VandasCatleyasPhalaenopsisDendrobiums 1 2 3 4Making Orchids Bloom

Dendrobium aggregatum

Dendrobium aggregatum

In this “chapter” on growing dendrobiums, we’ll cover the species in Section Callista. These are spring-blooming orchids native to mainland Asia, most commonly northeast India, northern Thailand and adjacent areas. Although the flowers typically last only about a week, they are among the most spectacular orchid displays you’ll ever see. Botanists include about 10 species in this section, and we grow most of them.

Cultural requirements

Fortunately, the most popular species in the group all have broadly similar cultural requirements. They grow robustly during spring and summer; growth stops altogether in late fall, but the plants do not drop their leaves during the winter resting period. Most of these orchids have upright pseudobulbs with a cluster of a few leaves at the top. The stems are usually spindle-shaped. Den. farmeri’s stems are 4-angled, Den. chrysotoxum’s are swollen and ridged. In the spring, the inflorescence develops from a node on the stem just below the leaves. Den. lindleyi and Den. jenkinsii have shorter, more flattened pseudobulbs and typically only one, or at most two, leaves per pseudobulb.

Dendrobium chrysotoxum

Dendrobium chrysotoxum

Temperature. In their native habitat, these orchids experience mild to warm daytime temperatures in spring and summer, and generally cooler temperatures in the winter. For most, the winter night temperatures range from about 45°F to about 55°F. Like the species in Section Dendrobium (described in “Part 1”),  these dendrobiums do well for us with our normal winter night temperatures, which are usually in that range. Den. chrysotoxum does best on the warmer end of that range, while Den. densiflorum, Den. lindleyi (aggregatum), Den. jenkinsii and Den. thyrsiflorum prefer the cooler end of it.

Dendrobium thyrsiflorum

Dendrobium thyrsiflorum

Light. While actively growing, give the plants plenty of bright but filtered light. Most will grow well under the same light conditions as most cattleyas. Many of these dendrobiums grow attached to deciduous trees in their native habitat, so they want higher light in winter. Den. chrysotoxum and Den. lindleyi need more light; provide as much light as possible without burning the leaves.  We grow these two under near-vanda light levels. It is normal for the pseudobulbs of Den. chrysotoxum to be more yellow than green.

Water. As with the species in Part 1, watering may be the most crucial element to success with these orchids. Their native conditions are fairly wet during the late spring, summer and early fall. Rainfall can be quite heavy for a few months, but it tapers off in the late fall, and winter months are fairly dry. Water the plants regularly when they are actively growing; mounted plants can be watered daily if air circulation is good. When the new pseudobulbs have matured in the fall, gradually reduce watering, and allow the plant to dry between waterings. There’s some moisture in the habitat at this time, so the plants do get a little water from dew or fog; a light watering every few days is all they need. It is normal for the pseudobulbs to shrivel somewhat during the resting period, but don’t allow the plant to dehydrate completely! During the coolest periods in winter, keep the plants fairly dry.

Den. farmeri

Fertilizer. These dendrobiums benefit from regular feeding when they’re actively growing. Any balanced, water-soluble fertilizer can be used, diluted according to package directions. Jack’s Classic 20-20-20 is a good option; we feed once a week during the growing season. Reduce feeding in the fall and eliminate it completely when the plants are resting in the winter. Resume feeding when the plant begins to grow again.

Potting. Because most species in this this group have upright pseudobulbs, the plants can be grown in pots or baskets, or mounted on a sturdy substrate like driftwood or teak root. Mounted plants can be watered daily in summer if the air circulation is good. In a pot or basket, use a very well-drained epiphyte mix; we prefer an inorganic product like expanded clay, as these orchids do best when their roots are undisturbed. We usually grow them in baskets, or on mounts. Den. jenkinsii and Den. lindleyi (aggregatum) do best mounted.

Den. jenkinsii

Problems. These dendrobiums have few really major pest problems. Snails and slugs may damage the leaves and canes, and occasionally during wet weather you may see some minor damage from leaf-spotting fungal diseases. Scale insects can be a problem, particularly on plants of Den. lindleyi (aggregatum) and Den. jenkinsii.  Mites are a possible problem on the softer-leafed species such as Den. farmeri. Stale or decayed potting medium can lead to root problems, as with any potted orchids.

If a mature plant doesn’t bloom in the spring, something in its environment is not correct. This may be too much water during resting; the plants need a little watering during this time but they must dry completely for two or three days between waterings. It may also be a temperature problem…these dendrobiums want mild to warm days in winter, but much cooler nights. And they need higher light in the winter, too.

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