GRASSES HAVE BEEN CALLED the hair of the earth; in fact, a variety of New Zealand sedge is known as `Frosty Curls.' While ornamental grasses don't require a trim every six weeks, most types do need an annual haircut.

Growing grasses is nearly foolproof. But when and how much to cut them back has always puzzled me. So I turned for advice to local ornamental-grass expert Eric Nelson. He chaired the grasses department at the 1986 Arboretum Foundation plant sale and has been an enthusiast ever since, growing more than 250 different species of ornamental grasses in his Fremont garden.

Nelson recommends them for beginning gardeners because grasses are rarely bothered by pests or diseases, are easy to maintain (sun, good drainage, avoid fertilizers), and most are fairly drought-resistant.

I grow a great many grasses for none of these practical reasons. I appreciate that most grasses bloom late and look their best all through the autumn. I love their boldly colored foliage, and use their inflorescences as long-lasting additions to flower arrangements. Whenever a grouping of plants seems dull, adding an ornamental grass or two often adds just the spark that is needed. A giant feather grass for vertical punctuation, a short mondo grass as an edging, or one of colored blades, such as a blue oat grass (Helictotrichon sempervirens), can tie a planting together and make all the other plants look their best. If that doesn't do it, I mix in a plant with great big leaves.

Before you select grasses, be sure you know the answers to two important questions: Are they invasive (through spreading runners, or too-generous seeding about)? And are they hardy? The gorgeous purple fountain grass (Pennisetum setaceum `Rubrum'), with flowers like little fox tails, dies over the winter. Its close cousin (Pennisetum alopecuroides) lacks the rich coloration, but has the same showy flowers and is dependably hardy.

For the purposes of care, it is convenient to think of grasses as falling into two categories, warm-season and cool-season.Warm-season grasses go dormant in the winter and don't begin growing again until the soil warms up in the spring. Cool-season grasses (sedges are an example) thrive in our climate, are often evergreen or semi-evergreen, and grow best in fall, winter and spring.

The most important maintenance task for growing healthy grasses is to cut back the foliage at least once a year. Eric Nelson has an easy answer on timing: Cut everything back in early March shortly before the new shoots appear.

The dead foliage and flowers of the deciduous grasses are looking bleached and tattered by now, so cut them back clear to the ground. They will grow to their full height during this year's growing season. They can be cut down earlier if they look so bad you can't stand them, but cutting late in the winter leaves a bare spot for the shortest possible time.

The cool-season, evergreen grasses resent being cut back too hard. If the older foliage looks messy, or the tips have been burned by winter cold, Nelson advises trimming them back by no more than a third. For years I've had clumps of golden variegated Japanese sedge along my front walkway which look great all season long with no care except some summer water and an occasional light trim.

The best book on the subject? "The Encyclopedia of Ornamental Grasses," by John Greenlee (Michael Friedman Publishing Group, 1992).

Valerie Easton is a horticultural librarian and writes about plants and gardens for Pacific Northwest magazine. Her e-mail address is vjeaston@aol.com

Now in Bloom: If they survive the slugs, delicate Iris reticulata come into bloom in February and March. Their flowers in deep plum (`Purple Gem') or blue (`Harmony') add welcome contrast to the creams and yellows of the early daffodils.