Ornamental Grasses

21 October 2009

In The Garden With the Fluvanna Master Gardeners
By Irene C. Burke - Fluvanna Master Gardener
Fluvanna County Extension Office: 434.591.1950

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The autumnal display of ornamental grasses lures us with their charms but what of their vices? There is the late winter chore of cutting them to the ground, which can be a challenge with large clumps. And there are the rhizomatous grasses (spreading by underground stems) which go where they are not welcome — blue lyme grass (Leymus arenarius), ribbon grass (Phalaris arundinacea), and switch grass (Panicum virgatum). Aggressive grasses spread only where the conditions provide for all their needs. Pampas grass is a garden thug where winters are warm but loses its courage in a cold snap.

Ive searched but cant seem to find any other drawbacks to these versatile landscape characters but I would confine my choices to the clump forming. Most worthy candidates for this area are warm season grasses, which accounts for their golden-hued dormancy during the colder months. This checks their invasiveness if they are prone to it.

Basics — There are some principles to follow when making your choices: size, texture, color, form, and function, which will help you abide by the cardinal rule — right plant, right location. Real estate matters in horticulture.

Form, Color, Texture, and Size — Fountain, spike and arcing are your form choices. Each has its purpose in your landscape design, from gentle to dramatic. Some spikes climb to 12 feet while some fountains cascade at three inches. With colors of creamy ivory painted with pale green stripes to deep crimson and luminous gold your options are mounting. Texture gives you smooth, ribbed, fine and fluffy to broad, bold, firm and dense. Though clumps expand when all is right and retreat when soil and sun are not in their favor, its best to plan for mature size when choosing a site.

Function — This is where your soil, sun and slope decide for you. Once established all will fare well but most benefit from clay enhanced with organic matter, preferably finished compost, mulched until firmly rooted — about a year. Clumping grasses prefer full sun but many will tolerate partial shade. All will control erosion on steep slopes but the shorter grasses are your best bet during that troublesome time when getting them started.

Culture — Late winter pruning is essential to having healthy clumps. In the wild this is achieved with fire, not a wise practice in the garden. For any size clump, but especially the large ones, encircle them tightly with twine, then cut 6-8 inches from the base with a pruning saw being careful to do no harm to the crown. After this trim they are also ready for dividing. Use the same pruning saw to divide the crown. The new location should have the same kind of organic material as was used at the first planting. Set at the same depth as the original site. Warm or cool season clumpers may be planted early spring, summer or early fall. To reduce your watering chores, mulch 3-4 inches deep, 3 inches from the base, watering every two weeks unless rain or snow soaks the earth to an inch.

Tip of the Week

For fine textured leaf mulch from your own trees, mulch that will not blow away with the winter winds, shred leaves inside a large garbage can with a string trimmer, then apply in moistened batches.