Native to Southeast Asia, the genus dendrobium is one of the largest of all orchid groups. There are about 1,200 individual species, and they grow in all manner of climates, from hot, wet lowlands to high-altitude, colder mountains. Growers usually divide dendrobiums into groups based on their growing conditions. All dendrobiums are epiphytes. Some are deciduous and some hold onto their leaves all year round. Serious collectors often favor the D. nobile, but the most common kind of dendrobium—the kind gracing grocery store shelves—is a hybrid dendrobium phalaenopsis. This article will focus on those plants.
These plants like strong, natural sunlight. They will grow in lower light conditions, but it is unlikely the plant will bloom well. The appearance of tiny plantlets on older canes (called keikis) often means the plant isn't getting enough light. These keikis can be potted up individually after they develop roots.
During the growing season, dendrobiums like high humidity and lots of water. As with all orchids, the frequency of watering depends on your growing conditions, but at least weekly is a good idea during the summer. After the growing season, cut water back somewhat (maybe every ten days), but do not suspend watering.
Feed heavily during the growing season with a weak fertilizer solution containing lots of nitrogen, or use a balanced fertilizer like Peters 20-20-20 at quarter strength with every watering. At the end of the growing season, reduce fertilizer by about half to help provoke a better bloom.
There are considered warm-house plants by growers, meaning they favor conditions in a warm greenhouse. The temperature for these plants should be above 60 degrees at all times, although experience has shown they can withstand a few nights down to 50 or so. But this should be avoided if possible. A slight drop in nighttime temperature will often stimulate a bloom.
These bloom for me throughout the year, depending on conditions. To help provoke a flower spike, slightly drop the water and nighttime temperature. Also, if a cane loses all its leaves, don't cut it off—they sometimes bloom from old canes. The flowers are long-lasting, up to six weeks.
Potting and repotting:
These are naturally epiphytic orchids that will thrive in hanging baskets with little or no potting media (in superb conditions), or they will do well in fast-draining media as a windowsill plant. These plants are shipped in quantity from Hawaii, Taiwan and throughout Asia, and some growers have started selling them grown in wood chunks. This is my least favorite potting situation because the plants are often top heavy and the wood gradually rots away. I prefer a mixture of clay aggregate, perlite, and coconut fiber. Repot at the beginning of the growing season when necessary.
I was initially confounded by these plants. I had several, and killed many, and could never coax them to bloom. I soon figured out my mistake: I was treating them like phalaenopsis. With the dendrobium phalaenopsis hybrids, think more of everything: more light, more water and more fertilizer. They are robust growers that send up at least one new upright can every year from creeping rhizomes. Don't cut off old canes as they will sometimes flower or produce keikis that can be potted up on their own.