If you allow your orchids the pleasure of growing outside during the warm season, they will reward you with an abundance of growth and blooms. You may have to make some adjustments to protect your orchids when the cool season arrives. Some orchids are very intolerant of cold and may need to be relocated to a warm winter home while others are more cold tolerant and might only need protection on the coldest nights. Each type of orchid has its preferred minimum night temperatures during the winter cool season, below which cold damage to the plant will occur.
- Most phalaenopsis, large two-toned vandas, evergreen dendrobiums and trichocentrums (mule-eared oncidiums) are the least tolerant of cold, preferring nighttime temperatures above 60 F (15.5 C), though some tolerate temperatures in the 50s F (down to 10 C).
- Most cattleyas and oncidiums prefer winter night temperatures in the mid- 50s F (12–13 C), though some tolerate temperatures in the mid-40s (above 7 C).
- Deciduous dendrobiums bloom better after a cooler, drier winter rest period with no fertilizer, tolerating temperatures in the low- to mid-40s (4.5–7 C).
- Dendrobium section Dendrocoryne and cymbidiums are the most cold-tolerant orchids of those that can grow in summer heat and are accepting of temperatures down into the 30s F (1–4 C).
CATTLEYAS
As a general rule, Cattleya Alliance plants prefer temperatures above 55 F (12.8 C) though many will tolerate temperatures into the mid-40s (above 7 C). Cattleyas from the Amazon such as Cattleya violacea prefer warmer temperatures, and there are many cold-hardy varieties that tolerate temperatures in the mid- to upper-30s (1.5–4.5 C), such as Cattleya coccinea, Cattleya loddigesii, Cattleya intermedia, Laelia anceps and Cattleya purpurata. As a general rule, protect your cattleyas when temperatures drop below 50 F (10 C), particularly if they are in bud or in bloom.
CYMBIDIUMS
Cymbidiums are perhaps the most widely grown orchids, beloved for their variety of color, long bloom period and ease of culture. Sadly for southern hobbyists, many are cool-growers that don’t grow or bloom well in summer heat unless you have carefully selected for warmth- and heat-tolerance. Cymbidiums are very cold tolerant. Their ideal minimum temperature is 40 F (4.5 C), although they tolerate temperatures into the mid-30s (about 1.7 C) and will survive light freezes with some cold damage.
ONCIDIUMS
While orchids in the Oncidium Alliance have a wide range of cold tolerances, many of the cool growers that can withstand near-freezing winter temperatures are not grown in warm climates because they cannot tolerate the summer heat. The thick-leaved trichocentrums (mule-ear oncidiums) such as Trichocentrum lanceanum, are from lowland tropical areas and resent temperatures below 60 F (15 C). Most that will grow well in the summer heat (such as brassias, miltonias [NOT Miltoniopsis], typical yellow-flowered oncidiums and similar genera and the myriad of intergenerics including Brassidium, Miltassia, Miltonidium, Oncidesa, etc. prefer night temperatures in the mid-50s F (around 13 C), but will tolerate temperatures down to the mid-40s (about 7 C).
PHALAENOPSIS
Phalaenopsis are fairly cold intolerant, but they enjoy the first fall cold snap. Allow them to chill down to 55 F (12.8 C) for two or three weeks during this period when the daytime temperatures go back above 70 F (21 C) during the day. This brief chilling will tell your plants that it is time for them to set their bloom spikes, after which they can be moved to their indoor winter home. As a general rule, phals enjoy nighttime temperatures above 60 F (15.6 C). If temperatures drop below this minimum, some phals will drop their buds, particularly the standard Phalaenopsis such as Phalaenopsis amabalis, Phalaenopsis schilleriana and Phalaenopsis stuartiana that are less cold tolerant. Phalaenopsis from higher elevations and the foothills of the Himalayas such as Phalaenopsis lindenii, Phalaenopsis lobbii and Phalaenopsis mannii are more tolerant of lower temperatures down to around 50 F (10 C).
DENDROBIUM
There are perhaps over 1,000 species of dendrobiums organized into more than 40 sections that grow in a wide variety of habitats and elevations. For those dendrobiums that grow in warm climates, cold tolerance ranges from the least cold-tolerant Dendrobium section Phalaenanthe (plants related to Dendrobium bigibbum) to the most cold-tolerant plants in section Dendrocoryne. The warm tropical growers will quickly drop leaves if exposed to conditions that are too cool. The more cold-tolerant dendrobiums actually flower better when exposed to cooler and drier conditions during the winter, though they shouldn’t be fertilized after Thanksgiving until the new growth begins in the spring. Minimum temperatures for various sections of the genus are:
Section Phalaenanthe
Characterized by flowers that resemble those of Phalaenopsis. Includes the species affine, bigibbum (phalaenopsis), compactum, dicuphum, schroederianum, superbiens and williamsianum. 60 F (15 C).
Section Spatulata
The so-called antelope dendrobiums; characterized by flowers with twisted petals. Includes the species antennatum, bicaudatum, canaliculatum, gouldii, johannis, lineale (veratrifolium), stratiotes, strebloceras, taurinum and undulatum. 60 F (15 C).
Sections Formosae and Foveolatae
Characterized by plants with cane-like pseudobulbs with black hairs on silvery sheaths. Includes the species bellatulum, cruentum, dearei, draconis, formosum, infundibulum, lowii, margaritaceum, sanderae, schuetzei, senile and spectatissimum. Mid-50s F (12.8 C)
Section Latouria
Characterized by mostly white, yellow and green long-lasting flowers. Includes the species aberrans, alexandrae, atroviolaceum, convolutum, forbesii, johnsoniae, macrophyllum, rhodostictum and spectabile. Mid-50s F (12.8 C)
Section Pedilonum
Characterized by bright pink, red or purple flowers borne on congested inflorescences on leafless canes. Includes the species bracteosum, bullenianum, capituliflorum, crenatifolium, goldschmidtianum (syn. miyakei), purpureum, secundum and smilieae. Mid-50s F (12.8 C).
Section Densiflora (Callista)
Characterized by showy pendent, usually yellow, flowers presented on grapelike inflorescences. Includes the species lindleyi (aggregatum), chrysotoxum, densiflorum, farmeri, jenkinsii, sulcatum and thyrsiflorum. Mid-40s F (about 7 C).
Section Dendrobium
Upright and pendulous inflorescences produced on leafless canes. Includes the species anosmum (superbum), aphyllum, chrysanthum, heterocarpum, loddigesii, moniliforme, nobile, parishii, polyanthum (primulinum), cucullatum (pierardii) and unicum. Mid- to low-40s F (5–8 C).
Section Dendrocoryne
Very freely flowering plants from Australia. Includes the species adae, aemulum, callitrophyllum, falcorostrum, finniganense, fleckeri, gracilicaule, jonesii, kingianum and speciosum. Mid- to upper-30s F (2–4 C).
VANDAS
Vandaceous orchids as a whole are fairly cold intolerant although some species from higher elevations withstand lower temperatures. If you are unsure of your plant’s genetic background, keep nighttime temperatures above 60 F (15.6 C). This is particularly true of the widely hybridized Vanda sanderiana with large two-toned flowers and species of the fragrant genus Aerides. Some Vanda species such as Vanda coerulea, Vanda denisoniana and Vanda tessellata are more cold tolerant, as are many members of the colorful small-flowered vandas previously known as Ascocentrum and Ascocenda, Renanthera and Rhynchostylis that are comfortable down to 50 F (10 C). Vanda falcata is probably the most cold- tolerant vandaceous orchid, accepting of temperatures in the lower 40s (4–5 C). If exposed to excessively low temperatures, vandaceous orchids will start dropping leaves, giving the plants an unappealing palm tree appearance.
Orchids are often categorized into general temperature groups that are based on their preferred winter night temperature below which growth slows. The three temperature groups are the warm growers with nights 60–65 F (15.6–18.3 C), the intermediate growers with nights from 55–60 F (12.8–15.6 C) and the cool growers with nights from 50–55 F (10–12.8 C). Many orchids that demand cool conditions in winter also prefer cooler summers than many locales offer so they are not good candidates for growers without special coolers in their growing area. Most plants suitable for growing in the summer heat are intermediate to warm growers that grow best when plants are protected from winter night temperatures below the 50s (less than 10 C). If you cannot protect plants during the cooler weather, be careful to choose plants that both thrive during the summer heat and are tolerant of winter cold.
— Sue Bottom started growing orchids in Houston in the mid-1990s after her husband Terry built her first greenhouse. They settled into St. Augustine, Florida, Sue with her orchids and Terry with his camera and are active in the St. Augustine Orchid Society, maintaining the society’s website and publishing its monthly newsletter. Sue is also a member of the AOS Publication Committee (sbottom15@gmail.com).
