Here's what Paul Black sent to me in an e-mail a while back when I asked him about their weed control. And I quote.
We do use a combination of preemergent and hand weeding. It takes both to keep on top of it. No preemergent herbicide will destroy all types of seed, nor does any preemergent destroy every seed of the type seeds that it does kill, therefore the need for hand weeding. We alternate two products, Ronstar and Snapshot. We use them after we work the beds and have the irises replanted. We dig and replant almost everything, every year. Don't incorporate it into the soil before planting. It will inhibit root growth. Treflan is one of the most potent. I don't use it. It tends to inhibit root growth whereas the others don't when used properly. Most of them come granular and liquid. Be sure to read and follow label directions.
