Plant Culture
A 48-in L × 48-in W × 80-in H (122 × 122 × 203 cm) grow tent has been modified for controlled-environment cultivation of Pleurothallidinae and other tropical high elevation orchid species.
The system is modeled after the Tropical High Elevation House (THEH) at the Atlanta Botanical Garden. Read more about the facility here.
Unlike the THEH, which uses a commercial air washer to lower the temperature, this system uses a portable air conditioner to cool the tent.
Cold air is blown into the tent while the warmer exhaust is directed upward toward the ceiling. The air intake is located on the unit outside the tent, so it doesn't remove humidity from the growing environment.
The air conditioner is self-evaporating, so there is no need for a container to collect condensation runoff.
The air conditioner operates continuously under two programmed temperature schedules. From 8:00 AM to 8:00 PM, the maximum temperature is set at 74°F (23°C). From 8:00 PM to 8:00 AM, the maximum temperature is reduced to 61°F (16°C).
These settings establish a controlled diurnal temperature cycle that reproduces the alternating daytime warming and nighttime cooling characteristic of tropical montane cloud forests.
During the daytime cycle, temperatures rise gradually toward the programmed maximum. At 8:00 PM, the lower nighttime setting initiates cooling, with temperatures declining through the evening and remaining comparatively low until the daytime schedule resumes at 8:00 AM. Actual nighttime temperatures can reach approximately 57°F (14°C) and fall even lower during the winter months.
This pronounced diurnal temperature fluctuation is particularly important for many high-elevation Pleurothallidinae, which inhabit tropical montane cloud forests characterized by comparatively warm days and substantially cooler nights. Rather than maintaining constant temperatures, the system recreates the daily thermal fluctuations characteristic of these environments.
Humidity is monitored and controlled using a 5-gallon capacity humidifier which is programmed to maintain 85% relative humidity (RH) .
The system maintains a consistently humid environment, with relative humidity typically ranging from approximately 85% to 98%. Humidity fluctuates throughout the day as the temperature changes.
The relationship between temperature and humidity is particularly important because warmer air has a greater capacity to hold water vapor than cooler air. As the temperature drops overnight, relative humidity rises, often approaching saturation. This helps reproduce the cool, humid conditions characteristic of tropical montane cloud forests, where orchids are frequently exposed to mist, fog, and high atmospheric moisture.
Vapor pressure deficit (VPD) provides another way of understanding these conditions. Unlike relative humidity, which measures how close the air is to saturation, VPD measures the difference between the amount of moisture present in the air and the amount it could hold at saturation. Lower VPD means less moisture is drawn from the plants through evaporation and transpiration.
In the tent, VPD has ranged from approximately 0.04 to 0.4 kPa, with higher values occurring during the warmer daytime hours and lower values occurring during the cool, humid nights. These conditions help limit moisture loss in many miniature Pleurothallidinae, which have limited capacity to store water and are adapted to environments where atmospheric moisture is consistently abundant.
Artificial lighting is provided by SEAOURA aquarium lights, which offer several advantages over conventional grow lights.
Aquarium lights are designed for use in wet environments and have sealed housings that protect their electrical components from moisture. This makes them particularly suitable for the high humidity maintained inside the tent and may contribute to a longer service life.
Aquarium lights offer a variety of lighting options, including the ability to customize the light spectrum. These lights allow the proportion of red light to be increased, which encourages flowering.
The photoperiod is maintained at 14 hours, from 8:00 AM to 10:00 PM, throughout the year. The lights are switched on and off automatically using a smart plug that can be programmed and controlled remotely through a smartphone app. Unlike conventional mechanical timers, the smart plug retains its programmed schedule following a power outage, eliminating the need to manually reset the timer when power is restored.
Plants are watered approximately every other day using a 2-gallon battery-powered sprayer. Ordinary tap water is used without modification, except for the addition of fertilizer when needed. The sprayer allows water to be applied gently and evenly, particularly to miniature orchids and mounted specimens with delicate root systems.
The majority of the plants are grown on cedar mounts.
Potted plants are grown in clear plastic pots in live red sphagnum moss (Sphagnum capiliifolia).
1. Living sphagnum provides a loose, free-draining growing medium that maintains moisture without becoming excessively compacted. Unlike dried sphagnum, which can become waterlogged and contribute to orchid root rot, living sphagnum maintains an open structure that promotes drainage and root aeration.
2. Living sphagnum continues to grow rather than progressively decomposing and compacting like dried sphagnum, allowing orchids to remain in the same growing medium for longer periods without requiring frequent repotting.
3. It is also plausible that living sphagnum provides other benefits to the orchid roots. Its photosynthesis and respiration may influence the air immediately surrounding the roots, while evaporation from the moss might help keep them cool and maintain humidity. These are possibilities I have not yet investigated.
