Gardening Articles: Flowers :: Perennials

Grasses in Containers (page 4 of 4)

by Rick Darke

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).

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