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Ornamental Grasses that Stay Put

  • Larry Hodgson
    • Flowers of reed grass
    Calamagrostis × acutiflora ‘Karl Foerster’. Photo: Daryl Mitchell, Wikipedia Commons

    Ornamental grasses have never been so 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 create islands, hedges or screens in the garden or simply fit here and there in a flower bed. And there are varieties that are suitable for sun or shade, dry or wet soil, clay, loamy or sandy soils, cold climates and 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…

    Ornamental Grasses by Category

    There are, in fact, 3 categories of ornamental grasses.

    Creeping Grasses

    Creeping grasses spread by means of rhizomes and form large colonies.

    The first is creeping grasses, also called rhizomatous 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. Now, that’s invasive!

    Clumping Grasses

    Clumping grasses grow in… clumps!

    These grasses grow in clumps. They do expand in diameter over time, as does any perennial, but only very slowly. So they really aren’t invasive. If they do get too big, all you have to do is divide them… just like you would divide any other perennial.

    Grasses with Short Rhizomes

    Grasses with short rhizomes like switchgrass (Panicum virgatum) spread out a bit, but don’t go very far. Photo: Matt Lavin, flickr.com.

    There is also an intermediate group: grasses with short, determinate 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.

    How to Recognize an Invasive Grass


    Just by looking at how this plant grows in its pot, you can see that it is a clumping grass: the leaves form a dense clump. Photo: thegardencorner.com

    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?

    Three Categories

    Here are three lists of popular ornamental grasses, categorized by their rooting type.

    Note that the “zone” (hardiness zone). It tells which climatic zones the plants is likely to thrive in. For more informaton on hardiness zones, read Understanding Hardiness Zones


    Clumping Grasses

    These grasses grow in clumps and are not invasive.

    Blue oat grass (Helictorichon sempervirens). Photo:
    Drew Avery    , flickr.com.
    Foxtail barley (Hordeum jubatum). Photo: Andrey Nagaycev, Wikimedi Commons.
    Reed grass (Calamagrostis arundinacea, syn. C. brachytricha). Photo:
    Krzysztof Ziarnek, Kenraiz    , Wikimedia Commons.

    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.

    Japanese forest grass ‘All Gold’ (Hakonechloa macra). Photo: KATHERINE WAGNER-REISS, Wikimedia Commons.

    Creeping Grasses

    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).

    Common reed (Phragmites australis) is the best known invasive grass. It produces rhizomes up to 42 ft (13 m) long! Photo  Jonathan Wilkins.

    Seedy Invaders

    Ornamental grass seeds. Photo: www.depi.vic.gov.au.

    So much for grasses with invasive rhizomes, but some grasses spread excessively through their seeds. The mother plant produces a huge number of seeds that sprout everywhere. This contrasts with the majority of ornamental grasses, which either only self-seed rarely under garden conditions 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

    Hairgrass (Deschampsia cespitosa) is very pretty, but can reseed itself too abundantly. Photo:
    Leonora (Ellie) Enking    , flickr.com.

    These grasses can be invasive by their overly vigorous spontaneous sowing in regions with cooler climates (hardiness zones 3 to 6) and therefore require some monitoring in Canada, the northern US and northern Europe.

    Grasses that self-seed in mild climates

    The spectacular pampas grass (Cortaderia selloana) is invasive by seed…but only in mild climates. Photo: Em?ke Dénes, Wikimedia Commons.

    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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