Well, it's going to be-3° in the Twin Cities tonight, that qualifies as a hard freeze and unless your greenhouse is attached.... Not sure what you mean by tender seeds. Most seeds can be winter sown, but in my experience tender seedlings need to be protected from really bitter cold. Despite the claims of winter sowers elsewhere, I've had more than one pot of seedlings succumb in the wild temperature swings of late winter/early spring. I'd be careful. GaryStPaul