From the Glossary of Rating System, Conventional Turf is defined as "Grass, typically a monoculture, that requires considerable watering, mowing, and/or fertilizers. What is considered conventional turf may vary by region." While root length may be an indicator, there is no universal root length that distinguishes native or healthy grasses from conventional turfgrass. The TASC likes the idea, but determined that using root length as a prescriptive determinant for this credit would not be appropriate.
Updated 10/1/13 for rating system applicability.