AS you stroll over the grassy brow of Battleston Hill, amiably planted with a natural woodland of rhododendrons and azaleas, and then see, set out below you across a field of six acres, the serried ranks of garden plants, you know that serious business is afoot.
The delphiniums, cauliflowers, dahlias, beans, chrysanthemums, and penstemons - among many other kinds of garden plants - are not here simply to look beautiful (though they do).
The garden plants here are on trial.
The majority of the plants growing on the trial grounds of the Royal Horticultural Society (RHS) at Wisley are cultivars (cultivated varieties) rather than species. Whether hybridized by bees or humans, certain sorts of plants are forever coming up with variations on their original shapes and colors.
Some - but only a few - of these cultivars may be deemed worthy of perpetuity by gardeners. The Wisley trials are a way of determining which new plants would be worth planting in gardens, and which would not.
After due deliberation by special committees visiting the site as many as five times during a particular plant's flowering season, a new cultivar may be given the RHS's accolade: the Award of Garden Merit (AGM).
Others, if they have not already been uprooted as rogues of one sort or another, may still be critically destined for obscurity.
Some may already have earned the AGM at a previous trial, and these are included in the current trial as standards with which new entries can be compared.
They are also included because Wisley is becoming more conscious of the ordinary visitor who may show an interest in its plant trials. A greater effort is now being made to explain cultivation and judging techniques to non-cognoscenti.
Dahlias are one of the popular garden plants that are given annual, or ``Permanent,'' trials at Wisley. In fact, dahlia trials began here as long ago as 1921. They are allotted 200 spaces on the field, three plants of each selected cultivar per space. The 600 plants in each annual trial cover the considerable range and variety found in garden dahlias, from bedders to giants. Other ``Permanent Trials'' are given to such groups as delphiniums, sweet peas, irises, and daffodils.
There are also ``Invited Trials'' for different plants, but these can be extremely rare occurrences. Penstemons, making an impressive patch this year, have not been tested for decades.
Besides assessing the merits of this beautiful plant's various cultivars, this particular trial offers a good opportunity to sort out the names horticulturists give different penstemons. Their nomenclature has long been in a state of confusion.
Mike Pollock, Wisley's technical- liaison officer, points out that the RHS trials, though they have evolved over the years, have represented one of the society's main aims throughout its 180-year history.
The RHS's gardens at Wisley (opened in 1904), though they draw 680,000 visitors a year, have also had this serious experimental purpose: to try out, and to select, new garden plants.
In line with this, the Royal Horticultural Society's trials department publishes sheets describing methods for cultivating the trial plants, and in its published reports gives specific descriptions of raising and growing methods used at Wisley. Such things as propagation, preparation of soil, and application of fertilizers are all detailed.
Wisley is not a botanical garden: Its aim is to encourage and improve scientific and practical horticulture. Derek Hewlett, chairman of the joint dahlia committee that judges the dahlia trial each year, explains that the plants are ``judged for garden use.'' He says the committee members look for ``freedom of flowering and quality of flower'' rather than ``exhibition quality.''
Some people do grow dahlias for exhibition, and some grow them commercially as cut flowers. But these are not the concerns at Wisley. The RHS basically wants to know how a particular cultivar will rate as a garden plant.
The trial report for 1993 lists further criteria: earliness, continuity of flowering, vigor, quality of bloom, and resistance to disease. To amateur devotee Hewlett, color is the chief virtue of the dahlia.
THE judging committee for dahlias echoes a balance that might be called typical of British horticulture: a balance of the amateur and the professional. The Dutch may grow dahlias as if they are tulip bulbs to be mass-produced and marketed in plastic bags. But in Britain, dahlias (no less than many other kinds of garden plants) are the stuff of intense passion and dedication.
The National Dahlia Society boasts ``three to four thousand members,'' Hewlett says. Though this membership is international, it is certainly Anglocentric.
The joint dahlia committee at Wisley is composed half of Dahlia Society members, half of Royal Horticultural Society members. Alternating chairman of the committee is a professional dahlia grower, Roger Aylett. Though he represents the RHS half of the committee, he is hardly alone in being a member of both societies.
Mr. Aylett may be a professional dahlia nurseryman, but, characteristically of the dahlia world today, if he introduces a new cultivar for trial at Wisley, it will not be his own work. ``We as a company do not hybridize dahlias ourselves,'' he says.
But many of the dahlias in Wisley trials have his name attached to them (he discreetly stands back when these plants are being judged) because he sometimes represents some amateur grower who (having allowed the bees to do their work) has sown his seed and come up with a plant he believes to be up to the test.
Aylett points out that propagation of new dahlias is ``very much in the hands of the amateur gardener,'' (which is the opposite of rose growing) and he mentions a grower named Pi Ensum, a longtime member of the joint dahlia committee who has raised over many years a host of highly regarded new dahlias.
``In his heyday, [Pi Ensum] was a fishmonger by trade. He has introduced the most wonderful dahlias - the `Hamari' strain,'' Aylett says.
Before new dahlias are planted on the Wisley trial grounds, they have already been through preselection processes - first by the enthusiastic growers themselves and then by committee.
Anyone can enter the contest, but by the time all the judging and vetting has taken place, only the Rolls Royces among dahlias are likely to get through.
Hewlett maintains it's all a matter of ``luck.'' ``First year I grew them I collected eight seeds. But I got one from them that won the `best seedling' in the country that year. On the other hand, 2,000 seeds may turn out all rubbish,'' he says.
Nurseryman Aylett certainly values the RHS's Award of Garden Merit. His catalogs always make a point of noting when a plant has earned the award and when.
``It's a very useful monitor for the amateur gardener,'' he believes. ``They'll know they're worth planting in their garden - worth looking into.''