It used to be that just being a gardener was trendy enough to keep your end of a cocktail-party conversation going. I mean, 10 years ago you could establish yourself as a serious player simply by declaiming the value of plopping your home-grown radishes into your wok. We all know what a bore it is to deal with someone who talks about woks; so it is with radishes and a host of other vegetables.

This is a competitive city. Gardening is a public activity by which all your neighborhood friends and enemies can and will judge you.

You are what you plant.

If you want to dazzle people with your elevator conversation this summer, pay attention to what's in and what's out in all the best gardens.

Let's dispose quickly of what's not in. Zucchini and yellow squash top the list. Squash borers make these vegetables a pain to maintain once the hot weather is here, and summer squashes take up far more space than they deserve. Besides, if you want fresh zucchini, there's plenty to be had from your less trendy neighbors who will insist on growing it. Cucumbers, unless you're inclined to make large amounts of gazpacho. Green cabbage -- even the best coleslaw gets tiresome as summer wears on. Yellow onions are so cheap to buy, why waste good garden space growing them?

There's a substantial gray area of vegetables that are neither particularly trendy nor embarrassing to grow. These include potatoes -- you can always point out to your most fashionable friends that you harvest them as tiny new potatoes and they'll forgive your gaffe in growing them -- and green beans -- again, point out that you pick them very young. Carrots are forgiven, especially the finger-size varieties, for they take up very little space.

And then there's a whole range of ordinary vegetables that even the trendiest person can grow because they're used for a specific trendy dish. Beets fall into this category, since they're the essential ingredient in a good borscht. Turnips in small amounts are allowable, especially if you point out how weary store-bought turnips are compared to a handful served 15 minutes out of the garden. Corn is definitely allowed because, while in itself a pedestrian vegetable, eaten right out of the garden, it surpasses the ordinary. Corn's biggest liability as a trendy vegetable is its excessive need for space.

Other vegetables that won't embarrass you: broccoli, spinach, peas, tomatoes (especially Italian sauce tomatoes such as Roma), red cabbage, asparagus, the melon of your choice (but limit it to one variety, they take up so much space), the winter squash of your choice (also limited to one variety), celery (which always surprises less fashionable gardeners who aren't aware that celery can be grown) and okra, if you point out that you never eat the stuff but it makes great thickening for spaghetti sauce.

Now, the important stuff. Here are some vegetables that you should seriously consider if you want to set the pace in your neighborhood.

Arugula, a lovely, peppery European green that appears on the menus of Washington's trendiest eateries. Easy to grow, it will reseed itself annually and it's maintenance-free.

Mache, a French salad green with small round leaves, which is winter-hardy if given some protection.

Radicchio. Grow this one and you have every right to be unbearable. This Italian chicory, sporting red-and-white variegated leaves and called the "rose of Travisio," is so trendy that it hasn't yet been discovered. In fact, it's so trendy that until recently you could find it only in the region of Italy where it originates. It appears in catalogues as Verona red chicory.

Cucumbers, but only the never-bitter white variety or the tiny French cornichon. Grow cornichons only if you're ambitious, because implicit in growing them is the task of turning the tiny finger-size cukes into pickles.

Fennel, while more on the edge of trendiness last year, is still extremely fashionable. This licorice-flavored vegetable serves a dual purpose: The attractive, fern-like foliage can be used in sauces and sandwiches, and the fat bulb, when mature at the end of the year, makes marvelous fennel soup and wonderful delicate sauces. In addition, seeds can be harvested and used in winter to achieve the licorice flavor so often called for in Italian and French cuisine. Fennel reseeds itself every year and in fact can become quite a pest.

Sorrel, like fennel, was on the cutting edge of fashion in past seasons, but it's still okay to grow it for citrus-smacked soups and lovely green sauces for delicate fish.

Purple cauliflower has replaced white- curded varieties. It requires no blanching, looks gorgeous in the garden and cooks up green.

White conical broccoli this year replaces common green varieties for beauty and taste.

And finally, if you want to blow minds all over the neighborhood, consider the cultivation of Shiitake mushrooms. They are virtually unavailable fresh from stores but called for ubiquitously in hopelessly trendy Japanese cuisine. According to the Montgomery County (where else?) Cooperative Extension Service, "The mushroom grows readily on hardwood logs, notably oaks, inoculated with the fungus Lentinus edodes, and requires little effort and expense when compared with commercial mushrooms. A lag time of about two years is required before an inoculated log will begin to bear mushrooms, but once started, a given log will continue to produce mushrooms seasonally for several years."

For more information on Shiitake mushrooms, contact the Mycology Laboratory, Department of Botany, University of Maryland, College Park 20742. SPECIALTY SEEDS GOURMET GARDENS, 923 North Ivy Street, Arlington 22201. JARDIN DU GOURMET, West Danville, Vermont 05873. NICHOLS GARDEN NURSERY, 190 West Pacific Highway, Albany, Oregon 09732. J.E. DEMONCHAUX, 837 North Kansas, Topeka, Kansas 66608.