The Köppen Climate Classification divides the world into five broad climate groups: tropical, arid, temperate, continental, and polar. To understand deciduous orchids is a lesson in understanding seasonal changes in the tropics and subtropics. When most folks think of the tropics, they think of a rainforest, but this is not entirely accurate. There are many climate types in the tropical zones of Earth, with the most common being:
- Equatorial – no dry season, just wet and rainy year-round
- Monsoon – The intermediate between equatorial and savanna climates, tropical monsoon climate has somewhat of a dry season where there is still rainfall, but not nearly as much as the wet season where it is near-indistinguishable from equatorial climates with daily heavy rainfall.
- Savanna – distinct wet and dry seasons where the dry season has little rain, and the wet season can be somewhat to very wet.
- These may be further affected by elevation to create ecosystems:
- Montane – a high-elevation environment which still has any of the tropical rainfall amounts above, but because of the elevation has much cooler temperatures. Many Masdevallia and other Pleurothallids in the New World are found in this ecosystem.
- Lowland – a near-sea-level environment that is perpetually high humidity, regardless of rainfall.
A tropical savanna landscape in the dry season (top) and wet season (bottom). University of Oregon.
Deciduous orchids have evolved to lose their leaves and “go into dormancy” during the dry season, with the most notable deciduous orchids being many Dendrobium of the Old World, and the many Catasetiinae of the New World. They drop their leaves to endure the dryness of the dry season. It’s important to understand that leaf loss is a coping mechanism for these plants to survive, as more leaves mean more water loss. During periods of drought, having leaves is a liability, as plants lose water through their leaves via transpiration. Because the sun is plentiful, the plants have the energy needed to grow new leaves once the rains return, so it’s an effective survival strategy.
Deciduous orchids time their flowering to environmental cues, and nothing else. This is because the seasonal changes are wide enough such that if a bloom happens out-of-sync with nature, the plant risks failing to reproduce. Deciduous orchids time their blooms so that they are in sync with others of their kind for successful pollination, and that their seed capsules will burst during the rainy season (orchid seeds are so tiny that they have no resources inside the seed and require a fungus and a wet environment to germinate). Therefore, these orchids look for one or more environmental cues to sync up such as:
- Dryness – Many orchids will simply take a reduction of water as a sign that it’s time to drop leaves and flower.
- Time – Many orchids have an internal clock where they will go dormant at a particular week in the year. Being deciduous by time alone is rare, but this may be synced up with other cues to create a blooming window where if any of the other blooming cues occur within the blooming window, then the plant will bloom[1]
- Light changes – Some orchids will take an increase in light as a sign that the days are getting longer (springtime), and that it’s time to bloom. Others will take increasing light as a sign that the trees that shade them are dropping leaves and that it’s time to drop their leaves too.
- Cooling [2] – Some higher-elevation orchids will take seasonal cooling as a sign that it’s time to bloom or drop leaves.
- Communication from other plants [3] – while research is still scant, it’s believed that to sync up flowering, plants will communicate via airborne volatile organic compounds (VOCs). If you’ve had a plant bloom out of season, but pretty poorly, chances are that it was induced by a volatile compound that another organism is making (could even be an unrelated plant around it!).
These environmental cues cause hormonal changes, which lead to leaf senescence (dropping) or flowering.
But what happens if one of these cues never happens?
You may have heard old-timey growers claim that your Dendrobium (or another deciduous orchid) did not bloom because you were “too nice to it.” In a sense, that’s true, but not because of the anthropomorphic aspect. It means that you kept conditions too favorable and summery for the plant to receive any signals at the right time to bloom. Many orchids will keiki instead of bloom if they have an internal clock that tells them that it’s in the blooming window, and the other cues/signals are not found.
To ensure successful blooming with your deciduous orchids, ask the grower typically which month your plant blooms in and which month it goes dormant. Give the plant the cues/signals just before those times, and you should be successful. Pay attention to when the grower starts to restrict water and when the grower’s seasons change. Reduce (but for almost all orchids DO NOT cut off) water, and try some other environmental cues to get your orchid to bloom. This is all under the assumption that your plant is getting enough light, and fertilizer and that the other conditions are met.
[1] Corroborated by many growers’ anecdotal and actual evidence and the fact that some plants just will not bloom at some times of the year, but keiki at the times they ought to bloom.
[2] EFFECT OF LOW TEMPERATURE ON FLOWERING AND ENDOGENOUS HORMONAL STATUS IN DENDROBIUM SCABRILINGUE L.S. Sarathum, S. Tantiviwat, M. Nanakorn, M. Hegele, J.N. Wünsche DOI: 10.17660/ActaHortic.2010.884.87
[3] Bonato, B., Peressotti, F., Guerra, S., Wang, Q., & Castiello, U. (2021). Cracking the code: a comparative approach to plant communication. Communicative & Integrative Biology, 14(1), 176-185.
