Although oncidiums constitute one of the largest genera in the orchid tribe, nearly 300 species having been identified, they have not achieved corresponding popularity. This is understandable because they were once considered temperamentally difficult; except for a half-dozen species their culture was often variable and usually tricky. Many continental growers still continue to regard them as shortlived, lasting for a year or two in cultivation; stock is renewed by constant importations. Since oncidiums bloom profusely, frequently flowering within a few months after being imported—and their flower stems are often hugely out of proportion to the small size of the plants—it was believed that they literally wore themselves out. In recent years, however, oncidiums have come to be important spray orchids in the United States and their requirements have been closely studied. Most of them fall into one of the three cultural divisions: cool, intermediate, or warm. Nevertheless, a considerable number veer slightly away from these convenient classifications and must be treated as individuals, and their rather distinct habits known, before they can be grown successfully.

In general, oncidiums have indicated that they are as voracious in their appetites as their closely related cousins the odontoglossums,miltonias, and brassias. Their historic tendency to succumb in cultivation is in large part due to being starved; when grown in leafmold composts instead of os-munda their growth is nearly always sturdy. However, osmunda should be used for the epiphytes—if only a little is used and a regular application of a nutrient solution is given.

As would be expected in so large a genus, covering the vast area from Mexico to Brazil and spilling over into the West Indies, the habits and appearance of oncidiums are surprisingly heterogeneous. All they actually have in common is a waxy excrescence or tubercle on their lips from which they take their name, their relatively small flowers (one-half inch to two inches across), their floriferous character (from one to several hundred flowers), and their predominantly chocolate-barred clear yellow blooms. Otherwise their characteristics are as variable as those of the countries and situations in which they are found. They range from hot tropical lowlands and damp river valleys to cool mountain ranges and higher peaks, where they become true alpines. Often they are localized in small, specific areas where they grow in rank abandon over trees; then there will be a jump of hundreds of miles to the spot where the next members of the genus congregate. Some oncidiums have pseudobulbs with thin, narrow foliage like odontoglossums. Others have thick, triangular leaves like short, broad bayonets, which are set on tiny round pseudobulbs, or none at all. Some are terrestrial, but most are epiphytic. Flower stems vary from six inches to six feet or more; they may be erect, nodding, or long and twining.

Only one member of the family doesn't produce a multitude of flowers: Oncidium papilio produces one flower at a time in nearly perpetual succession, a new flower appearing on top of the stem as soon as the old one dies. It is commonly known as the "Butterfly Orchid" and closely resembles the butterfly, on which it depends for reproduction. The dorsal sepals and petals are thin, elongated, and carried above the flower like an insect's antennae. Seen from a distance while a gentle wind moves it, the flower is hard to distinguish from a hovering butterfly.

There is no definite geographical, botanical, or climatic division into which oncidiums may be placed in order to determine their cultural requirements. The best approach is to list the species which come under each temperature classification and grow them accordingly. Even so, some confusion still exists about the correct temperatures; they are proving more amenable to lower temperatures than was believed possible. For example, Oncidium cavendishianum was long grown as a warm orchid, next as an intermediate one, and was then found to be more bountifully floriferous and healthy when subjected to cool treatment; it is an excellent lath-house orchid. Many other warm and intermediate oncidiums have shown a surprising adaptability to cooler treatment if given sufficient light and a nearly dry atmosphere during cold winter nights. Unlike their cousins the alpine odontoglossums, few oncidiums relish a saturated atmosphere. Humidity as low as 20 per cent does not inconvenience them. Neither do most oncidiums suffer from high temperatures although they must be kept as cool as possible in summer.

Happily, most oncidiums place themselves within intermediate temperatures, and unhappily, many of them are not in cultivation. Other than Oncidium flexuosum, which may flower any season of the year, and Oncidium sphacelatum, whose gay little flowers resemble ballerinas dancing, there isn't much of a selection open to you. They are grown in small collections along with cattleyas, and given much the same consideration and attention, although they are not as demanding as cattleyas and are slightly more insistent on their water supply. The cool oncidiums, though, offer a wider selection and are much lovelier. They are primarily natives of the Mexican table land and Central America, whence so many of the hardy orchids come. Only one, Oncidium varicosum (the loveliest of all oncidiums), is not from this area, but grows thousands of miles away in southern Brazil. Those members of the cool oncidiums which are reasonably available at present include the terrestrial Oncidium cavendishianum and the two semiterrestrials Oncidium splendidum and Oncidium tigrinum—the latter being known as the "Flower of the Dead" to the Mexican Indians who decorate graves with the flowers. The rest of the cool species are epiphytes, generally represented by Oncidium marshallianum, Oncidium leuchochilum, and Oncidium ornitharhynchum.

A fourth class is sometimes set up for oncidiums to include the alpine species Oncidium cuculatum and others that live, so to speak, in the clouds at elevations in excess of 10,000 feet. Unless you want to make a botanical collection, ignore these orchids; they are few in number, not spectacular, and are as rigidly exacting and unresponsive as alpine odonto-glossums.

In their native homes cool oncidiums thrive under a climate which, except for occasional relatively high humidity, is not too dissimilar to moderately subtropical sections of the United States. As a result, they have slowly gained in favor as coolhouse or outdoor orchids. In southwestern and southeastern United States they have weathered 28° and considerable frost if later protected from direct sunlight. In Santa Barbara, California, they grow in shade gardens where they can be kept cool and moist during the hotter summer days. They should be admirably adapted to window culture, but have not been so cultivated. Rarely exceeding eighteen inches in height, their foliage is in keeping with window requirements. The varieties that develop flower stems to three feet or more often are reluctant, in cultivation, to reach their normal stature, offering a splurge of yellow flowers a foot or two above the foliage.

However, until you learn more of their tolerances, oncidi-ums will do better for you when grown under moderate winter heat (about 50°), cool summer conditions, some humidity (40 per cent), and a good syringing once or twice a day in clear weather. Try to grow them in closed windows, orchid cases, or greenhouses during winter or other inclement weather. Take them outdoors in summer. Cool oncidiums benefit by such a vacation and are more likely to flower.

All oncidiums, no matter what class they fall in, are propagated by division of the pseudobulbs as soon as the root collar on new growth appears. Never leave less than two pseudobulbs or three leaves (terrestrials) to each propagation. Old orchid books record the fact that some oncidiums produce adventitious plants along the flower stem, much in the same manner as the weedy epidendrums. This production does not occur often enough in cultivation to be of value.

A compost composed of leafmold and gravel is suitable for terrestrial oncidiums, osmunda for the epiphytes though they may be grown as well in leafmold. Like odontoglos-sums, they do best in small pots, with plenty of drainage, and as little compost as possible. It is better almost to squeeze them into pots than to let them have too much room. Their roots are thin and short, take up little space. Overpotting tends to leave the compost too wet, decaying the roots. Place plenty of coarse rock in the bottom of your pots—at least half full, perhaps two-thirds full.

Build the compost up around oncidium roots with care; firm it but don't pack it. Sometimes there may not be sufficient roots to hold the plants erect, and they may have to be stapled to the compost or the last and lowest pseudobulb slightly buried if you don't mind taking a chance on losing it. A handier method, and one that helps prevent rot, is to add a thin layer of fine gravel to the top of the terrestrial compost. It helps hold the pseudobulbs erect until root growth is secure and also acts as a cooling mulch.

Water sparingly after repotting and after flowering. On-cidiums need a somewhat continuous supply of moisture about their roots, but it can be overdone. Heavy watering between periods of active growth may stimulate weak, immature growth which either rots or fails to flower. Orchidists of the nineteenth century believed this defect to be inherent in the plants and not a result of overwatering. As soon as new growth and roots appear, watering may be increased, though never to excess. Allow the top of the compost to become dry between applications of water, but not the bottom. A slight moisture content at the base of the compost is desirable since it helps to force roots downward.

Since recent experiments with nutrient gravel culture indicate that oncidiums have ravenous appetites, a planned feeding program is advisable. Growers who have added a level teaspoonful of dry guano to the compost comment favorably on the startlingly increased vigor of their plants. Dry guano, of course, is a bit on the dangerous side for orchids, being rather caustic. In its place try liquid guano, manure water, or a nutrient solution applied to the compost about once a month.

Oncidiums in one sense might be called the experimenters' orchid; because of old prejudices against them they haven't been popular and many lovely varieties are unknown to culture. In Mexico and Guatemala there are many species whose vivid flowers merit attention, and their record of hardiness in their native countries is excellent. But, until gardeners discover and import these oncidiums and determine their cultural tolerances, they will remain unknown to gardens.

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