For many orchid enthusiasts, achieving a successful bloom cycle with Dendrobium orchids can be a challenge. A key physiological trigger for flowering in numerous Dendrobium species, particularly the popular deciduous and nobile types, is a significant and sustained drop in temperature. From a botanical perspective, this is not a mere preference but a critical environmental signal that synchronizes the plant's reproductive phase with favorable conditions in its native habitat.
Many Dendrobium species are native to regions in Southeast Asia, the Himalayas, and Australia that experience distinct monsoon patterns. Their growth cycle involves a period of vigorous vegetative growth during the warm, wet summer months, where they produce new pseudobulbs (canes) and store water and nutrients. This is followed by a cooler, drier winter period. The drop in temperature serves as an unambiguous environmental cue that the wet season is ending and a period of lower competition and potential pollinator activity is approaching. By initiating flowering at this time, the plant ensures that its energetically expensive reproductive process occurs when resources are directed away from leaf and root growth, and when seed dispersal will be timed for the arrival of the next rains.
The process begins with the plant's perception of the cooling trend, primarily through its tissues. This cooler temperature acts as a signal that alters the plant's internal hormone balance. Specifically, the drop in temperature influences the synthesis and transport of key plant hormones.
Firstly, the production of gibberellins, hormones that promote vegetative growth and can inhibit flowering in some plants, is suppressed. More critically, the plant becomes more sensitive to other signals. For many Dendrobiums, the combination of cooler temperatures and reduced watering initiates the breakdown of cytokinins in the leaves and promotes the mobilization of stored resources in the pseudobulbs. This shift in resource allocation and hormonal balance signals the meristems (the growth cells at the nodes of the canes) to switch their developmental program from producing vegetative growths (keikis) to forming floral initials, which will develop into flower buds.
The required temperature drop is not subtle. For reliable flowering, a differential of 10-15°F (5-8°C) between day and night temperatures is often necessary for a period of 3-5 weeks. For many nobile-type Dendrobiums, ideal triggering conditions involve nights consistently around 50-55°F (10-13°C) and days not exceeding 60-65°F (15-18°C). It is crucial that this cooling period is accompanied by a reduction in watering and fertilization, mimicking the dry season. This combination of stresses is what fully convinces the plant to enter its reproductive state. Furthermore, this temperature signal often works in concert with photoperiod (day length). Shorter days in the autumn reinforce the message that the seasons are changing, providing a secondary, reinforcing cue for the plant to begin its flowering cycle.
Without this critical temperature cue, the Dendrobium orchid will often remain in a vegetative state indefinitely. The plant may continue to produce new canes and leaves if conditions permit, but it will not bloom. In some cases, particularly with nobile hybrids, the energy that would have gone into flowers is instead directed into producing aerial offshoots called keikis (plantlets) at the nodes along the cane. This is a clear indication that the plant has not received the correct environmental signals to initiate its reproductive phase and is instead attempting to propagate itself vegetatively.
