Tolumnia (Equitant Oncidiums)
The group of orchids now called Tolumnia were at one time called Oncidium section Variegata or commonly referred to as “equitant oncidiums”. The foliage seldom exceeds 6 to 8 inches in height, and a 4-inch pot can house a “specimen” plant. The leaves are arranged in pairs overlapping or straddling one another at the base, accounting for the popular term “equitant” (derived from the Latin meaning riding a horse). Most of the species produce growths at short intervals along the rhizomes, resulting in a compact, clumping growth habit. A few, however, possess elongated rhizomes that create rambling tangles of growth. Flowers are produced primarily in the spring on 12- to 18-inch inflorescences that are often branched on older plants. Some types have much shorter, bouquet-like displays. Their petite size and ability to adapt to a fairly wide range of conditions make them suitable for growing spaces under lights or on windowsills. And wait … don’t cut off that spike after the first blooms fade. There is often a secondary spike waiting to form and provide several more weeks of bloom.
The key to growing Tolumnias is understanding their natural habitat. The species are endemic to the Caribbean Basin with many confined to a single island. Most of the species involved in modern hybrids are found in intermediate to warm conditions growing on twigs where they are exposed to bright light and air movement. Moisture is provided by high humidity and by daily rain showers or heavy dews. Due to constant air movement by the trade winds, plants never remain wet for long.
Grow Tolumnias almost anywhere an intermediate range of temperature (55° to 90°F) and relative humidity of 50 to 70 percent can be provided. Those cultivated in windows or under lights benefit from summering outdoors where climate permits.
Light Provide bright, diffused light that is somewhere between the optimal for phalaenopsis and that for cattleyas. The general rule of thumb is if shadows on the orchid bench are just discernible, the light is about right. Plants that are growing well but reluctant to bloom usually need an increase in light intensity. Once conditioned to high light, tolumnias are fairly tough, but when moving tender plants to a higher light situation, increase their exposure gradually to prevent burning. This is especially true when moving plants outdoors for the summer.
Watering This is the most crucial aspect to success with tolumnias. There is no hard and fast rule for how often to water. Only close observation of your conditions will indicate frequency. Plants must dry out between watering. Drying will be faster outdoors than on a humidity-enhanced windowsill. Damp, cloudy days will retard drying while bright, breezy days will hasten it. Plants on mounts can be misted daily because drying is rapid, but those in pots must be observed more closely for complete drying. The adage “if in doubt, don’t water” applies here. Avoid misting or watering during the heat of the day. Water that collects in the overlapping leaf bases can reach “cooking” temperatures and severely damage plant tissue, especially the tender young growths.
Fertilizing In the natural habitat, plants are bathed with nutrients derived from decaying plant and animal matter with every rain. So, for cultivated plants, frequent and dilute feeding is the preferred approach. A balanced fertilizer applied every second or third watering at half to quarter strength should be adequate. Flushing with plain water between feedings is important because residual salts can damage the roots.
Potting and Mounting Frequency of watering and selection of substrate are closely integrated. The objective is to achieve the proper combination allowing for good irrigation with adequate aeration and rapid drying of the root area. Mounting is the method of choice; at least for a start. Twigs, cork bark, small wood or tree-fern plaques all work well. Place a pad of moss or coconut fiber around the roots and secure the plant to the mount with monofilament line or strips cut from nylon hose. A daily light misting will help establish growth. If plants on mounts show a tendency to shrivel despite regular watering, this may indicate conditions drier than optimum. Pare off some of the mount without disturbing the plant and simply set it in a clay pot (with no medium). This procedure may afford just the right amount of extra moisture around the root area. If conditions still seem too dry, sift potting mix into the container around the base of the plant. The medium used should be porous and drain readily.
For those just starting to grow the tolumnias, this step-by-step procedure causes minimal trauma to the plant while it and the grower are getting acquainted. As you observe the results you will be able to choose the method that works best in your conditions.
Problems The airy, bright and dry cultural preference discourages most disease problems. Mealybugs and scale and aphids on the tender inflorescence are encountered most and may be dealt with simply by direct removal using a cotton swab soaked with ordinary rubbing alcohol. For larger infestations, wettable powder formulations of Malathion, Orthene, a product containing pyrethrins or one of the newer imidacloprid products (used according to manufacturer’s directions) provide efficient control. Cygon seems to be toxic to the plants and should be avoided.
Unless plants are cultivated in a basket, a 3- to 4-inch pot full is the maximum size to which a plant should be allowed to grow. When this size is reached (every two years on average), divide and repot the plant. Otherwise, as the central part of the plant begins to decline it may affect the healthy portion through bacterial or fungal rot. Repotting should be done when new growth begins in the spring to assure quick establishment in the new quarters.