Pruning Ornamental Grasses

Tuesday, January 30, 2018 3:42 PM

When is the best time to cut down my ornamental grasses?  And, I have a lot of it, so what’s the best way to prune them?  Angela W., Tulsa

Grasses and grass-like ornamentals are great resources for gardens and landscapes.  Whatever the garden need, there is likely a grass to offer a solution.  They come in all sizes, from a few inches to more than 10 feet tall, and may be annual, perennial, evergreen or deciduous.  The spectrum of colors, leaf patterns and textures will provide something to please most any gardener.

Many of the true grasses have intriguing fall colors and evolve to an earthy tan color for winter.  So, for most of them like the large Pampas Grass, the show is not over in the wintertime.  They have large decorative flower and seed heads that catch the breeze which adds movement and sound to the winter landscape.  They not provide interesting landscape, but also provide valuable shelter from the weather for birds.  These seed structures can also be of value to use indoors for long-lasting floral arrangements.

In order for new growth to emerge in the spring, the old stems should be cut to a height of a few inches in late winter.  Do this before new growth begins. If you wait too long to trim in the spring after new leaves emerge, you will remove the tops of new growth.  Cutting back allows needed air and sunlight to get into the root zone and provides room for new growth.  The stems on these grasses are substantial and, in many cases, can be difficult to cut.  The use of a powered hedge trimmer seems to be most effective for the larger grasses, such as Pampas and Zebra grass.  

As for small ornamental grasses, Liriope is one of the most common grasses around, but is actually not a grass at all.  It is of the lily family from Asia.  The many varieties available are very durable and tolerant of most soils.  One of their greatest assets is shade tolerance.  They do well in partial to deep shade and will get sunburn in full sun.  These plants are not perennial but are evergreen and retain their usual color throughout the winter.  Be aware that both tend to get a fungus called anthracnose which causes brown spots.  By the end of winter, they usually are looking a bit ragged, but then predictably send up a flush of new growth in the spring, so late winter trimming is also needed.

Since the total height of these is less than a foot, lawnmowers or weed eaters are useful for removal of old growth.  Cut the leaves down to 3 inches or so and discard the old leaves if they are showing any signs of fungal growth.  Pruning time is also a good time to consider dividing the grasses.  This needs to be done every three to four years.  Doing so will help to maintain vigor and to prevent disease in the central root area.