IN THE GARDEN YARROW MAKES A STRONG, EASY GROUND COVER.

Byline: JOSHUA SISKIN

Q: A gardening show on PBS had one program about all kinds of ground covers aside from grass that are tough enough to walk on. Do you know of a thick ground cover that can choke out weeds, can be walked on, and doesn't need mowing?

- Grace Hampton,

Burbank

A: There are many ground covers that can occasionally be walked on without being damaged. Yet, if a ground cover existed that could take heavy foot traffic, we would probably all know about it. Bear in mind that even tall fescue, the most widely used lawn grass in Los Angeles, will die if planted in a nursery school play area where it is pounced on several hours a day, And a backyard fescue lawn that has been healthy for years will be in need of repair if a large party of guests were to congregate on that lawn during a long, hot summer afternoon.

Nevertheless, there are a number of testimonials about a certain flowering ground cover that, supposedly, is at least as tough as lawn grass. I am talking about yarrow (Achillea), of which there are many durable species and varieties.

My first experience with yarrow was on a sloping landscape in Granada Hills. A species with white flowers known as common yarrow (Achillea millefolium) was growing there. That slope was unevenly watered and full of mounds made by gophers, but the yarrow grew without let-up nonetheless.

The flowers of yarrow are known as cymes and appear as flat-topped clusters, up to 6 inches across in some varieties, of many small blooms. To picture the leaves of yarrow, think of the species name ``millefolium,'' which means ``thousand-leafed,'' referring to finely serrated, intricately divided, fern-like foliage.

Fifteen years ago, in the yard of the Charles Lummis Home - a historical landmark in Pasadena - a grand experiment was conducted with reddish-pink yarrow millefolium 'Rosea.' In search of a lawn substitute, 'Rosea' yarrow was seeded in the early spring. The rate of application was 1 ounce of seed per 3,000 square feet of planting area. To facilitate even distribution of the seed, it was mixed with sand at a 1:4 ratio and broadcast over the ground with a hand-held spreader.

Within 10 days, the yarrow seed had sprouted and, before three months had passed, the first 'Rosea' flowers were blooming as the rapidly developing plants covered the earth. Based on the Lummis Home experiment in Pasadena, it was found that mowing could be done between two and eight times per year, with more frequent mowing resulting in denser, more prostrate growth that lent itself to heavier foot traffic. Significantly, the yarrow's water requirement was just about half of what a lawn would have demanded.

Yarrow is also easily propagated by division and can also be procured as a container plant. There are green, gray-green and silver-leafed yarrow varieties, with flower colors including yellow, salmon, lavender-pink and red.

Often, the simplest solution to foot-traffic problems in ground cover is the placement of stepping stones or pavers, or laying down walkways of decomposed granite or pea gravel, through the planted areas. Rather than obsessing over finding a plant that can stand up to foot traffic, direct foot traffic through your yard or garden with hardscape pathways of your own choosing.

TIP OF THE WEEK: An application of Epsom salts, which contain magnesium sulfate, is recommended annually in the rose garden. Sprinkle two tablespoons around the drip line of each rose bush at this time of year.

Roses are heavy feeders of magnesium, an element required by the chlorophyll pigment that makes roses and every other plant green. Sulfate keeps down soil pH, facilitating root uptake of iron, zinc and manganese.

Lightly cultivate the salts so they go just beneath the soil surface; water well. Tomato crops also benefit from Epsom salt application, especially large-fruited varieties that may otherwise have trouble reaching maturity without blemish.