Cheap Hacks: Native Seed Mixes

I bought a North East native perennial mix from American Meadows. Seeds were planted in May in an area where we removed Forsythia. We covered the area with cardboard boxes, top soil and compost. What came up appears to be mix of Partridge Pea, Swamp Milkweed, Rudbeckia and some likely volunteers, Devil's Beggarticks, Blue Wood Aster, Evening Primrose and of course, crab grass.