Answer: Depending on which variety of ornamental grass you have it may be only marginally hardy in your area, but in either case there is not too much to be done to the grasses in the fall. Apply several inches of organic mulch such as shredded bark around the plant but not over top of it. Since the seed heads are so decorative many gardeners leave them on the plant until early spring, other gardeners will trim the plants back by about half in late fall because the plants may flop open and look untidy under ice or snow, this is a matter of personal preference. Leaving a good amount of foliage in place does help protect the crown from the cold, but then in early spring you should cut the entire plant off as short as you can manage -- about three to six inches tall is a good level to try for. This allows the new growth to start easily when the weather warms. You might also apply a balanced fertilizer or a top dressing of compost or well rotted manure in early spring.
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