As I mentioned before. I have more collards than I can eat. That may not be accurate. It would be more accurate to say I am growing more collards than I am eating.
So, when to harvest collard greens? I wait for the plant to be established, and this timing is completely variable. The seeds that I started in January are now plants barely large enough to transplant. Some plants will be destined to produced leaves the size of your mailbox, some will only grow paperback novel sized leaves. So, it isn’t about the size of the leaves but instead the sturdiness of the plant. Just talk to the plant, she will tell you when she is robust enough to loose a few leaves. I wouldn’t harvest any leaves until at least at least 10 leaves are on the plant
On a collard plant the leaves grow in a spiral around the stalk. When two layers of leaves are the same size, that is the maximum size leaf that plant will produce for the time being. So, harvest all the leaves of that size. Let smaller leaves toward the center say on the plant and grow. I use kitchen scissors to cut the base of the leaf from the stalk. If you just pull he leaf you will likely damage the stalk and impact the plant’s production. Collards can grow for a long time if you harvest in this way. My grand mamma plant, which will receive a proper introduction in the future is two and a half.
And when it is it too late to harvest? You will notice that the oldest leaves on the plant start to become shiny and brittle looking. In this case you have waited too long. Leave those old flakey leaves on the plant for the bugs and harvest the prettier, flexible leaves above.
So, harvest and be happy.