Ornamental Grasses:  easy, beautiful — and invasive?

Ornamental Grasses: easy, beautiful — and invasive?

  • By Catherine Caldwell
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  • November 2017 - Vol 3 No.11
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  • 0 Comments

The “New American Garden” style of Ohme Van Sweden popularized ornamental grasses. Photo courtesy of UK Garden Photos.

Ornamental grasses took the garden world by storm in the 1970’s and 1980’s when the “New American Garden” concepts of the now-famous landscape design team of Oehme and Van Sweden — featuring naturalistic gardens dominated by ornamental grasses — began to appear in public and private gardens and on magazine covers.  That popularity continues, fueled in no small part by the easy nature of most ornamental grasses, which need little attention from the gardener to thrive.  But for the newbie gardener — and even for the veteran — there are new grasses to consider and some unwelcome facts to learn about some old favorites.

The favorites that began to appear in nurseries and gardens a few decades ago and which continue to be readily available are mostly natives of Asia.   At that time, the native/non-native status of a plant received little if any attention.  The most popular grasses employed in the “New American Garden” were cultivars of Miscanthus sinensis, Calamagrostis brachytricha and its relatives, and Pennisetum alopecuroides.   These varieties, like most ornamental grasses, are extremely easy to grow, often thriving in dry, poor soil and requiring only a once-a-year haircut in later winter or early spring.  But today, a gardener can find a wide variety of native grasses, so let’s start there.

Pink Muhly grass (Muhlenbergia capillaris)
Photo: Catherine Caldwell

 

Two warm-season grasses you should definitely consider are native to the Southeast.  The first is pink muhly grass (Muhlenbergia capillaris), which puts on a massive disply of pink panicles in autumn, and is drought-tolerant to boot. It wants a sunny, well-drained site and will grow to 3 feet tall.

 

 

 

 

Muhlenbergia capillaris ‘White Cloud’ 
Photo: Toby Gray

There’s a new variety of muhly grass named ‘White Cloud’ — which has white panicles.  Either variety of Muhly grass will make a striking statement in the garden.  And if you’re wondering about plants that would make good companions for muhly grass, you can take a cue from the gardens at Martha Jefferson Hospital, where pink muhly grass has been grouped near purple New England asters, Symphyotrichum novae-angliae. For more information about these tall New England asters, look at www.missouribotanicalgarden.org

 

Another native warm-season grass is known as switchgrass or panic grass (Panicum virgatum).  It is tolerant of most soils, including sand and clay, even wet soils, but needs full sun to perform best.

Panicum virgatum (switchgrass)
Photo: Matt Lavin

Several varieties of switchgrass are available, including ‘Cloud Nine’ (tall, upright, to 8 feet tall),  ‘Prairie Fire’ (burgundy coloration, 3 to 4 feet tall) and ‘Northwind’ (very upright, olive green, 5 feet tall).  Switchgrass forms tall, slowly-spreading clumps that can be divided every 3 to 4 years.  For more information on this easy grower, see www.clemson.eduplantprofiles/panicum-virgatum

A long-time favorite in the tall and upright category is Calamagrostis, a non-native which looks great in masses and can lend your garden a needed upright element. A famous cultivar is Calamagrostis x acutiflora ‘Karl Foerster’, which is a hybrid of European and Japanese species.  For more information about this grass, take a look at the Plant Finder feature of the Missouri Botanical Garden website, www.missouribotanicalgarden.org/PlantFinder.

Calamagrostis x acutiflora ‘Karl Foerster’

 

One genus of natives worthy of more garden use is Andropogon, including Andropogon virginicus and Andropogon gerardii,  both of which are native to our area and which you’ll often see on roadsides and fields.  Its common name is broomsedge, but this humble plant can add a striking element to your garden.

Native broomsedge (Andropogon virginicus) in a garden bed.
Photo courtesy of Hoffman Nursery

 

Splitbeard Broomsedge (Andropogon ternarius)
Photo:Tom Potterfield
Mt. Cuba Center.

 

Another member of the Andropogon genus now being sold by native plant vendors is splitbeard bluestem, Andropogon ternarius, which grows up to 3′ tall with a 2′ spread. Its seed heads are sparkly little white puffs.   ncsu.edu/goingnative

If you’re looking for a shorter grass — 2 to 3 feet wide and tall — you might be directed to fountain grass, Pennisetum alopecuroides, which is is one you’ll at least want to be aware of as you wander the nursery aisles.

Pennisetum alopecuroides showing off its autumn colors. Photo: SB Johnny, courtesy of Wikimedia Commons.

But fountain grass has been so widely planted that the risk of invasivenss is increased, so ask for alternatives.  Our nurseries and garden centers can offer us many alternatives, especially if plenty of us are asking for them.

 

 

One of those alternatives is a native — prairie dropseedSporobolus heterolepis, a showy easy-to-grow grass that tolerates deer,drought, erosion, dry soil, shallow-rocky soil, black walnut,  and air pollution.  I found this long list of positive traits at the Plant Finder feature of the Missouri Botanical Garden website.  http://www.missouribotanicalgarden.org.

Prairie dropseed in fall.

 

Little bluestem, Schizachyrium scoparium, is an under-used native that is unfussy about soil or moisture and apparently needs only a sunny spot to perform well.  It’s quite small — averaging about 2 feet wide and 2 feet tall — and in summer, it is a blue-green color that changes to rusty rose in fall.

Schizachyrium scoparium ‘The Blues’ at the Battery Conservancy, New York City

Several cultivars have been developed, one of the most popular being ‘The Blues’ — which, judging from this photo taken at the Battery Conservancy, lives up to its name.

 

A very small grass popular in the nursery trade is Festuca glauca, which has striking blue-gray foliage and forms neat mounds.  Perhaps the most popular variety is ‘Elijah Blue’ —

Festuca glauca
Photo: Peggy A. Lopipero-Langm

but keep in mind its limitations:  short-lived, needs regular dividing and not tolerant of warm humid weather.  So there has to be something good about it, right?  It IS tolerant of dry

Carex pensylvanica
Photo: Susan Harris

conditions.  But if your garden needs a diminutive grass, you might try instead one of our native sedges, such as Pennsylvania sedge (Carex pensylvanica), which grows only a few inches tall, is a bright green, and which tolerates dry, poor soil.   www.fandm.edu/center-for-the-sustainable-environment.

 

The Ornamental Grass You Should NOT Plant:  

Miscanthus sinensis
Photo: Miya.m

 Miscanthus sinensis — newest plant invader of the Mid-Atlantic Region

A number of years ago, I planted two cultivars of Miscanthus sinensis in my yard —  ‘Variegatus’ and  ‘Gracillimus’.  These are two of the popular cultivars of Miscanthus — which is known by many names, including Chinese silver grass, eulalia, and maiden grass.  To my surprise,  I soon had Miscanthus seedlings popping up all over my yard, and at first I thought this was terrific.  These volunteers were filling in empty spots in my garden beds and, with their tall, waving leaves and plumes,  creating lots of drama.  But these mini Miscanthus plants eventually grew to gigantic proportions, and even worse, they were appearing in my neighbors’ yards and on the edge of a nearby woods.  I began to worry.  So naturally, I started Googling.

My research led me to the conclusion that the two cultivars in my yard had been up to some cross breeding and had produced the species form of Miscanthus sinensis.  And that turns out to be a very bad result because the species form of Miscanthus spreads rapidly via seed and has been labeled an invasive species here in Virginia and in the greater mid-Atlantic region.  www.dcr.virginia.gov/natural-heritage/invasive-plant-list ; www.virginia.edu/blandywww.invasiveplantatlas.org.

Miscanthus sinensis covers a large portion of the Tonomine Highlands in Japan.
Photo: 松岡明芳

Miscanthus sinensis is native to eastern Asia, and in fact, it is something of a bully in Japan.  The species was brought here for the ornamental garden trade and it has been the subject of much hybridization, with over 50 cultivars developed over the years.  Some of these cultivars became tremendously popular in the 1980’s thanks to their use in the gardens of Oehme and Van Sweden.

But then observers began to notice miscanthus spreading into areas where it had not been planted, mostly in the Mid-Atlantic region.  Early on, scientists noted that it seemed to have “escaped” from cultivation.  Still, some of the early scientific research indicated that miscanthus spread mostly through rhizomes,  and the accepted wisdom for many years was that the cultivars of miscanthus were sterile and did not produce seed.    But in 2010, research at the Chicago Botanic Garden proved that notion wrong.  Scientists examined the seed set of many cultivars of miscanthus, and found that almost all set viable seed, some of them in very high numbers, a factor that can enhance invasiveness. As the researchers explained:

“Most cultivars set filled seed, ranging from 14 to 349,327 seeds per plant; only four produced no seed over the course of the trial. Most cultivars of the species represent a high risk for self-seeding in Zone 5. Because Miscanthus sinensis is self-incompatible (8), risk of self-seeding increases when two or more cultivars are grown together.”

—“Differences in Seed Set and Fill of Cultivars of Miscanthus Grown in USDA Cold Hardiness Zone 5 and Their Potential for Invasiveness,”  Journal of Environmental Horticulture, www.hrijournal.org (March 2012).

As noted above, Miscanthus is “self-incompatible” — meaning it requires two or more cultivars or species to set seed.  My research revealed that the “wild” miscanthus species that have naturalized in my yard are each a unique individual or genotype.  This means that these “wild” plants can breed with each other, and thus, set a lot of seed.  “Miscanthus: Ornamental and Invasive Grass,” HortScience (Mary Hockenberry Meyer, 2004).  Aha! I seemed to have solved the mystery of my expanding population of miscanthus seedlings.  Now I knew what I had to do.

First of all, I knew that one of my cultivars had to go.  Because I was so smitten with the ‘Variegatus’ foliage, I got rid of the ‘Gracillimus’.   Then I set about removing the giant species plants that were dropping their own seed. Not so easy!  The species sets deep rhizomes that are extremely resistant to shovels!  Plus, they had apparently shed a lot of seed because new seedlings continue to appear every year.  At least the new ones are easier to dig out.

Should you continue to plant Miscanthus?

  • First and foremost, DO NOT PLANT the species form of Miscanthus sinensis. Few nurseries off the species form these days, but it’s wise to ask. And don’t take one from a neighbor!
  • Avoid planting miscanthus cultivars, and most important, do not plant more than one cultivar on your property.
  • And if you simply must plant a miscanthus cultivar, use only a vegetatively propagated cultivar that has shown little or no evidence of self-seeding in our area.  Be sure to ask about the cultivar you’re considering — how it was propagated and whether it has been self-seeding in our area. Asking these questions of staffers at garden centers and nurseries will not only spread a greater understanding of the problem, but may even turn up some helpful information.
  • Remember, there are plenty of alternatives that provide the height and presence of a miscanthus.

One researcher in this field has created a miscanthus website to help gardeners and others dealing with miscanthus questions and problems; she plans to update it regularly. Check it out at miscanthus.cfans.umn.edu.  There’s something else we gardeners need to know about miscanthus:  it is being developed and genetically manipulated for bioenergy fuel crops.  Grounds for worry?  Perhaps, and that seems like one more good reason to avoid using miscanthus in the garden.

Are you wondering about the invasion potential of miscanthus cultivars you’d like to plant or are already in your yard?  You can compare the seed set of various miscanthus cultivars at the website of Fine Gardening magazine, which has a chart summarizing the research at the Chicago Botanic Garden, www.finegardening.com/which-grass-invasive-which-isnt.  Keep in mind that the seed set in Chicago — where the research was conducted — will probably be different in our region because environmental factors such as climate have a major impact on the number of viable seeds set.  Still, I found the chart very interesting.  The ‘Variegatus’ cultivar, which was once thought to be sterile, in fact produced 211 viable seeds in the Chicago trials.  But compare that number to ‘Silberfeder’ — 3,975 viable seeds per plant.  Or consider the whopping number of viable seeds produced by ‘Malepartus’ — 203,699.

What if you already have two miscanthus cultivars in your yard?  Well, I’m sorry to say you’ll need to keep a watchful eye out for miscanthus seedlings —  or get rid of one of those cultivars.  Remember, when two or more of these cultivars are grown together, seedlings are possible, and almost always revert back to the “wild type” or species, which can become quite aggressive, especially here in our area.

Armed with my new knowledge, I did a little sleuthing at local nurseries and found the usual suspects, i.e., the well-known varieties.  But there was more to intrigue me:  a  new cultivar of miscanthus sinensis which was advertized to be non-seed-bearing.  Hallelujah!  Could this be true?  Well, almost.

Scientists have indeed developed a number of seedless plants through the creation of triploids.  You’ve no doubt eaten some triploids — seedless watermelons, for example, and also seedless grapes and oranges.  A triploid will look normal, but it has three sets of chromosomes — hence the name triploid — and reproductively-speaking, that makes all the difference.  To get really scientific about it, those 3 sets of chromosomes “cannot be divided evenly during meiosis, yielding unequal segregation of the chromosomes (aneuploids) or complete meiotic failure.”  If this stuff intrigues you, read more at  “Developing Non-Invasive Nursery Crops,” mountainhort.ces.ncsu.edu (N.C.State Extension, Mountain Horticultural Crops Research and Extension Center).

Efforts to develop a non-invasive triploid miscanthus have apparently been successful. Research at North Carolina State University — a hotbed of miscanthus invasion — indicates that “some triploid Miscanthus display substantial reductions (greater than 95%) in fertility compared with diploid controls.”  A 95% reduction would be greatly appreciated in my yard, but it’s still shy of complete infertility.    Nevertheless, the North Carolina researchers concluded that several of the triploids they examined had such “substantial reductions in fertility” that they may provide desirable, noninvasive substitutes” for the ever-popular cultivars now on the market.   “Fertility and Reproductive Pathways in Diploid and Triploid Miscanthus sinensis,” hortsci.ashspublications.org.

So apparently, these desirable, noninvasive triploids have indeed been developed and are now available at our local nurseries.  One of these triploids is named ‘My Fair Maiden’ — and we should all hope it is as infertile as claimed.  Read more about it at  www.ces.ncsu.edu.

The fact that miscanthus has become invasive in our area should not deter you from creating the kind of movement and drama that ornamental grasses provide.  Just look at the native alternatives identified above, and for more detailed information on native grasses, see our previous article, “Meadow Gardening,” Meadow Gardening, The Garden Shed.  The photographs above of the Muhly grasses ought to be inspiration enough!  But don’t stop there.  You can learn from the masters — Oehme and Van Sweden.  Their public and residential garden designs still demonstrate the many ways to employ ornamental grasses to exquisite effect — despite the fact that they often included miscanthus cultivars.

And we have a special opportunity to learn more about the “New American Garden” style of Oehme and Van Sweden because a traveling photographic exhibit of their many gardens is now in Charlottesville!  The exhibition opened two years ago at the National Building Museum, and is now on view at Campbell Hall at the University of Virginia until November 21, 2017.  For details about the exhibit, go to http://calendar.virginia.edu.

The New American Garden Exhibition Poster

 

SOURCES:

The Encyclopedia of Grasses for Livable Landscapes  (Rick Darke, 2007)

“Selection and Use of Native Warm-Season Grass Varieties for the Mid-Atlantic Region,”www.nrcs.usda.gov/Internet/FSE_PLANTMATERIALS/publications (Natural Resources Conservation Services, US Dept. of Agriculture, 2008)

“Delightful Ornamental Grasses: Coming to a Garden Near You,” The United States National Arboretum, Gardening Page (2007);www.usna.usda.gov/Gardens/faqs/OrnamentalGrasses.html

“Miscanthus: Ornamental and Invasive Grass,” HortScience, hortsci.ashspublications.org (Mary Hockenberry Meyer, July 2004) (abstract)

“Miscanthus: Ornamental and Invasive Grass,” www.hrijournal.org/doi/abs/

 “Fertility and reproductive pathways in diploid and triploid Miscanthus sinensis,” Hort-Science 46:1353–1357 (Rounsaville, T.J., D.H. Touchell, and T.G. Ranney, 2011), hortsci.ashspublications.org

www.invasive.org

“The Relative Risk of Invasion: Evaluation of Miscanthus x giganteus Seed Establishment,” Invasive Plant Science and Management (2014) (evaluating another species of miscanthus – Miscanthus x giganeus —  used in bioenergy fuel production) weedeco.ppws.va.tech.edu

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