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Overcoming Problems With Ornamental Grasses

In-ground, grasses are perfect for planting in dry stream beds and among wildflowers. (Photo by Maureen Gilmer)

When landscape grasses take on full autumn color in the western states, they are always the focal point of the late season. It is the driest part of the year when their life cycle peaks after flowers pollinate, seeds form and are finally released into the wind to repopulate the land. These annual reproductive structures are why ornamental grasses own the fall garden when few other plants bloom. Even in the early winter, the standing flower stalks offer attractive interest through the snow.

It’s the less desirable grass habits that are less understood. These influence selection, placement, and other issues you won’t hear about elsewhere. Here are some tips to help you select and design grasses into your landscaping, so they don’t become problems later on.

Grass Litter

When this Pennisetum sheds flower parts and seeds, it goes right into the pool. (Photo by Maureen Gilmer)

After pollination, grasses shed their flower parts. When the seed is released, they shed their hulls. A lot of fine litter is dispersed over a long period. If the grasses are located upwind from a swimming pool or water feature, the litter is blown directly into the water. This can make it challenging to keep pumps and equipment clear and the water quality sparkling.

Therefore, know the direction of your prevailing winds and storm winds before you decide where to plant grasses. Limit planting areas downwind or away from the pool.  However, it’s common for wind direction to change with the seasons, so if you plant them poolside, planting them downwind is not foolproof. Cutting the seedheads back may be necessary.

Invasive Grasses

Native deer grass (Muhlenbergia rigens) is quite long-lived and fairly trouble-free.

The reason you hear so much bad press about fountain grass (Pennisetum species and varieties) is that they love our climate and sprout anywhere there is enough moisture to grow. There are many ornamental species with weedy tendencies. Some garden favorites are hardy perennials, like foxtail fountain grass (Pennisetum alopecuroides), but in milder western climates tender perennial forms, like purple fountain grass (Pennisetum setaceum ‘Rubrum‘), will survive several seasons, too. Perennial forms don’t die back and are long-lived. Pennisetum such as these are displacing less aggressive native species in low, moist areas.

The same applies to your yard. If the seeds fall near irrigation heads, they sprout into weeds. It may have been open ground, but now it’s become a longterm weed problem. Such introductions are hard to stop and take a few seasons of dedicated handwork to clean out.

Runner Grasses

Runner grasses, like Japanese bloodgrass (shown) and Bermuda grass, will invade and become intertwined with perennials and other ornamental grasses.

Runner grasses spread, unlike stayput bunch grasses. The common southern lawngrass, Bermuda grass (Cynodon dactylon), is the poster child for a host of aggressive runner grasses that spread fast and invade perennials and well-behaved bunchgrasses. Another ornamental grass to add to the equation is Japanese bloodgrass (Imperata cylindrica), with its red-tipped blades and fast-spreading runners that will quickly overtake moister beds. The problem is the worst when runner grasses overtake bunch grasses. They creep unseen beneath a garden grass, and then once well rooted, the runner grass becomes nearly inextricable. If the bunchgrass is large and broad, the two grasses will forever be bound together, foliage plaited into a nest, and there’s no separation once established. Prevention is everything. Beware adding these, or any aggressive runner grass, to your yard or garden.

Short-Lived Grasses

This fine-textured Mexican hair grass in full flower and nodding in the breeze. (Photo by Maureen Gilmer)

Grasses are ephemeral plants by nature, adapted to range fires in the wild, grazing, floods, and landslides. Those that evolved with a long life span prove that they have adapted to climate change, since well before the Pleistocene, and are still super adapted for the future. The most long-lived, resilient grasses to grow in arid gardens are native deer grass (Muhlenbergia rigens) and its kin (Muhlenbergia group). But, more short-lived species, such as the windswept Mexican hair grass (Nassella tenuissima) or purple fountain grass, die out in just a few years.

Clump Splitting

These newly planted blue fescues have not suffered crown split yet.

Early in the life span of blue fescue (Festuca glauca varieties), the mounds of icy blue needles are perfect hemispheres. Like many other grasses, fescues grow too tall and heavy then split down the middle, allowing light to reach the root crown at the center of the clump. The direct sun sears those formerly shaded crown stems, causing premature aging while the rest of the plant is perfect.  Replacement is often required if the plants are depended upon to create full geometric domes. This is a natural process for fescues, so they are best planted with other species that take up the slack visually if they decide to split.

Midwinter Decline

In warm-winter climates, grasses are cut back midwinter, in colder ones, late winter. (Photo by Maureen Gilmer)

As dramatic as sweeping monocultures of grasses are, they are best used with ever-beautiful support plants due to an unattractive period in midwinter, even if not fully dormant. The grasses are routinely cut back to just a few inches to simulate a cold event. This removes dead and dormant growth as well as detritus inside the clump to make way for the renewal of foliage. To avoid the barren ground, it’s wise to choose other evergreen plants to carry this composition until green grass shoots start up again in spring. Renewal is part of grass biology, so cutting back is regenerative and makes them healthier overall.

Ornamental grasses are an important cornerstone of today’s arid-zone gardens. Those species adapted to warmer climates without summer rain offer a change in texture as well as wind-blown beauty in containers on porch or patio. They require lots of nutrition, so be sure to use Black Gold Moisture Supreme Container Mix when planting for efficient water-holding potential and water conservation. The best grass for containers and garden at higher elevations or further north are Miscanthus varieties, which ask for a bit more water. (Choose low-seeding or sterile forms, such as giant miscanthus (Miscanthus x giganteus).)

While shrubs and succulents can be inanimate, the slightest breeze begins the gentle sway of a thousand soft grass blades. The animation of the nodding flower spikes liven up a dying landscape in the dry autumn winds.

 

Bold Grasses for the Fall Garden

Pennisetum ‘Jade Princess’ is a spectacular variety with huge, showy plumes.

With many summer-flowering annuals beginning to fade, autumn is the season when many ornamental grasses take center stage. I do not want to dismiss those ornamental grasses that look quite good in the garden during the summer, and some that look good all year round, but by careful selection, a gardener can pick certain grasses that are at their prime in the fall season. In my neighborhood, I see ornamental grasses being used much more frequently than in years past. Nowadays, there is such a large assortment available that I will highlight favorites that have consistently performed well in my Pacific Northwest garden.

Keep in mind that most grasses, these included, grow best in full sun and require well-drained soil with average to good fertility. Hardiness varies among the perennial species. Adding a little Canadian sphagnum peat moss at planting time is always welcome.

Ornamental Millet

The bold ‘Purple Majesty’ ornamental millet stands out in the garden.

An annual grass that I regularly see in garden centers is the bold purple ornamental millet (Pennisetum glaucum ‘Purple Majesty’), which reaches up to 4 feet. It provides purple foliage, which can be quite striking from summer into fall. If planted in a full sun location, the blades will be the darkest. Upright purple flower spikes appear above the foliage, usually in mid to late summer. More compact ‘Jade Princess’, which reaches 2-2.5 feet, is another exceptional variety with lush green blades and large, puffy purple-red spikes that curve at the tips. Ornamental millet spikes are often cut when fresh and dried for Halloween or Thanksgiving arrangements. If the seed heads are left on, they are an excellent source of food for wild birds.

Chinese Feather Reed Grass

The striped Chinese feather reed grass ‘Gold Bar’ is a real stand out all season long.

If I had a favorite Chinese feather reed grass, Miscanthus sinensis ‘Gold Bar’ would be top on my list. It is an introduction by Joy Creek Nursery in Scappoose, Oregon as well as a Great Plant Picks selection. As the name indicates, ‘Gold Bar’ has golden, zebra-stripe bars that begin at the base of the green blades and continue to the tips. It is ideal for large containers or planted in a garden in need of striking foliage throughout the season. ‘Gold Bar’ is more compact and upright for feather reed grass, with a maximum height of 4-5 feet. It is also hardy, surviving in USDA Hardiness Zones 5-8. The foliage turns tawny brown in winter and has nice structure, so wait to cut it back to the ground in late winter.

Purple Moor Grass

Variegated purple moor grass has attractive variegated foliage. (Image by Daderot)

For an easy-to-grow, small, clumping grass, variegated purple moor grass (Molinia caerulea subsp. caerulea ‘Variegata’, Zones 4-9) is an outstanding choice. The yellow and green variegated foliage grows 1-2 feet tall and works well as a border plant along a flower bed. In late summer and early fall, clusters of airy yellow to purple flower spikes appear about 2-3 feet above the clump.  The purple flower spikes contrast well with the variegated foliage, and as they age, they turn yellow and look quite attractive into the autumn season.  Cut the plant to the ground once the flower stems have faded. During the growing months, moor grass needs additional water when it is dry.

Tall Moor Grass

Another worthy garden grass of the same species is tall moor grass (Molinia caerulea subsp. arundinacea).  This one has broader gray-green leaves that can form a clump about 3 feet tall with blooming stems up to 8 feet.  One of its most popular, best forms is ‘Karl Foerster’ (to be distinguished from the more common feather reed-grass, ‘Karl Foerster (Calamagrostis × acutiflora ‘Karl Foerster’), which has 2-3 foot leaves and feathery stems that can reach up to 7 feet.  The form ‘Skyracer’ has plum-colored plumes that reach straight into the air. Tall moor grass survives in Zones 5-8.

Giant Feather Grass

Giant feather grass looks like a glistening cloud when in full bloom.

Native to Spain and Portugal, the tall flower spikes of giant feather grass (Stipa gigantea) rise high above the foliage and can reach up to 6 feet in height from clumps that are 2-3 feet high. It is drought-resistant and hardy in Zones 6-10. Be sure to plant it where it can be appreciated as a specimen plant.  It makes a very bold statement in a gravel garden.  Airy stems of yellow flowers appear in summer, and a large plant in full bloom almost looks like a glistening cloud.  It is a real attention-getter when in full bloom. Cut it to the ground in late fall or when the leaves begin to fade.

This listing presents a small sampling of some of the ornamental grasses that are available at local garden centers.  If there is a particular public garden in your area that features perennial grasses, fall is an ideal time to visit to get an idea of what they look like at the end of summer in your area.  Fall is also a good planting time.