I am often asked questions about care of Ornamental Grass in the garden. I came across this excellent Article by the University of Illinois at http://www.urbanext.uiuc.edu/grasses/understanding.html
which sheds light on the whole subject and which I recommend to all gardeners.
"With the popularity of ornamental grasses and their use in the landscape, it is a good idea to understand how these plants grow. This understanding will lead to better use of these plants and often avoid disappointment or frustration.
Grasses respond and start to grow based upon temperature. Some grasses will start to grow in early spring when temperatures are still cool and others will wait until the soil is warm and temperatures are more stable.
Cool Season Grasses
Cool season grass will start to grow early in the spring and may even remain semi-evergreen over the winter. Cool season grasses also seem to do better and have better foliage quality when temperatures are cool or if they are given sufficient water during drought periods. If they are not watered during drought, they tend to go dormant resulting in brown foliage.
These grasses may require more frequent division to keep them healthy looking and vigorous. If not, they tend to die out in the center. For the ones that remain semi-evergreen, you should only cut off the brown or winter injured foliage in the spring. Some of the more popular cool season grasses include, Fescues, Blue Oat Grass (Helictotrichon), Tufted Hair Grass (Deschampsia), and Autumn Moor Grass (Sesleria).
Warm Season Grasses
Warm season grasses will do better during warmer times of the year and remain good looking even when temperatures are high and moisture is limited. Warm season grasses do not begin to show growth until the weather becomes stable and the soils warm. The previous seasons growth usually browns out in the fall requiring the cutting back of plants to about 4-6 inches in the spring.
Warm season grasses usually do not require as much regular division as cool season grasses. Some warm season grasses include Northern Sea Oats (Chasmanthium), Japanese Silver Grass (Miscanthus sp.), Hardy Pampas Grass (Erianthus), Perennial Fountain Grass (Pennisetum), Switch Grass (Panicum) and Prairie Cord Grass (Spartina).
Before planting you should also understand the growth habit of the grass. Grasses can be either clump forming or rhizome forming. The latter is often called "running" grass. The clump forming grasses will grow in very nice, neat mounds or clumps. They tend to mix very well with other perennials and will not become invasive. They will increase in girth slowly over time.
The rhizome forming grasses spread by underground stems and can become very aggressive and invasive. These grasses have their place but it may not be in a well-tended perennial border since they can soon take over an entire area. Before selecting a grass, be sure to understand how it grows so you won’t be planting a future problem. Some attractive but aggressive grasses include Blue Lymegrass, Cordgrass, and Ribbongrass.
As with any other perennial, success depends greatly on soil preparation before planting and having good drainage. Ideally, the planting areas should be prepared in the fall, beginning with deep tilling of the soil. Fall tillage facilitates freezing and thawing action during the winter, and improves soil tilth and workability. If this is not possible, spring tillage is also satisfactory.
Incorporate ample organic matter during the tillage process. Ornamental grasses do not require high amounts of fertilizer. Adding about one pound of a general-purpose fertilizer (like 10-10-10) during soil preparation per 100 sq. ft. of planting bed should be sufficient.
Ornamental grasses can be planted in the spring or the fall. The advantage of spring planting is to give the plants adequate time to develop a good root system before winter. Fall planting is often not as reliable without some additional precautions, particularly in years with early or severe winters. You should try to complete fall planting during August and September.
Then provide a light cover of straw or hay during the first winter for best results. Apply the mulch after several hard frosts. Plants should be planted no deeper than their previous growing depths and should be well watered after planting. Maintaining uniform soil moisture around the plant hastens establishment. Plants planted too deep tend to develop root diseases or simply rot in the ground.
Ornamental grasses require relatively low levels of fertility. By keeping the level of nitrogen low, lodging or flopping over can be kept to a minimum. Leaf color and vigour are good guides to nitrogen requirements. Application of one-half to one pound of 10-10-10 fertilizer per 100 sq. ft. of garden area or about one-quarter cup per plant is sufficient. Apply fertilizer just as growth resumes in the spring. An application of a slow release fertilizer in the spring is enough to take care of the plant's needs throughout the summer. Fertilizer should be watered in thoroughly.
Plants should be well watered the first season after planting so they can develop a good root system. Established plants do not need regular watering, but may need supplemental watering during drought periods. The amount of water will depend on the grass species, the site, and on the quality, size and growth rate desired.
Cultivate around grass plants to control weeds. Application of mulch will greatly reduce the need for cultivation as well as watering. It also tends to keep grasses in check that have a tendency to be heavy reseeding types.
Winter Protection and Spring Clean up
Grasses do not need to be cut down before winter. In fact, they are attractive when left standing and the foliage helps to insulate the crown of the plant. Cut back the foliage to about 4-6 inches in the spring before growth resumes. When foliage is removed, spring growth will begin earlier. Old foliage left on the plant can delay the crown’s warming and subsequent growth by as much as 3 weeks.
Division depends on the spacing and visual appearance of the plants as well as the overall health. Plants suffering from die-out in the center should be divided to improve appearances. Division is done in the spring before growth resumes or in the late summer or fall after the growing season. Plants that bloom late could be divided in the spring."