"Greens" is a very diverse category of plants that encompasses all plants that produce a green leaf that you eat (instead of say, a seed pod or a fruit). Hearty cold loving greens like kale are best planted in the fall, and even less heat tolerant greens like spinach and lettuce have to wait till after Thanksgiving to be planted in your home garden.
We sell "quick growing" greens that are eaten young like arugula, mustard & bok choi in 6 packs, so you can plant more for your buck, and full season greens like chard & kale in 4" plants that are approximately 6 weeks old.