Yes and no.
If it is a cool season grass like Calamagrostis arundinacea “Karl Foerster”, it is safe to divide. Why? It is a cool season grass that actively grows during cool weather like fall, winter, and spring. It grows best when temperatures are between 60 and 70 degrees F. Cool season grasses also tend to be evergreen
If it is a warm season grass like Miscanthus sinensis “Gracillimus”, it really depends on a few things.
- Is it currently flowering?
- How late in the fall is it?
The fall is actually a great time to divide a cool season grass, as is spring. Just be sure to keep it watered well and it should do just fine.
Warm season grasses grow best between 80 and 90 degrees F and go dormant in the fall. Also, warm season grasses are usually beginning to slow down their growth and putting out flowers in the early fall. Dividing a grass while flowering will cause unnecessarily stress on the plant which may result in it dying.
Dividing a warm season ornamental grass in the late fall will not give it enough time to form roots that will allow it to survive over the winter. The cool weather slows the plants growth down and the roots will not spread into the surrounding soil, anchoring the plant and providing moisture to it as well.
So how do you know if your grass is a cool or warm weather variety? I usually reference the following books:
Good luck dividing your ornamental grass!