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Ornamental Grasses That Stay Put

  • Larry Hodgson
    • 20150623A
    Diamond grass (Calamagrostis arundinacea)

    Ornamental grasses have never been more popular… and for good reason. They have attractive summer foliage that dances in the wind and often persists, after changing color, through the autumn and even the winter. Plus many have feathery flowerheads that are just as long-lasting. They can be used in multiple fashions, from hedges or screens to filling hell strips to simply being dropped here and there in the perennial border. And there are grasses that are suitable for sun or shade, dry or wet soils, clay or sandy soils, cold or hot climates, etc. In fact, whatever your requirements, there is always a choice of ornamental grasses that will meet your needs!

    Landscape architects and city planners have been at the forefront of the popularity of ornamental grasses. Many in fact include them in all their projects. Still, many home gardeners hesitate to try ornamental grasses. After all, aren’t they invasive?

    The answer is yes… and no. Let me explain…

    Three Categories

    Ornamental grasses can be divided into three main categories, according to their root structure: three categories that also help determine their potential invasiveness.

    Creeping grass

    The first is creeping grasses, the ones that produce long rhizomes and quickly form large colonies. Think of the common reed (Phragmites australis), a grass often seen in roadside ditches where, in certain cases, it has been known to spread for dozens of miles (kilometres), all from a single original rhizome.

    All creeping grasses are considered at least somewhat invasive and some are so dominant that you shouldn’t even think of releasing them into a cultivated zone without surrounding them with an impenetrable barrier of some sort. (Try sinking a large pot or bucket with the bottom removed into the ground and planting the invader inside: that will keep it in check).

    Clumping grass

    But there are also clumping grasses, that is to say, grasses that grow in dense tufts. They do expand in diameter over time, as does any perennial, but only very slowly. If they do get too big, all you have to do is divide them… again, just like any other perennial.

    Finally, there is also an intermediate group: grasses with short rhizomes, sometimes called sod-forming grasses. They do expand outward, but don’t go very far, forming a tuft perhaps less dense that that of a clumping grass, but nothing truly invasive. Again, all you need is shovel to cut out any excess growth: that will put the plant back in its place.

    Just look in the pot: this grass already clearly forms a clump! Photo: www.lookup-beforebuying.com

    How to Recognize an Invasive Grass

    It is easy to distinguish between a potentially invasive grass and a clumping grass at the time of purchase. Just look in the pot. If the stems already form a dense clump, you’ll know it’s a clumping grass; if the stems are relatively close together, but without forming a real tuft, it’s a grass with short rhizomes; and if there are well separated individual stems that emerge here and there from the pot, it’s a creeping grass, likely to be at least a little invasive. Simple isn’t it?

    Here are lists of popular ornamental grasses classified according to their growth habit.

    Calamagrostis x acutiflora ‘Karl Foerster’. Photo:
    Matt Lavin.

    Clumping Grasses

    These grasses grow in clumps and are not invasive.

    Japanese forest grass (Hakonechloa macro ‘Aureola’) is a typical short-rhizome grass. It will spread via its short rhizomes, but ever so slowly! Photo:
    Megan Hansen.

    Grasses with Short Rhizomes

    Yes, these grasses are creepers, but they only spread very slowly. They’re sometimes referred to as sod-forming grasses to distinguish them from the more invasive creeping grasses. They can easily they can be planted in a flowerbed without fear of them trying to take over.

    Grasses with Creeping Rhizomes

    Common rush (Phragmites australis) is probably the most invasive of all grasses: rhizomes can reach tup to up to 43 feet (13 m) long.

    The following grasses may be beautiful, but they are highly invasive. In most cases, it’s best to plant these within a good root barrier.

    Seedy Invaders

    Some grasses spread by seed.

    So much for grasses with invasive rhizomes, but some grasses are spread excessively through their seeds. The mother plant produces a huge number of seeds that sprout everywhere. This contrasts with the majority of grasses, which either only self-seed rarely or don’t do so at all. It’s worth dividing these plants into two groups, according to climate.

    Grasses That Self-seed in Cold Climates

    Foxtail barley (Hordeum jubatum) is an attractive hardy grass, but can self-sow too abundantly. Photo:
    Jonathan Wilkins    .

    These grasses can be invasive due to their abundant self-sowing in cooler climates (zones 1-6). A good mulch will help keep them from spreading too abundantly.

    Grasses That Self-seed in Mild Climates

    Pampas grass (Cortaderia selloana) may be strikingly beautiful, but its culture has been banned in many mild climates because of its excessive self-seeding. Photo: Em?ke Dénes.

    The following grasses also can self-seed excessively, but only where the winters are mild (zones 7-12). There is no risk in the North, because either the grass is not cold hardy (several grasses, such as purple fountain grass, are grown as annuals in the North and don’t survive the winter there) or because, even if the grass is hardy, its seeds fail to ripen in the North (the case with most miscanthus grasses).

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