There are a couple of different approaches you can take depending on what types of weeds and saplings you have and also how frequently you want to mow; but I think most sources recommend mowing in the early spring. This accomplishes a couple of things: removes old growth to make way for the new, the clippings act as a mulch which helps retain moisture and crowd out weeds, it knocks down saplings, and allows the standing growth late in the year to provide over-wintering habitat for wildlife. I might mow once every other year or so in mid-summer though to try to keep annual weeds from going to seed and help control their population.

Bon, another approach is to do a high mowing-perhaps 6 to 8 inch height-early July or really whenever objectionable weed species are beginning to produce flowers, the idea being to interrupt seed cycles for these species. And your Rudbeckias and a few other native species will often still have time to bloom/produce seed. Just watch your site and see when thistles, wild carrot, reed canary grass, or whatever you've got there that you don't want, is beginning to bloom. You don't want to wait too long into this stage though: Many plants can produce viable seed if their flowers are mature, even if beheaded from the mother plant. So get them early.

+oM