Congratulations on your lawn, we are glad it is doing so well. We are sure you read our How To Article Native Lawns: Habiturf - The Ecological Lawn before you began, but we suggest you read it again to refresh your memory. Of particular interest relevant to your question are the paragraphs on Mowing and Sowing. We concluded from re-reading these that you do not want to be mowing or sowiing seed in late Fall or Winter, and that the best time for seeding is Spring. A lawn kept at about 6" will be producing a few seed heads all during the growing season.