foot high, you're not going to smother anything. You are not mowing this to the ground or even just off the ground. The idea is to take enough off the top to keep the canopy & sunlight open to new seedlings for as long as possible. When the canopy gets clogged up again, time to clip it off again. I usually mow mine around 8" or so depending on growth stage of plants. you probably could take half the growth off the weeds, so 6" if stuff just getting growing. later- If the desired plants are getting a foot tall, I'll raise that up some as they are farther along. If they are just getting started and I have a lot of weeds out of bad luck or bad preparation, I'll mow shorter. you aren't going to smother them and if weeds are super bad, your seedlings are sure struggling now so good time to get them freed up.