What's a rare plant? Some people think anything that doesn't grow down the neutral strip of a highway is rare. Others think any plant not listed in any of the 30 largest garden catalogues is rare.

But I would say a plant is rare if it is unobtainable from any nursery of the Western world. Such a plant is Viola guadalupensis.

This little half-inch-wide yellow flower on six-inch stems was found three years ago and its discovery announced recently. Brent Wauer, a ranger of Guadalupe Mountains National Park 100 miles from El Paso, noticed it in bloom on a high cliff in the park. Only a scant handful of plants have been found. It is supposed this violet became extinct elsewhere about 8,000 years ago.

Gardeners probably do not need one more yellow violet, but you never know. Certainly seed should be collected by park authorities and the plant should be propagated by a botanic garden. It may have, and probably does, some peculiar virtues (as violets go) and besides, in this day of widespread plant exterminations, every hitherto unknown flower should be coddled.

I had a great-great-grandfather, a doctor, who cured people right and left with violets, no doubt using the flowers and paste made from the leaves and perhaps the roots. Violets contain ionone, which is not in our modern list of useful and therapeutic chemicals, so presumably the cures were lucky or semi-miraculous. But then most cures are.

Mid-November is a fine time to plant hardy perennials, assuming you can find them locally in pots. Otherwise, better wait till late March.

There are a number of more or less hardy verbenas that I have admired here and there over the years but that I have never investigated or grown. Some are from Chile, some from America. They come in magenta, rose, red, white, lavender and purple. Gardeners who don't let weeds get started, and there are some such folk, might find them handy for growing in rose beds. Some of these verbenas are quite hardy but others are unreliable north of Zone 8, which is south of Norfolk and Vicksburg.

This may be the place to say hardiness zones are chancy things. Gardeners should not hesitate to try plants said to be hardy only in the next warmer zone. Thus Zone 6 gardeners should freely try Zone 7 plants and Zone 7 adventurers should try Zone 8 plants, but only if they are prepared for occasional losses in winter.

You take our native scarlet honeysuckle, Lonicera sempervirens, that George Washington grew at Mount Vernon (where it has been used lavishly along the arcades, I am told). It is sometimes listed as hardy to Zone 3, or else to Zone 6 -- a tremendous variation.

And in Zone 8 in England, which is of course warmer in winter than Washington, it is often killed in winter. English gardeners might conclude, therefore, that this honeysuckle is not safe anywhere that ice forms frequently in winter.

The truth is that this honeysuckle like most American and Chinese plants requires heat in the summer to ripen its new growth. Given that, it takes subzero temperatures well. Lacking that, it will perish in winters of little frost.

Within a single zone there are substantial variations, some of them magical, I believe. For example, gardenias refuse to grow outdoors in Washington, but they are easy enough in Memphis, in the same zone. Furthermore, Memphis may have lower minimum temperatures in winter -- occasional drops to 12 below zero, for instance -- than Washington. On the other hand, summer temperatures there are higher and more prolonged than here. Spring starts earlier there.

So it's not just a matter of the lowest temperature in winter.

Hardiness depends on the plant, too. Gardeners in parts of New England can readily grow the ginger lily, Hedychium coronarium, and the coral tree, Erythrina crista-galli, though both are commonly considered Zone 9 plants. That is because both plants winter over nicely in cold regions if given plenty of mulch. Their storage roots are such that though the plant is killed to the ground every winter, it sprouts vigorously every spring.

There is also the matter of wind. Tea roses, those tender parents of today's hybrid teas, have been grown nicely (in a few varieties) on Cape Cod, though they are readily killed in exposed sites in inland South Carolina.

A successful grower of teas in Delaware tells me the secret is shielding them from the wind, not the cold. It probably also helps if they are on their own roots, not grafted. The class of tea roses is not considered hardy in most of England, not even in Surrey or Kent, but the great Victorian gardener, William Robinson, said his were never killed in a quarter-century of variable winters. He attributed this to preparing their planting sites 30 inches deep, and to growing them on their own roots from cuttings, not on understocks.

He also refused to give his roses any cow manure (or horse, either) except in preparing their planting stations to begin with. He used no manure mulches, or mulches of any kind, except small plants beneath the roses. He was virtually mad for violas, tufted pansies, pinks and practically any small plant. He carpeted his rose beds with these.

He confessed to growing his roses superbly, and died (in his mid-nineties) before the gods of modesty sent a winter that killed them all.

Gardeners occasionally tell me "the books say" this and that. In fact, books say endless things, they say all manner of contradictory things about roses and all else. You have to weigh and sift what you read and you have to judge whether the writer knows anything first hand or not.

This same Mr. Robinson -- whom I would never for one second believe when he gets off on his favorite fantasy of woodland gardening (he says that, once planted with vast quantities of things he recommends, the woodland will increase in beauty even if the gardener goes away for years) -- I am inclined to trust fully when he speaks of particular plants he grew for decades with great success.

We have wandered afield. But that's one good use for fields.