Generally, ornamental and native grasses are grown for their extreme fall and winter interest, so do not cut your grasses down in the fall.
Leave your grasses standing to create movement and interest during the fall and winter months in your landscape. Hold off cutting your grasses until spring just before they start to grow.
What about your perennials and mixed borders?
A good rule of thumb is that if any of your annuals or perennials had some diseased foliage this year, then cut and remove all infected debris (including the leaves) and dispose of them at your local yard waste site to prevent disease from overwintering on last year’s foliage.
Plants in your border that were not diseased can be allowed to stand in the landscape, so the birds can eat the seeds, and it allows beneficial insects places to overwinter. Leaving your plants standing also creates some winter interest and protection for wildlife too.