Now is the time to look beneath established Berberis bushes and observe germinating seedlings. As can be seen from the attached image of Berberis aristata the viability of the seed crop is good. If you are not in need of these juveniles then at the earliest convenience take a digging fork and swath it through the soft growth. If possible choose a dry day when desiccation will prevent re growth. Wait much longer and the seedlings become woody, will have a developing root system that also anchor each into the soil, making eradication that little more difficult.