Proper pruning of ornamental grasses

I have to admit, this article is about one of my pet peeves, plant butchering. I hope to educate homeowners and commercial landscapers about how the specific growth habits of different plants should dictate the appropriate pruning methods.

If you remember only one rule from this article, it should be this: some plants should NOT be sheared into unnatural balls or boxes!

This caveat especially applies to tall grass species and similar plants. Their leaves are showy, and if they’re chopped halfway off the attractive form of the plants will be ruined permanently or for many years to come. If grasses are “shaped” like hedges, the result is unsightly, ragged balls of half-length leaves from which the points of longer, newly emerged leaves stick out, looking like a horrible haircut.

Ornamental grasses and other plants with a similar appearance have characteristically long, narrow, strap-like leaves, and they’re frequently used as focal points in a landscape. Depending on the species, their foliage can be stiffly upright, gracefully arching, or tufted. They don’t have branches with leaves scattered along their length; instead, their leaves all emerge like a fountain from a central, ground level clump or “crown.” Their leaf buds are at the root-shoot junction at or near the soil level.

So how should these plants be properly maintained? Let’s start with the grasses.

Ornamental grasses can be grouped into two categories: warm season and cool season.  Each has different pruning needs.

Warm season ornamental grasses

Warm-season grasses grow best at temperature between 80 and 95°F. They grow robustly during spring and summer, flower in fall, and go dormant from late fall through early spring. Some commonly planted decorative species of warm season grass include Deergrass (Muhlenbergia rigens), Blue Grama (Bouteloua gracilis), Fountaingrass (Pennisetumspecies), and Maiden Grass (Miscanthus sinensis).

As a general rule, warm season grasses need a pruning only once a year or less. Oftentimes, they can be “combed” with gloved hands or with a small rake to remove dead leaves and neaten their appearance, without the need for severe pruning. When grooming the plants, start at the base then pull upward to remove old growth. If necessary, the clumps can be renewed by cutting them down to a height of 2 to 4 inches (not flush to the ground, to protect the crowns).

The best time to cut back warm season grasses is in late winter or very early spring, just before fresh leaves begin to emerge. (The exact timing of new growth depends on climate and precipitation.) The exception to this seasonal trimming rule is if you live in a fire-prone area; in this instance, cut back dried grass clumps in the fall, and protect their tender crowns with a layer of loose organic mulch.

Warm season grasses are best undisturbed through winter. Their dried flowering stalks, seedheads, and leaves change color to subtle shades of golden-brown, tan, and white during dormancy, adding a decorative touch and visual interest to the winter garden. The old leaves also protect the tender crown of the plant from frost damage, and they provide shelter for birds and beneficial insects.

Poorly pruned deergrass, with new leaves emerging from improperly shaped balls, contrasted with a beautifully maintained deergrass specimen. (Photos © Kathy Ikeda)












Cool season ornamental grasses

Cool season grasses favor temperatures between 60 and 75°F. They begin new growth as temperatures drop in the fall, and they prefer more moisture than warm season grasses. Many cool season grasses flower in spring and stop sending out new growth in summer. In our mild climate they grow year round, although their growth slows significantly in heat. Some common ornamental cool season grasses include Feather Reed Grass (Calamagrostis x acutifolia), Blue Fescue (Festuca glauca), and Blue Oat Grass (Helictotrichon sempervirens).

Periodic upkeep to remove dead leaves and spent seedheads is sufficient to keep these grasses attractive for many years, without trimming/pruning. Use the same grooming method as with warm season grasses.

Cool season grasses often don’t need cutting back unless the foliage is damaged or dominated by old leaves. If it’s necessary to renew these grasses, cut them down by about 2/3 before the fall growth spurt, or early in summer if fire danger is a consideration. Trimming cool season grasses too severely or frequently or at the wrong time of year — such as during peak fall and winter growth or during severe cold weather — will stress the plants and make them more susceptible to disease or frost damage. Once new growth fills in, the older, shortened leaves can be selectively cut out with pruners to improve appearance if needed.

Cool season grasses are also pretty in winter, when cold temperatures often leave their foliage tinged with hues of bronze, purple, red, or gold.

Some final notes

Make sure your pruning tools are clean and sharp to prevent disease and ragged cuts. Hand pruners work well for small grass species, but if you have large grass clumps to cut back, use manual hedge shears, electric hedge trimmers, or a weed trimmer fitted with a blade attachment.

One useful trick is to bind the leaves of each grass clump together before pruning; this keeps them out of the way and makes clean up easy. If you use natural jute twine instead of man-made materials, the entire bundles can be composted or put in green waste bins.

Some excellent sources of information on ornamental grasses and their care are:

  • “The Encyclopedia of Ornamental Grasses” by John Greenlee
  • Pruning Ornamental Grasses,” an online article by the UC Master Gardeners of Sonoma County
  • A short YouTube video entitled “Pruning Ornamental Grasses” by the University of Illinois Extension

Next month, we’ll take a look at how to properly maintain grass-like plants.

For gardening-related questions, call the UC Master Gardener office at 209-953-6112, or visit our website.


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