Pampas grass: good or bad?

By Marianne C. Ophardt, Special to the Herald

KENNEWICK -- We can blame ourselves as gardeners for some of the troublesome weeds in our yards, gardens, roadsides and natural areas.

Many noxious weeds are non-natives that were brought into this country as ornamental garden plants that later escaped. They now threaten native wild plant and animal life or cause us trouble in our gardens and landscapes.

Pampas grass is one of those non-natives ornamentals that escaped. While some will adamantly avow that pampas grass isn't invasive, others will swear that it's already a serious invasive weed in parts of the U.S. Part of this disagreement is the result of the confusion resulting from the two related types of grass commonly called pampas grass. One is the common garden pampas grass (Cortaderia selloana) that has been considered non-invasive and its close relative is Jubata grass or Andean pampas grass (Cortaderia jubata) that's known to be highly invasive. Both are native to South America.

The Andean pampas grass is the "bad" cousin of the two. It's very aggressive in open ground that's bare or has been disturbed. In California, Andean pampas grass is a serious concern in coastal areas. The seeds of this grass develop without pollination and the plant readily seeds itself. The "good" cousin, the garden pampas grass, has been considered non-invasive in the past. That's because it's dioecious with separate male and female plants. For viable seed to occur, the female flowers need to be pollinated with pollen from male flowers. Because the female flowers or plumes are whiter and showier, nurserymen and gardeners have traditionally only grown female plants. These are easily propagated by vegetative divisions of other female plants.

As long as male plants aren't grown in the same area, few viable seeds are produced and the garden pampas grass is not invasive. However, in recent years some unwitting or unscrupulous nurseries have propagated garden pampas grass from seed and sold the resulting plants not knowing if they're male or female. This has resulted in more of the male plants being planted and has resulted in this "good" cousin becoming very invasive in areas of California, Hawaii and other areas.

In warmer regions it's understandable that garden pampas grass can thrive. However, as a warm-season grass rated as hardy to USDA Zone 8, it shouldn't survive and grow well in colder regions, but it does. In warmer climates its leaves stay green during the winter, but in colder climates they turn brown in winter like many other ornamental grasses. The plants will survive down to 10 degrees and the crown will only be killed if the soil freezes deeply during the winter.

Personally, I think the term "garden" pampas grass is a misnomer. This ornamental grass is not for the ordinary garden. The clumps can grow up to five feet wide. The plants may grow to 10 feet tall with flower stalks with plumes reaching to 12 feet tall. It takes about three years before it produces flowers. In late winter, the plants are cut back to about 18 inches from the ground. This is no easy task! The edges of the leaves are extremely sharp and the plant is very dense. The best tool for cutting it back is a small chainsaw. That's what I call extreme gardening!

Of course, like with so many garden plants, there is a dwarf form of this gargantuan grass. It's dwarf pampas grass, Cortaderia selloana "Pumila." Keep in mind though that "dwarf" is a relative term. Dwarf pampas grass plants reach a size of 4 to 6 feet wide and 5 feet tall. It produces 10-foot-long flower stalks with silvery white plumes. Once established, it does well in scorching sun and tolerates drought and wind. It's also hardy down to USDA Zone 6. This one is also cut back to 18 inches from the ground in late winter with a chainsaw.

There's also another grass that's commonly referred to as pampas grass. The hardy pampas grass (Saccharum ravennae, previously known as Erianthus ravennae) is not related to the bad and good "cousins." The hardy pampas grass is native to northern Africa and the Mediterranean. It resembles true pampas grass with plants that grow 6 feet wide and 10 to 15 feet tall with silvery white flower plumes that look somewhat like the fluffier pampas grass plumes. There are also concerns about growing this grass because it's already shown itself to be invasive in parts of the West.

Environmentally conscientious gardeners should avoid planting any of the pampas grasses. They pose potential threats to our native wildlife, and they're pretty big for the average gardener to manage. I know I won't be planting one anytime soon. If it becomes a noxious weed here, I don't want to have that on my gardening conscience.

* Marianne C. Ophardt is a horticulturist for the Washington State University Extension Office in Benton County.

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Saturday, Nov. 15, 2008

Pampas grass: good or bad?

By Marianne C. Ophardt, Special to the Herald