No they don't grow that much. Real slow. You have to start them early and plant extra.

I was just reading on the site from a MD farm that grows in winter (zone 6 as well) that you can cut up to 30% of the leaves of the greens every 10 days during mild weather, and every 6 weeks during the dead of winter. I'm not sure how to figure the percentages, but I basically just cut a few of the outside leaves whenever I uncover the bed.

In my experience, the plants do grow new leaves, and get more active in that growth as the days become longer later in the winter. It's the cutting that makes them grow. So, for example, I cut spinach when the leaves are the size of quarters, and the plants grow new leaves. Don't wait for the size of leaves you're used to during the summer growth months.

I also focus on salads rather than cooking, since it takes many fewer leaves to make a nice salad than it does to make a serving of cooked greens.

The technique is to get your greens to harvest size before daylight falls below 10 hours. Then they basically sit there while you harvest. Once daylight lengthens again, they'll start to grow noticeably.

Regarding the type of green grown through the winter, my wife grows radicci and it seems to drow well except in the very cold spells. She covers it when it is very cold with plastic and it picks up the growing process as the weather moderates.

I am thinking about growing leaf broccoli or "spigariello" that way. Any advice?