My experience has been to wait for spring and sow. Most grasses do not need cold stratification and the seeds rot if you sow to early. The cool season grasses can be started earlier. I'm sure Donn will post some of his recomendations. He is more of an expert.

mickey_dee has it exactly right. Very few ornamental grasses need cold stratification. The cool season grasses, like Festuca and Deschampsia can be winter sown successfully, because they germinate at cooler temperatures, and don't have to sit in the moist containers as long. Warm season grasses don't germinate until much later in the spring, and if you keep them out in moist potting soil for months in advance, they may rot.

Since you gathered these from commercial landscaping, chances are they are fairly tall, which usually means they're warm season grasses, like Miscanthus and Panicum. They could also be Calamagrostis x acutiflora, which is a fairly tall cool season grass. In any case, I'd still wait until the end of winter, or early spring. Then sow them in containers, just like winter sowing. The potting mix will warm more quickly, and if the seeds are viable, they'll germinate earlier.

I said "if the seeds are viable" because you may have collected seeds which never had a long enough season to fully ripen. You are in a less-than-warm climate, and it's entirely possible for grasses to go into a nice bloom, and even set seed, but not have time to ripen the seed. Also, some of the more popular grass cultivars are sterile, and never produce viable seed.

Another potential complication is what you may grow from these seeds. Most of the named cultivars will not come true to the parent plant when grown from seed. You may get a variegated Miscanthus seed to germinate, but the resulting seedling will likely be a regular green species version of the grass. The same is true with many other examples. They still make nice grasses, but be ready for a possible surprise.

Thanks, Mickey Dee and Donn. Not calamagrostis x acutiflora, the ones I saved from were much shorter (seedheads were hip-high or shorter), but I understand the information about sterile/immature seed and/or possibly not coming true to seed.