Grasses in Containers
It's a delight to see the rising popularity of grasses as gardeners discover the striking array of textures, forms, sizes, and colors they offer. The multi-season interest of ornamental grasses is unsurpassed, and their luminous quality and fluttering response to winds and breezes add to their appeal.
The passion for growing plants in decorative pots coincides perfectly with this interest in grasses. Combine the beauty and versatility of ornamental grasses with the freedom afforded by container gardening, and the possibilities expand to the farthest reaches of your horticultural imagination. Most ornamental grasses readily adapt to container culture; some, such as miscanthus, are actually easier and more practical to grow in pots.
The idea for this pairing first dawned on me while exploring nurseries in Japan, in search of new plants for Longwood Gardens. In Japan, nurseries specializing in woody bonsai typically offered herbaceous plants grown in shallow containers as display companions. Common among these were the red-leafed Japanese blood grass (Imperata cylindrica) 'Red Baron', green-leafed Hakone grass (Hakonechloa macra), and its golden-variegated form (Hakonechloa macra 'Aureola'). Their colorful foliage played against the variously hued Japanese ceramics, creating exquisite miniature scenes. These grasses, shallow-rooted and naturally diminutive, seemed obvious choices for container growing, appearing quite content in pots as shallow as 2 inches. But I was intrigued to find a narrow-leafed, white-striped miscanthus in a slightly larger container, forming a fountain of foliage about 18 inches high. Though an apparent miniature, this grass was later revealed to be the superb variety we now call Miscanthus sinensis 'Morning Light', which can grow more than 6 feet tall when planted in the ground.
Going Tropical with Grasses
One of the joys of container gardening is experimenting with plants that are not hardy in your region, including a number of beautiful tropical and subtropical grasses. Three distinct purple-leafed fountain grasses -- Pennisetum setaceum 'Rubrum', P. s. 'Eaton Canyon' (also sold as 'Rubrum Compactum'), and P. 'Burgundy Giant' -- make dramatic container specimens in the summer and early autumn garden. None is reliably hardy below 40°F, but during winter they can be held in semi-dormancy in a basement or a garage attached to a house if you don't have a greenhouse. If fountain grass isn't exotic enough for you, try one of the purple-stemmed varieties of sugarcane, such as Saccharum officinarum 'Pele's Smoke'. In their semi-dormant state, the grasses will survive. Just don't let the soil in the pots dry completely.
For those of us fond of the aromas of Asian cuisine, it is delightful to have lemongrass (Cymbopogon citratus) in the garden. The crushed leaves of this subtropical native of India and Sri Lanka are strongly lemon scented. Lemongrass is cold hardy only through USDA Hardiness Zone 9, yet it makes an attractive pot plant in the sunny summer garden. Purchase starts each spring, or overwinter potted plants in a greenhouse or sunporch.
Arizona native Muhlenbergia dumosa is called bamboo muhly for its bamboolike stems and fine-textured foliage. Though not winter hardy in the ground beyond zone 8, it thrives in a container, luxuriating in the summer sun and adding a unique look to the garden. In warmer climates such as those of California and Texas, I've seen it growing with desert plants like cactus and agave. The imaginative container gardener can recreate these striking textural combinations anywhere in the country.
The current craze for bold-textured tropicals, such as cannas and bananas, also suggests exciting combinations with fine-textured grasses in containers. Even on a small scale, container grasses can be enlivened by interplanting them with broad-leafed plants, such as a mix of golden-variegated Hakone grass with bronze-leafed sweet potato vine.
Using Container Grasses
The dwarfing effect of containers on grass plants makes pots the ideal way to enjoy grasses that would be too big for some garden spaces if grown in the ground.
Though many grasses look best when grown individually in pots, you can create exciting combinations by planting grasses with other flowering or foliage plants in the same pot. Because many grasses have a fairly upright, fountainlike form, plant a tumbling, spreading, or cascading plant, such as moss verbena (Verbena tenuisecta), bronze-leafed sweet potato vine (Ipomoea batatas 'Blackie'), or one of the spreading silver-leafed sages such as Artemesia schmidtiana at the base of the container to create an elegant scene.
Large grasses are useful for organizing garden spaces, creating outdoor "rooms," or serving as focal points in the same way that specimen shrubs or hedges typically do. Miscanthus, for example, is particularly well suited to cultivation in containers ranging from clay pots to half whiskey barrels to antiques such as copper boilers and tin milk cans. A tubbed specimen of miscanthus may be only half the size of an in-ground plant, yet it can serve a similar purpose. The Delaware Center for Horticulture uses a matched pair of containers planted with 'Morning Light' miscanthus to flank the front courtyard entrance doors. The fine texture of the grasses softens the bold surfaces of the building. Container-grown miscanthus can be used in similar ways to organize space in rooftop gardens and on suburban patios.
Containers are also an excellent way to enjoy spreading grasses that might otherwise be weedy nuisances in a flower border. While visiting legendary garden designer Penelope Hobhouse's garden three years ago, I was stunned by her dramatic use of blue grass (Leymus arenarius) in a large copper urn. This notorious spreader was perfectly controlled in the urn -- its colorful, straplike foliage beautifully complemented the natural patina of the copper urn and the surrounding perennials. The ribbon grasses (Phalaris arundinacea 'Picta', 'Feesey', and 'Tricolor') are all strong runners and superb candidates for containers.
Giant reed (Arundo donax) can grow nearly 14 feet tall if planted in the ground, and its stout rhizomes can cover a few feet in a single summer. In a container the size of a half whiskey barrel, however, it can be maintained at a very manageable 5 foot height. The white-striped 'Variegata' makes a particularly beautiful, modest-sized plant when grown in a container.
Growing grasses in containers sometimes reduces flowering, but in many cases that may be desirable. Blue lyme grass is less likely to bloom when confined in a pot, but the colorful foliage is this grass's main feature anyway; the flower stalks are relatively coarse and unattractive. Miscanthus is also less likely to flower when confined by a container. Although the plumy flowers are beautiful, the form, color, and texture of miscanthus foliage is quite striking by itself.
Another advantage of container gardening is the ability to shuffle plants, making new combinations over the course of a season, as plants go on and off peak. A strategically placed clump of blue oat grass (Helichtrichon sempervivens) can add a cool touch to the mid-summer garden. Since the textural beauty of grasses often remains constant throughout the year, they are especially useful for providing a reliable backdrop to various flowers and broad-leafed plants. In addition, the autumn flowering and foliage colors of many grasses can be the highlight of the late-season garden. I particularly value ornamental grasses for the way the way they are illuminated by the sun's rays, especially in fall, yet it takes careful design when laying out garden beds to ensure that grasses will be naturally backlit or sidelit. Container grasses can easily be repositioned to make the most of autumn sunstreams.
Hardiness and Culture
Hardiness. All of the grasses mentioned here are heat tolerant in warmer climates, even in summer in eastern zones 8 and 9, with the exception of Hakone grass and Karl Foerster's feather-reed grass (Calamagrostis acutiflora 'Karl Foerster'), which suffer some in the summer heat of zone 9.
Winter hardiness of container-grown grasses is not known with much certainty. Containers offer minimal protection from low temperatures, which leaves the vagaries of weather to play a more significant role. A general formula for overwintering container grasses successfully is to have one or two zones of reserve to ensure survival. For example, 'Morning Light' miscanthus is cold hardy in the ground through zone 5. In an unprotected container, it will most always survive winters in zone 7. Normally hardy to zone 4, 'Purpurascens' miscanthus, an excellent subject for a large container and a fine substitute for 'Morning Light' miscanthus when organizing garden spaces, will usually withstand zone 6 temperatures in a container. If you're looking for narrow, upright grass sturdy enough to survive winter as far north as zone 6, try Karl Foerster's feather-reed grass.
While much more research is needed in this area, I encourage you to experiment. Look for sheltered spots in courtyards or against the south wall of the house -- any shielded area that might afford that critical bit of extra protection in winter. This is especially important with larger grasses and heavier containers that may be impractical to move into winter protection. Also, since many large grasses remain attractive even in dormancy (if not cut back), it is nice to be able to leave them in place in the winter garden.
Pests. The majority of grasses are pest and disease free and will grow well in a container if a few basic needs are met. Though generally drought tolerant, grasses must have adequate moisture if their foliage is to look clean and crisp through the season. Watering frequency will vary with the type and size of the grass, the type and size of the container, the soil mix, and the weather. Check soil moisture frequently, and don't allow soil to dry completely.
Exposure. Though sun and shade requirements are similar to those of plants grown in the ground, the typically smaller size of container-grown grasses permits moving them farther into shade without fear of flopping.
Soils. Most grasses will tolerate a wide range of soil types and prefer a pH between 5 and 7. The potting mix must be porous enough to allow good drainage, yet heavy enough with organic matter to retain moisture. A good all-purpose mix is two parts Metromix 510 plus one part screened (20-grit) and washed sand. You can also start with a light, peat moss-based inorganic mix, then add an equal amount of the above sand to it.
Containers. Some of the smaller grasses, such as Japanese blood grass and Hakone grass, have shallow, modest-sized root systems and can be grown in shallow containers as small as 5 to 6 inches wide and deep, but they must be watered frequently. Many larger grasses have deep-reaching roots (which is why they are so naturally drought-tolerant).
For larger grasses such as miscanthus, North American native switch grass (Panicum virgatum), and giant reed, choose pots that are one to two times as deep as they are wide. Larger grasses do need larger containers, but to a great extent gardeners can limit the size of the grass by limiting the size of the container. For example, a full-sized specimen of 'Morning Light' miscanthus might exceed 6 feet in height, with similar spread, if grown in the ground. To grow a container plant to this size would require a container at least the size of a full whiskey barrel. However, a half whiskey barrel will limit a specimen of 'Morning Light' to about 3 feet tall and wide. If you plan to grow large grasses in containers over a number of years, consider planting them in a durable concrete container that will withstand the pressure from the expanding root system of the grass.
Dividing plants. Plan to remove large grasses from their containers and divide them periodically to maintain an appropriate size. Because most clump-forming grasses grow outward from their edges, divisions taken from the perimeter are best for replanting.
The possibilities of grasses in containers are so many and so exciting that I encourage all my friends to experiment. Start with a division taken in spring from an existing clump in your garden or a friend's. In the case of 'Morning Light' miscanthus, a 4- to 8-inch division will grow into a beautiful, fountain-shaped mass of fine-textured foliage, light green with narrow cream-white variegated edges, in the same season.
Rick Darke is a horticultural consultant living in Landenberg, Pennsylvania. He is author of The Color Encyclopedia of Ornamental Grassses (Timber Press, 1999; $50).