We have the black variety lining each side of our front walk. In Ohio, October is fantastic weather for growing, while November starts to get cold. How late can we split these clumps up? Will splitting them up late make then less hardy for the upcoming winter freeze?
It's best done in spring I'm afraid. I believe most of Ohio has now been moved into Zone 6 - the UK is Zone 8, and the recommendation here is to divide Ophiopogon nigrescens (mondo grass) in Spring, from March to May, rather than at this time of year. First frost date is roughly end of October, depending which bit of Ohio you're in, and given you get colder earlier, and colder altogether than we do here, I'd suggest you wait till spring as recommended here http://www.thegardenhelper.com/black_mondo.html