Cut everything to a height of 4-5 inches and mow as frequently as your site demands; usually that is when growth reaches 8-10 inches or weeds put up seed heads. Hand-held string trimmers are ideal for small areas or steep slopes.
You can usually stop mowing at the end of the first season. If weeds are thick in the beginning of the second season, mow or spot-mow once or twice.
If your first-year planting looks like this, mowing is overdue. Mow now! You may be encouraged by the Black-eyed Susan and not want to mow. But also pictured, Daisy Flea Bane, Canada Goldenrod and Queen Anne's Lace are weeds and need to be mowed, along with the Black-eyed Susan, to allow light in.
Set your mower to a high setting; 4-6" on most mowers. Refer to Step 7 in Growing Your Prairie for more information.
Here is a first-year planting next to a third-year prairie. We are practicing maintenance mowing multiple times this summer to allow light to infiltrate and to prevent weeds from going to seed. Mowing will not harm young native plants. Refer to Step 7 in Growing Your Prairie for more information.
Use a hand-held string trimmer on steep slopes. Refer to Step 7 in Growing Your Prairie for more information.