Dendrobium primulinum, also known now as Den. polyanthum

I have had Dendrobium primulinum for about five years and I have found it is among the most beautiful and easy to grow of all Dendrobium. Unfortunately easy to grow doesn't necessarily means it is also easy to bloom. All the plants I have brought have grown well with what I consider standard care for members of the Dendrobium section of the genus Dendrobium. However getting these plants to bloom has been at times a source of frustration and disappointment. It appears that I am growing this species at the very upper limit range of their temperature tolerance for blooming. That means that relatively small variations on the night temperatures at certain critical parts of the year can have profound influence on the quality and quantity of the blooming.


Growers that grow primulinum in the lowland coastal plains of Puerto Rico report that the plants bloom with few flowers or even a single one. Other plants may fail to bloom at all. Irma Selles,a well known local orchidist or renowned skill growing plants showed in an Internet forum a large healthy plant with only a single flower. Dr. Julio David Rios, an accomplished grower of many types of orchids says that his primulinum plant has never bloomed. Don Alfonso a Mayaguez grower that had a large collection of orchids in the seventies and eighties that had a plant of primulinum var. giganteum for many years would complain that his plants bloomed very rarely and often would produce just a single flower. All the plants in the previously cited cases are growing under a climatic regime where night temperatures rarely fall much under 75F at night even at the depth of the tropical winter (which actually is not that cold and could be more properly regarded as the dry season). Where I live, in the mountainous interior of the island, at a height of about a 1,000 feet, temperatures fall to the top sixties at night in winter and may go down to the high fifties for a few nights during the coldest part of the year.
This 10 degree difference between the night temperatures in the coast and the interior makes all the difference in blooming primulinum. But it has to be said that even thought my plants bloom better than the plants in the coast, they could probably bloom even better if the temperatures dropped down consistently into the fifties. I know this because I have seen photos of primulinum growing outdoors in Hong Kong, China and there they produce many more flowers in a single cane than plants in Puerto Rico. In the year 2009 my plant bloomed poorly, with the exception of a single cane that produced two flowers all the other canes failed to bloom. Some of my primulinum, a plant labeled var. Leon, seems to be less sensitive to excessive warmth, however even this plant has, on occasion, failed to bloom.  So if you have cool winters where temperature dips into the fifties and forties, and you also have sunny, warm and wet summers, this plant will probably do well for you.

I have my plants in a place where they get full sun during the morning. Due to the orientation of the shadehouse where they are growing they get more sun as autumn goes into winter, this is similar to what happens in their native habitat where they are exposed to brighter light conditions when the trees where they live lose their leaves.
My plants are watered daily from May to November and then the watering frequency tampers down until they may not get water for two or three weeks at the height of the dry season. I watch the canes and if they get too wrinkled I water the plant lightly.
During their growth phase, in my garden it is from May to November, I fertilize every week. I also put a few pieces of dry cow or horse manure on top of the media, this seems to help produce larger canes. Because the plants are grown outdoors and get a good deal of rain I don’t worry about fertilizer salts build up on the baskets where they are grown.

I grow them on plastic and wire baskets that allow for excellent exposure to air of the roots. The roots die if deprived of oxygen and this can spell doom for a plant. My plants are grown hanging from the bottom of the baskets, this allows for a more natural presentation of the flowers and in my view for easier watering and fertilizing. I don’t grow my plants in plaques of any kind as local conditions means that they dry up very fast necessitating too frequent watering.
They seem to grow well in all the media I have tried bark, cork and coconut. I would guess that these plants will grow well in most orchid planting media as long as watering and fertilizing is done taking in account the properties of the media.
I have three types of primulinum, one has bluish petals and sepals and a white lip, another has al white lip with two yellow eyes and baby pink sepals and petals. The flower of the third type I have has so much yellow in the lip that it resembles a Dendrobium farmeri flower, the petals and sepals of this flower are light pink. I have heard about some plants that have been offered for sale as primulinum alba, but the ones I have seen appear to be white individuals of a similar species Dendrobium cretaceum.
Although primulinum has very beautiful flowers, if care is not taken to groom the plant the flowers can suffer from a poor presentation. This mainly consists of flowers in each side of a cane pointing in opposite directions and flowers from different canes getting all bunched up. I have seen photos of plants exhibited in orchid shows in Asia, in particular in Japan and Taiwan which were groomed with skill that the intrinsic beauty of the flowers was greatly enhanced by the delight artistry of the presentation.

When my plants bloom next year I will try to emulate the high standards set by exhibitors in Asia to make my plants a living work of Art. Wish me luck!

Notice the excellent presentation with all the flowers pointing to the same side

An unusually dark blooming of var. Leon

Note the huge round lip characteristic of this species