not ready to start myself. Gonna see what else I have at the end of this week.

Don't wait. The sooner they get in the soil, the sooner they can start putting down roots. Don't expect a lot of top growth right away. As the soil warms up, they'll grow their roots as they grow topside. Water them in after planting, and if the soil is dry in the garden 1" deep. I didn't baby mine last year and they managed just fine.

I only remove the tops if they're in sun, and it's warmer than 70 degrees. Hasn't happened yet, and all my sprouted containers are in semi-sun under an old oak tree.

The only thing you need to wait to plant out are tender perennials and annuals. Everything else, especially in our zone, can handle the cold if they were wintersown.

Oh, and most will even handle snow, just in case it happens.