Orchids are one of the most popular houseplants because they are easy to grow and flower – for some of us. But many people struggle to get them to re-bloom a second time. Over the years I have heard a number of myths for reblooming orchids and I have even been guilty of passing some of these on to others. In this post I will separate myth from real science to better understand how best to flower orchids.
Orchids Vs Phalaenopsis
The term orchid is used to describe some 25,000 species and as many hybrids in the orchid family (Orchidaceae), including one genus called Phalaenopsis (phals). Phalaenopsis are the most popular orchid houseplant because it likes the warmer temperatures found in our homes and it gets by with less light. Few other types of plants can match its long flowering period, except maybe my new love Streptocarpus which blooms even longer.
When most people use the term orchid, they are referring to phals as if there are no other orchids.
My focus in this post is on phals because they are so popular and because most of the research has been done on them. I suspect that much of this information also applies to other orchids that are native to tropical regions.
The Secret to More Orchid Flowers
The single most important thing you can do to get more flowers is to grow good orchids. Larger plants with larger leaves produce more flowers. A plant with 5 leaves or more is easy to flower although a plant with fewer leaves can flower. A plant with poor leaf growth or a small root system will simply not flower or not flower well. Step one is to learn how to grow the orchid with the right temperature and light level. Proper watering ensures good growth and a good root system.
New leaves on phals should be at least as big as the older ones. If they are getting smaller, your cultural conditions are not as good as they could be.
In a lab test, phals that were force-fed sucrose, using foliar spray, produced flower spikes sooner provided that temperatures were not inhibitory as explained below. Healthy plants contain more natural sugars.
Library of Videos on Orchid Culture
More Light Produces More Orchid Flowers
Phals are known as low light orchids, which is true. This means they can grow and flower with less light than other orchids, but it does not mean they will grow well in low light. Phals need more light than most indoor plants.
A lot of common advice says that they should not be given direct sun, but that is not correct. My orchids go outside in a shady location each summer were they get full sun for 3-4 hours each day. In spring they are conditioned slowly to this light and they are watered properly. The leaves go from a medium to deep green in winter to a very yellow green by mid summer and they flower every year.
If leaves get too hot or if they are exposed to sun too quickly, they will burn.
Studies with phals show that increased light intensity reduces the time for flowering, results in multiple flower spikes with more side branches. Another study kept plants in the dark for 2 to 6 weeks and then exposed them to higher light which caused them to flower in 35 days. This implies that you can grow them at low light part of the time and they will flower provided they get higher light levels for a month or more.
More Fertilizer Produces More Orchid Flowers
Fertilize weakly weekly has been the mantra among orchid growers for a long time but this is a complete myth. I have discussed this in more detail in Orchid Fertilizer Myths.
Calculating 100 ppm
A teaspoon of soluble fertilizer weighs about 5 g (will vary depending on the product).
One tsp of a 10% nitrogen fertilizer added to one gallon of water will produce a 132 ppm nitrogen solution. The same solution will be produced by using 1/2 tsp of a 20% fertilizer or 1/3 tsp of a 30%.
Do Temperature Changes Produce More Orchid Flowers?
Historically, orchid enthusiasts believed that a cool down at night will cause orchids to initiate flowers. The science disagrees with this for Phals, but temperature is important.
“Phalaenopsis originate from tropical and subtropical areas of the South Pacific Islands and Asia, and thus have unique temperature and light requirements compared with other common potted flowering plants. In their native habitats, tropical conditions persist throughout the year with temperatures ranging from 28 C to 35 C (82 F to 95 F) during the day and 20 C to 24 C (68 F to 75 F) at night.”
One study looked at growing phals at different temperatures (no night cooling) and found that a temperature of 29 C prevented the flower spike from forming while temperatures of 14, 17, 20, or 23 C resulted in flowering. Tests on Phalaenopsis amabilis found that temperatures over 28 C produced only vegetative growth.
Phals grown at 29 C during the day, with a night time drop to 17 or 23 C did not flower. A cool night temperature will not overcome a day time temperature that is too high. For some phals an 8 hour duration at 29C is enough to prevent flowering, but other clones may need as much as 12 hours to have flowering turned off.
When plants are grown at lower temperatures, a drop in temperature at night is not needed to induce flowering. A constant temperature below 26 C (79 F) will cause them to flower.
How Does Day Length Affect Flowering?
An 8-hour day length of good quality of light (PPF at 200 μmol·m−2·s−1 ) will produce a high-level of photosynthesis and good a quality bloom in phals. A 12-hour day length will produce a higher flower count and earlier flowering. Day lengths longer than 12 hours do not improve flowering.
The number of inflorescences (flower spikes) per plant and the total number of flower buds were greatest when the average daily temperature was 14 or 17 C (57 or 63 F) and less when daytime temperatures were higher.
Temperature Affects Bud Count
Temperatures above 26 C (79 F) after spike initiation can reduce the number of buds formed and the size of flowers. After bud development it can even lead to bud blast (bud drop).
Prolonged exposure at high temperature can cause keiki (new plantlets) growth on the flower spike.
Best Growing Temperature
The above information has focused on flowering and not vegetative growth. Phals grow faster at higher temperatures. Commercial growers keep young plants at 28 to 32 C (82 to 90 F) during the day with a drop in temperature at night. Once they are at a mature size, they are cooled down for best flower production.
These high temperatures are not required to grow good sized healthy plants in a home. Normal house temperatures work fine especially if they are coupled with warmer days outside in summer.
Provided plants are growing well and are in a suitable environment, temperature controls the initiation and development of flower spikes. A temperature above 26 C (79 F) prevents flowering in most cultivars and it is the day temperature that is most important. Night temperature has little effect on flowering.
As temperatures drops below 26 C, the number of buds and spikes increase. A minimum of 8 hours of light is required and increasing this to 12 hours will increase bud count.
Once the flower spike starts growing, plants should be maintained above 11 C (52 F) and the rate of development increases with temperature, up to about 25 C.
Plants that receive the proper temperature for flowering and still don’t flower are either not grown properly, are too small, have too few leaves or don’t receive enough light.