The wide range of colors and forms exhibited by roses is the result of their ability to cross-pollinate freely, both in the wild and in cultivation. Complex hybridizing over thousands of years, along with the poor record keeping and once-secretive practices of hybridizers, has made it difficult to establish the lineage of many roses. As a result, their classification sometimes stymies even the experts.
All roses belong to the genus Rosa, of which there are some two hundred species and many thousands of hybrid varieties. Hybrids do not belong to a particular species and thus lack conventional botanical names; instead, they are identified by the variety names their hybridizers have assigned to them.
To help simplify matters, the American Rose Society has arranged all roses under two broad categories: old garden roses and modern roses. Old garden roses belong to classes that existed before 1867, the year the first hybrid tea was introduced. Modern roses consist of hybrid teas and other classes created after 1867.
A “class” of roses is not the same as the class category in taxonomy (kingdom, phylum, class, order, and so forth), but rather a grouping of species or hybrid varieties with similar characteristics and sometimes a common heritage. For example, hybrid teas are typically bushy plants with large, showy, solitary flowers; all descend from a nineteenth-century crossing of a tea rose with a hybrid perpetual. Climbers generally have large flowers and long canes; unlike hybrid teas, however, they represent various lineages. New classes of roses are continually emerging; any distinctive breakthrough in plant form, flower form, or blooming characteristic can serve as the basis for a new class.
In choosing a class of rose, you should look for one whose habits and requirements harmonize with those of your garden.