After 30 years of being a horticulture educator with University of Illinois Extension the towering pampas grasses still amaze me each fall. What energy and power that plant must have to grow over 12 feet tall each summer, just to die back in the fall and start over again the next year.
Also called plume grass, hardy pampas grass (Erianthus ravennae) is hardy in zones 5-9 and grows 8 to 12 feet tall. Hardy Pampas Grass is very upright and open in habit, with large, white plumed flowers. Flowers appear from now to late October and are probably why they stand out for me at this time of year.
This grass needs a lot of space. It forms large clumps up to five feet in diameter. Because of its large size it makes a great specimen plant or screen.
Beware because there are grasses with the common name pampas that are not particularly hardy here. The only true Pampas grass (Cortaderia selloana) does not grow in our climate. Always look for the scientific name to assure you get the correct plant, if you want it to regrow each year.
Although the hardy pampas grass grows here, it does have weaker stems than the true pampas grass. As a result, they sometimes break and bend in high winds.
This grass is a sun lover and needs well drained soil. If you have trouble over-wintering this plant, it might be because your site is too wet in the spring. Wet sites shorten its life considerable and lead to extensive winter injury.
Japanese Silver Grass or Maiden Grass (Miscanthus sinensis) is another tall ornamental grass that is hardy here and not invasive (thus a clump grower). Depending on the cultivar, this grass grows 3-6 feet tall. There are many different cultivars of this grass, each with different features. 'Autumn Light' and 'November Sunset' are two excellent varieties for zone 4 climates. All have nice fall flower displays and outstanding fall color.
'Morning Light' Maiden grass gets its name because sunshine tends to highlight its silky plumes and variegated leaves. Pink flowers rise above foliage in late summer and persist for winter interest. Leaves have a white band along each margin.
'Strictus' is also known as porcupine grass due to the creamy-white vertical stripes on its green leaves. It grows over six feet tall, reaching nine feet in flower. Its plume flower has a very pretty rosy-plum color that turns to silver in late fall.
All these grasses do well in dry soil and drought conditions. They also make excellent erosion control. Always select ornamental grasses carefully because some can be invasive.
For more information ornamental grasses that grow in Illinois go to http://urbanext.illinois.edu/grasses.