Whether phalaenopsis truly need distinctly cooler night temperatures is debatable. Photo: jaycwolfe.com & bellefioriflorist.com
One persistent semi-myth about growing orchids is that they need a distinctly cooler night temperature in order to bloom. It’s only a semi-myth, because there are indeed orchids (and in fact, many of them) that do need truly cool night temperatures: think of hardy orchids like slipper orchids (Cypripedium spp.) or many of the dendrobiums (Dendrobium nobile and others). However, 99% of orchids sold today are moth orchids (Phalaenopsis cvs) and their need for distinctly cooler night temperatures is … debatable.
Things Have Changed
The official recommendation I was given when I first started growing phalaeopsis orchids back in the 1980s was that they needed 70–80ºF (21–27ºC) days and 55–65ºF (13–18ºC) nights starting in the fall to order to initiate bloom. (Nighttime temperature dips during the spring and summer have always been considered less vital to flowering.)
The first part was easy enough: 70–80ºF (21–27ºC) are average indoor temperatures. But 55–65ºF (13–18ºC) nights: geez! I gardened at the time in an apartment with pretty much no temperature control. There was no way I could offer them nights that cool. So, I just gave my phals the best care I could and hoped for the best. And guess what? They did bloom!
The reason is partly because such strict temperature ranges really weren’t that necessary to start with. (Early orchid growers were likely a bit too zealous in their advice!), but also because phalaenopsis have, over time, adapted to “normal indoor growing conditions.”
The original species, phalaenopsis taken directly from the wild, may indeed have had distinct night temperature preferences (at least, some of them did), but as hybrid varieties came to dominate the market, that became less and less true. The ones that bloomed the best were kept and those that weren’t so satisfactory were eliminated. And thus gradually “easier to grow” phalaenopsis, often ones that bloomed more than once a year, began to take over the market. And one factor in making orchids “easier to grow” included the lesser need for a distinct night temperature change during the fall and winter months, so hard to give in the average home.
This is actually common with almost any cultivated plant. By choosing the most successful subjects from even a “difficult” plant, using those in crosses and keeping, again and again, year after year, the best performing plants of each generation, our garden plants have become less wild and more adaptable to cultivation. Some are no longer even capable of growing in the wild anymore! And so it is with orchids as well.
Even by the 1980s, this was already happening, whence my success with phalaeopsis orchids in spite of having no temperature control. It’s even more the case today. It’s hard to find a hybrid phalaenopsis today that really needs at nighttime temperature drop to 55ºF (13ºC) in order to bloom!
If You Can…
A bit of a nighttime drop is still helpful. Cooler nighttime temperatures allow orchids to slow down and store carbohydrates rather than expending them all night. But they’re unlikely to need a precipitous drop. Even a 5ºF (3ºC) drop from the daytime temperature (whatever it is) usually suffices these days, and that’s usually easy enough to give … with no effort on your part!
First, sunlight during the day warms up leaves that then cool down when the sun goes down in the evening. You don’t have to do anything special to take advantage of that. Plus, cooler nighttime air outdoors in fall and winter tends to cool the air just inside the window, so plants on a windowsill typically do profit from a bit of a temperature drop even on cloudy days. Even plants growing under lights in a basement far from any natural light tend to cool off a bit when the lights are turned off at night.
That’s why most phalaenopsis automatically profit from somewhat cooler nights even if you don’t do anything special … and that’s generally enough for them to bloom.
That said, if your phalaenopsis is growing vigorously, but still fails to flower, you might want to try and find a way to give it a good 10˚C (5ºC) temperature drop at night. You just might have a recalcitrant phal not as adapted to modern ways!