I just bought some Chasmanthium laxum from a grower who used an unheated greenhouse. I left the plants in my unheated garage at night and then out in the relatively warm daytime for a week (also zone 7a...but perhaps a colder one than some) and just planted them out yesterday. I had emailed the grower, and she seemed to agree that grasses are tough and can take more temperature fluctuations.

So, I cannot say for sure as I don't know how large your plants are. Mine were filling the proverbial four inch pot and had heavy roots.

I'm not growing this particular grass right now, but the company I get my native seeds from will only ship Big Bluestem/Andropogon gerardii as a plant from early May through late September.

Some of the folks on the Meadows & Prairies Forum on GardenWeb may have some suggestions on the best time to plant it out in your Zone (copy and paste link below into your browser):

http://forums.gardenweb.com/forums/prairie/

Here's some other info I found on growing Big Bluestem (info is from a seed company). They suggest planting out from late May to early October:

"Collect big bluestem seeds when the seed head no longer has a creamy center, usually in September and October. Dry seeds in paper bags for 2 to 4 weeks. Seeds can be stored up to 7 months at 50 degrees F and 50 percent humidity. Cold stratification (40 degrees F, 35 percent humidity) may improve germination uniformity. Fill germination trays or pots with moist soil, compacted at bottom. Sow seeds by hand, covering with a thin layer of soil. Keep soil evenly moist during germination and do not use fertilizer.

Greenhouse establishment will occur at alternating day/night temperatures (set at 75/65 degrees F) and 12-14 daylight hours (may be extended artificially). Transplant seedlings into plug cells. Soil does not need to be consistently moist at this stage. In early to late spring, move plugs to cold frame. Seedlings are ready for outplanting when the plant and soil can be completely pulled from the pot as one unit. Outplanting can take place from late May to early October.

Seeds can be sown directly outside from late winter to early spring. Emergence will occur in 4 weeks with several irrigations."

Hope this helps...

Here is a link that might be useful: Big Bluestem (Andropogon gerardi)

You should be able to plant them out once the ground has thawed in the spring. I've put plugs out as late as November and as early as late March. Now would seem like a bad time though. The roots won't have a chance to take hold (assuming the ground isn't already frozen) and you might find your plants on top of the soil come spring due to frost heaving.