A min-max thermometer will show you
the temperature rangeover a given
period of time.
Unlike most of our familiar houseplants, orchids are adapted to conditions that experience not only seasonal variation in temperatures but also significant day/night variations as well. These seasonal and diurnal variations are often critical to proper growth and flowering and, when not provided, may weaken the plant and allow the development of temperature-stress related disorders.
Cool, Intermediate or Warm?
Orchids are usually classified as either cool-, intermediate- or warm-growing depending on their temperature needs and the usual definitions of these ranges are: Warm 80-90F days and 65-70F nights, Intermediate 70-80F days and 55-65F nights and Cool 60-70F days and 50-55F nights.
First, these temperature ranges are for winter conditions. Obviously, cattleyas and many oncidiums tolerate summer days into the 90s or no one in three-quarters of the country would be able to grow them. However, temperatures above the low 90's do cause physiological stress on the plants and their ability to tolerate it is tied to a significant drop in night temperature as well as the typically short duration of summer heat. The same plants constantly exposed to high day temperature with little diurnal variation rapidly develop symptoms of heat stress such as black rot and calcium-deficiency induced leaf-tip dieback.
Second, these ranges are SAFE estimates. We all know individuals who routinely expose their plants to temperatures above and, more importantly below, these ranges without apparent damage but the ability to survive temperature extremes is closely tied to many other environmental and cultural practices as well as the duration. A few hours of temperatures in the 40's will have a remarkably different effect on phalaenopsis than a whole growing season of too-cool conditions. Think of these ranges like traffic hazard warning signs. Does a sign for a curve ahead at 35mph mean you can't take it at 50mph? Of course not, you might be able to but it helps if you are an experienced driver. The same is true of plants. If your plants have been grown under uniformly warm conditions they will be much more stressed by sudden cold snaps than they would be if they were allowed to gradually become acclimated to lower temperatures and wet plants can be more easily damaged by cold than dry plants. Some clones are much more tolerant of extreme conditions that others. That fact is the basis of the development of warmth tolerant or especially cold tolerant landscaping plants.
These temperature ranges overlap somewhat and many genera are either adaptable to a wide range of conditions or have species with different cultural requires such that, with careful selection of micro-climates in your growing area, it's possible to successfully grow a wide range of plants. For those that want nights a bit cooler, moving them closer to the windows will help while those that want to be a bit warmer can be staged further away provided adequate light can be provided.
The Need for a Day/Night Differential
Most of the orchids we grow do best under intermediate temperature conditions. Given adequate humidity and air movement, many will tolerate higher daytime temperatures than the ranges would indicate as long as they cool off at night. Night temperatures that are too warm or too cold are more often than not, much more damaging to plants than day temperatures.
Appropriate night temperatures are critical to good growth and flowering. Most orchids do best with a 10-15F fluctuation between day and night temperatures with those from lower elevations and more tropical climates needing somewhat less but without this day/night temperature differential the plant's respiration and metabolism are impacted. Cool nighttime temperatures allow orchids to store rather than expend the carbohydrates they manufacture during the day. Night temperatures that are too high or day/night fluctuations that are insufficient are perhaps the second leading cause of failure to bloom. If your plants are growing well, with strong vigorous growth but fail to flower and you are sure that your light is adequate, try dropping your night temperatures by a few degrees. You may be surprised by the results. In some cases, plants will not flower unless both the day and night temperatures are below a certain threshold regardless of the day/night fluctuation. For example, phalaenopsis grown under 90F/80F (day/night) temperatures will not flower even though there is a 10F differential. This is because day temperatures above 85F and night temperatures above about 75F inhibit flowering independently of each other. Conversely, if your plants are not producing vigorous growth, try raising or lowering your night temperature a few degrees.
Know thy orchids. Many orchids, especially species, are adapted to significant seasonal variations and without them will either not flower or may not grow at all. This is especially true of plants from higher elevations or more northerly climates. While Dendrobium lindleyi (aggregatum) grows perfectly well during the summer months with temperatures in the 90's during the day and 70's at night, it will not flower without a sharply colder (and virtually dry) winter season. The same sort of seasonal variation is at play in the flowering of nobile dendrobiums and plants like Dendrobium kingianum. In their native habitat, summers can be very hot with temperatures even exceeding 100F but winters are cool and dry. Without this cool winter, flowering is inhibited and the plants produce numerous keikis where there should have been inflorescences.
For every orchid that needs a wide seasonal variation there's an orchid adapted to consistently warm or cool conditions. In some cases these species occupy localized habitats that do not experience significant seasonal variations because of altitude or equatorial location or they may come from high or low altitudes. This is where a little research into the native habitat of your plants will go a long way to successful culture.