What Is Wrong With This Picture?

By: Mark Arena, Clemson Extension Service Horticulture Agent and Barbara H. Smith, Consumer Horticulture Agent, Home and Garden Information Center

What is wrong?

One question that returns every fall season is: “when is the best time to prune ornamental grasses?” Pictured above is an ornamental grass commonly referred to as Maiden Grass (Miscanthus sinensis). It was pruned in mid-October, and the picture was taken on November 15. The location of this planting is Upstate South Carolina. What are your observations, comments, and conclusion?

In the picture, there are several observations that may be made. First, the grass is already starting to regrow since it was pruned too early in the year. Secondly, the plants were and still have a portion of actively growing foliage (this is the green portion of the stems). Thirdly, the plants were not cut at the recommended height. Finally, the height of the pruning varies between the grasses. My only comment is that Maiden grass is now considered a non-native invasive plant and is not recommended for use in South Carolina. The first conclusion is to prune when the grass is completely brown or in the early spring before the new foliage starts to emerge. Secondly, cut all grass at the recommended height of 4 to 6 inches. Finally, in the spring remove all dead debris within the grass to prevent it from rotting in the center.

Let’s have a brief overview on the proper pruning practices for ornamental grasses. Grasses respond and start to grow based primarily on temperature. There are two types of ornamental grasses which are classified based on the time of year they start to grow. Some grasses will start to grow in early spring when temperatures are still cool, and others will wait until the soil is warm and temperatures are more stable. Therefore, they are categorized as “cool season” or “warm season” grasses.

The following are cool season grass pruning recommendations. Trimming back the dead stems of cool season ornamental grass should wait until the early spring when the threat of spring frost in your area has past, unless the plant becomes unattractive through the winter months. For the coastal area, this is around March 15, in the mid-lands about March 30, and approximately April 15 for the upstate. If winter temperatures remain seasonably warm along the coast, these grasses may even remain green through the winter. For the ones that remain semi-evergreen, you should only cut off the brown or winter injured foliage in the spring. The recommended height to cut these grasses back to is approximately 4 to 6 inches. Trimming cool season grass more than suggested might irreparably harm the plant.

Some of the more popular cool season grasses include, Fescues (Festca sp.), Blue Oat Grass (Helictotrichon sempervirens), and Tufted Hair Grass (Deschampsia cepitosa).

Warm season grasses are the next group. This group grows actively during warmer months of the year and remains attractive during periods of high temperature and limited moisture. Warm season grasses turn shades of brown as the weather turns colder. Once warm season grasses turn completely brown, you can trim them back at almost any time. However, the recommended time is in the spring once the threat of frost has long passed. If you like a tidy looking landscape, feel free to prune them back in the fall when they are completely brown. The recommended height for fall pruning is to remove 2/3 of the upper portion of the plant.

Examples of warm season grasses include Upland River Oats (Chasmanthium latifolium), Japanese Silver Grass (Miscanthus sinensis), Pampas Grass (Cortaderia selloana), Perennial Fountain Grass (Pennisetum sp.), Switch Grass (Panicum sp.), and Muhly Grass (Muhlenbergia capillaris).

Some general advice and recommendations:

Leaving the dried grasses and seed heads in your landscape for winter interest is always an option! Be aware that not all ornamental grasses look good through the winter; therefore, trim back those that don't look good in the fall.

The best time to cut back pampas grass is in late winter, just before the plant begins sending up new foliage. Waiting until the end of winter allows you to enjoy the plumes all year.

Burning grasses should never be done as it pollutes the environment and creates fly ash that is not healthy to breathe. In addition, burning can kill the grass.

The recommend method for getting an even cut across the grass is to use a rope. Simply tie the grass with the rope above where you are going to cut. This makes it easier to cut the grass and bundles up the cut grass for easier removal.

The best time to cut back fountain grass is in the late winter or early spring. The exact timing is not as important as making sure that you prune fountain grass back before it starts actively growing. Avoid cutting back fountain grass in the fall, as the plant has not yet died back all the way.

Hopefully, this information has given you a better understanding of pruning practices for ornamental grasses. If you have any questions, concerns, or comments, please feel free to send me an email at marena@clemson.edu. For additional information on ornamental grasses please feel free to visit Clemson Home and Garden Information Center (HGIC) website (http://www.clemson.edu/extension/hgic/) and place the words ornamental grasses in the “search” box.

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