Bad Boys of Ornamental Grasses

Many ornamental grasses are graceful garden ornaments, and then there are others that you'll wish you'd never planted.

'Moudry' fountain grass (Pennisetum alopecuroides 'Moudry') — photo by Rick Darke

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Those chocolate-brown seed heads look so striking, I thought. It would be fun to mix them with white-flowered forms. And that's how I came to buy three fine specimens of 'Moudry' fountain grass and take them home. A year later there were plenty of seedlings that wound up sporting brown flowers and four years later I'm still digging 'Moudry' up. The more sedate white-flowered 'Hameln' has never multiplied.

We love ornamental grasses for their texture, motion, flowers and multi-season interest. But even in this splendid family of plants, there are some black sheep: a few are invasive.

Loved for its form and its bristly seed heads, fountain grass (Pennisetum sp.) is a staple of many gardens. Selected cultivars behave nicely but several species self-sow freely in warmer climates. Pennisetum alopecuroides ‘Moudry’ and 'National Arboretum' can be a particular nuisance, if not very invasive. That's why most fountain grasses don't pose a problem in Rick Darke's Pennsylvania garden but they multiply with abandon in California.

River oats (Chasmanthium latifolium) — photo by Rick Darke

Invasiveness is a matter of region, culture and genes, says Darke, author of the new Timber Press Pocket Guide to Ornamental Grasses. "Casmanthium latifolium [river oats] normally grows along rivers and floodplains. In Kentucky, Tennessee and parts of the Carolinas, you have a long season and a moist climate; if you put it in there, it will seed and go everywhere. But it's been in my garden for 14 years and it's never been a pest." Not only is the season shorter in his locale, but as a matter of long-standing practice, no plant in Darke's garden gets supplemental water.

And even if you live in an area where Casmanthium could be a problem, you can control it to some extent with culture. "If you have a south-facing area, a big tree with surface roots and soil compaction, that grass is going to be fine, but if you add lots of mulch, drip irrigation and fertilizer, that grass is going to be all over the place."

The grass that's gotten the worst press lately is the venerable Miscanthus sinensis. Appreciated for its fine texture, density and showy flowers, it's the problem grass of the mid-Atlantic and southeast, not only creating a problem for the gardener but sometimes escaping cultivation altogether. But find a region that's either dry or short-seasoned, and there's no problem. "In the Tennessee bottomlands where it's moist or wet, you're crazy to plant it, but it's a non-issue in New York," Darke says. "And in California, unless you water it, it dies. There, Miscanthus is not a problem."

The species is the worst offender; many cultivars are pretty safe to use. The attributes that help ensure that a Miscanthus is not going to get away from you is sterile seed or early bloom. "To be invasive on a seed basis, the plant has to flower early enough to complete the maturation of seed so that it's dropping fertile seeds," Darke says. "Old-fashioned Miscanthus such as zebra ('Zebrinus') or 'Gracillimus' need a long season to complete the whole cycle, so they're safer than many of them. If Miscanthus blooms in late September or October, you won't have a long enough season to set viable seed, but if it blooms in July or August, chances are really high that it will complete the cycle and set fertile seed."

Other invasives

Ribbon grass (Phalaris arundinancea). This old standby has often been used as a quick and easy groundcover, but homeowners usually regret planting it. Tolerate of a wide range of soil types in sun and shade, this pest can be very invasive. If you want to try ribbon grass, however, choose one of the more easily controllable cultivars such as ‘Feesey’s Form’ and ‘Picata’. Three to four feet tall, with white seed heads from June to October. USDA Zone 4 to 8.

River oats (Chasmanthium latifolium). A native of moist terrains but tolerant of drought, this clump-forming grass begins flowering in summer. Flowering begins in early summer and the seedheads later turn rusty-brown or bronze. River oats can be invasive, spreading by rhizomes or by seed. The oatlike seed heads are ornamental on the plant or in dried arrangements. Two to three feet. USDA Zone 3 to 9.

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