Dendrobium nobile orchids (nobile means "noble" or "showy" in Latin, so this species is named "the showy dendrobium") are native to southeast Asia and the Himalayas. They are grown for their spectacular display of multicoloured, fragrant flowers produced in late winter or early spring. A large plant may have as many as 100 flowers at a time, and they last for two to three months. These dendrobiums have quite specific care requirements, and they require different care in different seasons.
Growing Season--March to August
Because they grow flower buds in fall and bloom in winter, nobile orchids need to recover in the spring and summer. Give them lower light (bright indirect light is good), lots of water and regular fertiliser during this season. Depending on the size of the pot and the temperature, they may need water every day. They should be watered in the morning, preferably with distilled water or rainwater, and fertilised every two to three weeks with a water-soluble fertiliser, preferably one formulated to work with distilled water. Use 20-20-20 (or a similar balanced fertiliser), dissolving 1 tsp in 1 gallon of water.
Nobiles do best when there is at least a 10-degree difference between daytime and nighttime temperatures. You might be able to achieve this more easily by putting plants outdoors or on a porch at this time of year.
Fall--Flower Buds Form
Stop fertilising nobiles at the end of August to promote flower bud formation. Over-fertilising will produce more foliage growth but few flowers. Move plants to a sunnier spot and reduce watering to once a week, just enough to keep plants from shrivelling.
Cooler temperatures and a 10-degree differential between daytime and nighttime temperatures will promote formation of flower buds. Try to keep plants below 15.6 degrees C at night. Put nobiles outdoors (bring them in before freezing), in an unheated room or even in the refrigerator at night to achieve the necessary cool conditions. In cool temperatures like this, overwatering can be lethal, resulting in fungus problems and rot. Water sparingly.
Flower buds develop over about three months, produced along the sides of the canes, usually opposite the leaf axils. When you see flowers, increase watering to every three to five days. Flowers usually open in January or February and should last for two to three months. Protection from direct sunlight will prolong bloom. Resume fertilising, reduce light and increase watering only in March, after the flowers are done.
Orchids in general like to be root-bound, so keep them in a rather small pot. Repot only about every two years into a slightly larger pot, using standard orchid potting mix or sphagnum moss.