General Conditions
Dendrobium nobile and its relatives are Asiatic orchids native to Burma, India, Thailand, and Indochina, where they grow on trees from the lowlands all the way up to the cool highlands of the Himalayas at elevations of 4,000 feet. The species are extremely hardy, surviving temperatures ranging from warm to downright hot as well as enduring freezing temperatures in some locales. If kept dry, these species and hybrids will survive winter temperatures of 37-39 F (3-4 C) and will flower around April. If temperatures are maintained at about 62-64 F (17-18 C) as soon as buds appear, they will flower in January or February.
Flowering
Poor flowering in spite of robust growth and the appearance of offshoots (keikis) is the most common problem with nobile-type dendrobiums. This can be solved by taking the following precautions:
- Light and Air: Provide plenty of light to flowering-size plants. As long as there is good air circulation, they can be grown without shade even in summer. In Hawaii, plants are grown under full sun (no shading) with good results. If air circulation is poor, about 30-40% shade should be provided in July and August to prevent leaf burn. In any case, from September on, plants should be grown under full sun to produce strong canes and leaves and prepare for flowering.
- Fertilizer: There are many types of orchid fertilizers. The most common cause of poor flowering in this type of Dendrobium is the accumulation of too much nitrogen. For flowering-size plants, use fertilizers with a low percentage of nitrogen. It is important to stop application of any fertilizer in the first part of August. Slow-releasing fertilizers in pellets (e.g., Osmocote) are not good for flowering -size plants.
- Temperature: For differentiation of flower buds, it is important to expose plants to low temperature. Canes (pseudobulbs) which grew from spring through summer and have matured in the fall require approximately one month of low night temperatures. Therefore, in the autumn when it becomes cool, do not rush to bring your plants into the greenhouse, as with cattleyas. Leave them outside to cool, and they will bloom much better.
When in full bloom, flowers will last longer if the plant is placed in a cool, dry spot away from any draft and out of direct sunlight. A night temperature of 40-50 F (5-10 C) is ideal. Water the plant enough to moisten the surface of the medium once every 5-7 days during the warmest part of the day. When given the right amount of water, the media should dry before evening.
If you follow these cultural hints for nobile-type dendrobiums, you will be pleased by the results.
Potting
- Tree fern fiber, osmunda, fir bark, sphagnum moss, and other media appropriate for cattleyas can be used for nobile-type dendrobiums. A mixture of 3 parts perlite, 1 part vermiculite, and 1 part peat moss also can be used. A slightly acid medium (pH 5) which drains well but still retains some moisture is ideal.
- Clay pots are recommended for sphagnum moss or media that retain moisture. For media that drain well, plastic or polyethylene pot are recommended.
- Appropriate pot sizes are as follows: For a small plant up to 3” tall, a 2-1/2” pot is good; for a plant 5” tall, a 3” pot is adequate, and for a plant 10” tall, a 4” pot is fine. Overpotting is not good for growth of small plants.
Watering
There is a direct relationship between temperature, light, and watering. When temperatures begin to rise in the spring, gradually start watering. In the summer, when temperature is high and sunlight is strong, water almost every day to keep the plant from drying out. From about late September, when temperatures begin to fall, gradually reduce watering. When night temperature falls below 50 F (10 C), water only enough to keep the canes from shriveling. Once a week should be enough. When night temperatures fall below 41 F (5 C), keep the plants dry. If you have a greenhouse in which night temperatures in winter can be kept above 59 F (15 C), water lightly whenever the plants are dry. It is important that the amount of water applied in the morning to the media be dry by evening time.
Fertilizing
Low-nitrogen fertilizer should be used for flowering plants. Be sure to stop fertilizing after early August if you wish to have many flowers. For small plants grown without supplemental heat and where night temperatures fall below 46 F (8 C) in winter, apply fertilizer high in nitrogen when night temperatures rise to about 50 F (10 C), probably in March or April. If night temperatures in winter are higher than 50 F (10 C), then fertilize from January. The easiest way to fertilize small plants is by using slow-release fertilizers that are effective for more than half a year. Fertilization for blooming size plants should not be time-release type as it may cause over fertilization.
Lighting
For small plants, no shade is required in winter. However, 30-40% shade is needed from late spring for good growth and beautiful canes and leaves. For medium-sized or flowering-size plants, no shade is necessary at any time during the year as long as the leaves will not get burnt. Plants will grow vigorously and bloom profusely with full sun.
In areas where there is little breeze in summer, about 30-40% shade is needed at that time. If ventilation is inadequate in the greenhouse during the flowering season, buds will be damaged, and flowering will be poor. Therefore, 30-40% shade is recommended from the time flower buds appear until the end of the flowering season. However, no shade is needed if temperatures are low and sunlight is weak.
Overgrown or large plants that have finished flowering should be repotted. This should be done only when night temperatures remain above 55 F (13 C). In case the lower temperatures prevail, transplanting should be postponed. To repot, remove decayed potting medium and discolored, black, or decayed roots by washing them with water. Be careful not to damage the live roots. Repot into a larger size container. If the root ball has decreased due to removal of decayed roots, pot the plant into a smaller size container.
Plants with more than 7 or 8 canes can be divided but not necessary. If healthy plants with only 4 or 5 canes are divided, the following year’s growth will be poor. Transplanting small or medium-size plants that have finished flowering should be done only when the pot has become too small to support the height of the canes. The best time for planting or transplanting is when new shoots grow to about 4-6 inches. The roots from the new shoots should take to the media very well. Plants should not be replanted when no new shoots are growing, or plant has stopped growing.
After repotting, plants should be kept relatively dry for two weeks. Water once every 3-4 days, just enough to moisten the surface of the medium. When new roots appear, the plant should be given ample water to soak through the bottom of the pot. The plants should be kept in 40% shade for about 3 weeks after transplanting.
From spring to early summer, offshoots (keikis) may appear on the upper nodes of canes due to damage of the new shoot by breaking or by slugs or because of excessive application of nitrogen. The offshoots that appear in spring will produce thick canes and mature during the summer. When the roots are 3 to 3-1/2” (7-10 cm) long, the offshoot can be removed from the cane. Soak the offshoot in water to soften the roots and plant it in a 3” or other size pot, depending on the size of the offshoot. If bottom main shoots and keikis appear simultaneously, pluck off keikis to make main shoots grow stronger.
