Spring is always my favorite time of year. Flower boxes, gardens, and storefronts are filled with colorful blooms heralding the arrival of summer. It’s also when rhododendrons are at their most beautiful. The breathtaking display of azaleas, a subcategory of the Rhododendron genus, is less fleeting than that of their outdoor cousins: they can be seen in stores for several months and can bloom for up to two months, provided they are well cared for. That’s what we’ll be discussing in today’s article.
A Family Story
Azaleas have been cultivated for hundreds of years. The cultivars we see today are mostly complex hybrids, hence the nickname “florist’s azalea,” a reminder of their horticultural origins.
Azaleas belong to the genus Rhododendron, which includes a thousand plants in the Ericaceae family. The genus Rhododendron has undergone various revisions after each generation of botanists attempted to bring order to this large family. To reflect this complexity, the genus Rhododendron is now divided into subgenera, each with a section name, eventually leading to discrete scientific names such as Rhododendron sub. Azaleastrum sect. Tsutsusi, followed by the species name and cultivar… (It is easy to understand why the name is abbreviated most of the time.)
Despite its relative complexity, this system has at least the advantage of dividing rhododendrons according to tangible characteristics, such as foliage (deciduous or evergreen) or the presence of scales, i.e., the tiny dot-like structures that cover the surface of the leaves of some plants of the genus Rhododendron (especially the underside of the leaves). For the small section of highly specific vocabulary development, we will refer to lepidote rhododendrons (with scales) and elepidote rhododendrons (without scales). Other criteria are involved in this complex task of dividing the genus.
Rhododendron
The Rhododendron genus belongs to the large Ericaceae family, which includes certain fruit bushes such as blueberries (Vaccinium angustifolium, Vaccinium myrtilloides, and Vaccinium corymbosum, in particular) and cranberries (Vaccinium macrocarpon and Vaccinium oxycoccos).
Plants from this family are not generally grown indoors, except for azaleas, which are not exactly the best houseplants—perhaps it is wiser to consider them migratory plants, which spend the winter indoors but must be taken outside in the summer, like bougainvillea.
Florists’ Azalea
If some of you are as confused as I am on this beautiful spring morning, don’t worry: this article is only about florists’ azaleas. All distinctions of family, genus, subgenus, and section are unnecessary: as complex hybrids, florists’ azaleas are almost never accurately identified, and their parentage is lost in the maze of hybridization…
Origin
The main relatives that contributed their genes to the florist’s azalea are Japanese azaleas (R. sub. Azaleastrum sect. Tsutsusi) and Chinese azaleas (R. sub. Azaleastrum sect. Pentanthera). In the wild, these small shrubs grow to only one or two meters in height, a striking contrast to some rhododendrons, which can become absolutely enormous (and, consequently, populate my wildest dreams).
Japanese azaleas live in subtropical and temperate regions; they are also found in other countries in the surrounding area. Like all rhododendrons, they need acidic soil and prefer to grow in partial shade rather than full sun. Azaleas are widely cultivated in Japan, and many hybrids originate from this thousand-year-old culture.
Chinese azaleas are very similar to Japanese azaleas, both in appearance and in their growing requirements. They thrive particularly well in the Gaolignong Mountains, where there is an azalea that is over 500 years old and nicknamed “The King of Azaleas in the World” due to its circumference of nearly 10 meters! Azaleas are used in traditional Chinese medicine (a surprising fact, given that rhododendrons, including azaleas, are highly toxic).
Azaleas or Rhododendrons?
The terms “azalea” and “rhododendron” are sometimes used interchangeably, but they are not entirely interchangeable. All azaleas belong to the genus Rhododendron, but not all rhododendrons are azaleas.
There are several criteria that separate the two, with some inconsistencies depending on the source consulted. Size is one: although azaleas are generally smaller, there are large azaleas as well as small-leaved rhododendrons, which does not help to distinguish between the two.
Another suggested distinction is the characteristic of the foliage in winter; most rhododendrons are evergreen, meaning they remain leafy during the winter, like firs or spruces. However, not all azaleas are deciduous: the term “azalea” is used more specifically to refer to the Tsutsusi section (evergreen or semi-evergreen azaleas) and the Pentanthera section (deciduous azaleas).
We can also mention the presence of scales: all azaleas are elepidote, meaning they have no scales. However, although many rhododendrons are lepidote, some rhododendrons—which are not azaleas—are also elepidote, so this distinction is not consistent either.
Finally, we can try to distinguish azaleas from rhododendrons by the number of stamens in their flowers: azaleas have fewer stamens (usually five), compared to ten or more in rhododendrons. But again, these are somewhat general rules.
If, when trying to distinguish between azaleas and rhododendrons, you find that it all seems rather arbitrary, you’re right! This is because it is mainly in horticulture that attempts are made to distinguish between the two, with varying degrees of success. In terms of botany, we return to the (not much simpler) system of genera, subgenera, and sections mentioned above. And once again, I’ll leave it at that!
Description
The florist’s azalea is a small shrub measuring 20 to 60 centimeters (8 to 24 inches), sometimes a little taller, but never exceeding one meter indoors. It has a bushy habit and is highly branched at the base. The branches are covered quite densely with small, glossy oval leaves that are very dark green. They are sometimes even hairy and generally shiny.
The main attraction of the azalea is its stunning flowers. The flowers are large for the size of the shrub, measuring five to seven centimeters, and funnel-shaped. The five petals, parchment-like or slightly wavy, are deeply cut. The flowers, with their five clearly visible stamens, last for one to two weeks. During the flowering period, the flowers appear in quick succession. A well-cared-for azalea can remain in bloom for about two months.
Flowering usually occurs in late winter and spring, rarely throughout the year except for a few particularly attractive hybrids. This spring flowering explains why azaleas are available at this time of year. The flowers are generally scentless, although some plants are intensely fragrant.
Florists generally distinguish between azalea varieties by the color of their flowers: white, the entire range of pink, red, and orange, and even magenta and lavender.
Growing Tips
Light
As a general rule, plants need bright light to produce flowers. Azaleas are no exception. Since they develop their buds during the darkest season, at least in the northern hemisphere, more intense light may be necessary during the winter.
Watering
Azaleas are thirsty plants, and their soil must remain moist at all times. When in bloom, they lose even more water through evapotranspiration, so it may be necessary to water them more than once a week. During the dormant period (after flowering), reduce watering slightly, allowing the soil to dry out very slightly.
If possible, water with non-calcareous water, such as rainwater, distilled water, osmosis-filtered water, or water from a dehumidifier. Note that letting the water stand for several hours has no real effect, except on the temperature. For more information, check out this article.
Atmospheric Humidity
Azaleas do not tolerate the humidity in our apartments very well, which is generally too low, especially when temperatures are too high (above 16°C/60°F). They would not be the worst plants if it weren’t for the fact that they are very fragile during the months when atmospheric humidity is at its lowest. In heated rooms, the relative humidity should be kept at around 50-60%.
Potting
Azaleas are among the few plants that absolutely require acidic soil (between 4.5 and 5.5 pH). A soil mix based on horticultural peat is naturally acidic. For more information on soil mixes, check out this article.
It is best to avoid repotting during bud formation and flowering, unless the plants need it so badly that watering has become difficult—which is often the case with azaleas sold in garden centers: they are in pots that are too small for the size of the plant. If watering is not compromised, wait until just after flowering to repot.
Fertilizer
Azaleas can be fertilized with an acidifying fertilizer every 2-3 weeks during the summer. When the plant is forming buds or in bloom, fertilization can cause the buds to fall off and should therefore be avoided. Caution: azaleas are often heavily fertilized to promote a stunning first bloom… obviously, it is best to fertilize sparingly or even wait a year before doing so.
Temperature
This is the main problem with azaleas and often causes them to die. Unless you have a slightly cooler room, it is very difficult to keep these delicate plants in a “normally” heated apartment, and they often end up losing their leaves, even those that are kept sufficiently moist.
Azaleas often require a cooler temperature than we find in our homes. When they are in bloom, aim for a temperature between 60°F and 68°F (15°C and 18°C) and higher humidity. After flowering—i.e., during the dormant period—temperatures can even drop to 7°C/45°F without harming the plant.
To stimulate bud development, temperatures between 7°C and 13°C/45°F and 55°F are necessary, which is why a short outing in the fall after the summer outing is recommended (see below).
Maintenance
In terms of general care, you should pinch back the stems regularly to keep the plant compact. It is normal for the flowers to bloom one after the other and even for the plant to lose its leaves relatively quickly, especially if you forget to water it regularly (remember, this is a delicate plant).
After flowering, allow the plant to rest. It is best to move it outside in the shade as soon as temperatures allow (minimum 7°C at night). It can then enjoy full shade (which is still brighter than most indoor rooms), high atmospheric humidity, and rain, which washes away any limescale or other minerals that have built up in the soil.
Azaleas particularly like to stay outside longer than most other indoor plants (again, minimum temperature 7°C/45°F) as this period of cool weather stimulates flowering.
Since azaleas require a mandatory rest period, this means managing temperature changes. The simplest solution is still to take them outside so that you only have to manage the dates when they go in and out. Nature works in wonderful ways: a few weeks outside in the fall will be enough for azaleas to rest and start budding.
It is for this reason—and because of the delicate nature of the plant—that I consider azaleas to be outdoor plants that are kept indoors to help them survive the coldest months.
Propagation
It is technically possible to take cuttings from semi-woody stems, i.e., stems that are half covered with bark, but this process is extremely delicate and should only be attempted by professionals.
Problems
- Leaf drop can be caused by a number of problems. Sudden changes, insufficient watering (which can happen very quickly, as azaleas need a lot of water), dry air, or high temperatures—the latter being the most common cause.
- The leaves are discolored and falling: check for signs of red spider mites. A good weekly shower for two or three weeks is often enough to get rid of a small infestation.
- The tips of the leaves start to dry out: the ambient humidity is insufficient. A humidifier may be necessary. Lowering the heating in the room where the azalea is located may help, if possible, of course.
- The leaves are yellowed, growth slows down or stops completely: there is too much lime in the soil, it is time to wash it out.
- Be sure to look carefully for signs of the most common indoor plant pests: scale insects, thrips, whiteflies, aphids, and even the rarer cyclamen moths. (It’s not for nothing that this plant is described as demanding!)
Toxicity
Like all rhododendrons, azaleas are toxic due to grayanotoxins, which are present in all parts of the plant and can cause gastrointestinal, cardiovascular, and neurological problems.
Note on Difficulty
Some books describe azaleas as fairly easy plants, which seems overly optimistic to me. In my experience, they are difficult to keep indoors and impossible to keep without a little summer treatment. I’m sure some people have had positive experiences with them and will disagree with me in the comments—but I’ve never been able to keep an azalea alive for more than two months!
So why do some books underestimate the difficulty of keeping azaleas? I believe that the main prerequisite, which is also the most difficult to meet, is temperature. In fact, few people, at least in Canada, keep rooms below 16°C/61°F in winter. The only rooms that people consider not heating are garages, entryways, and cold rooms, which are typically rooms with little window light. All other rooms are heated to at least 19 °C/66°F, which is already too warm for azaleas.
However, this is not necessarily the case in Europe, where houses—at least some rooms—are not as heated as ours (well, their winters aren’t as harsh, either!). And I think that’s the secret to growing azaleas successfully. If I manage to fulfill my dream of having a cold greenhouse, I will definitely try my hand at a collection of azaleas in all colors, and I will let you know if I now consider azaleas to be easy plants!
Buying Advice
After choosing the plant with the flowers you like best, it is best to choose a specimen with lots of buds that have not yet bloomed, which will ensure long-lasting blooms at home (and not long-lasting blooms… in the store). To ensure your new friend is healthy, choose a plant with moist soil and shiny leaves. A few dried or drooping leaves are acceptable, but these signs of hardship should be kept to a minimum.
Conclusion
Few sights are more impressive than an azalea in bloom. If you are lucky enough to have a well-lit, cool room, kept at around 16 °C/60°F, it would be almost criminal not to buy an azalea from a florist! If the azaleas are already in full bloom, it is likely that the flowering season is coming to an end. Photo by Leonid Dzhepko.
We grew them as simple “azalea”. “Florist azalea” refers to the fancier types that are grown as potted florist types plants, which we did not grow. What we consider to be “florist azalea” are flashy as a potted plants, but do not perform as well in landscapes as more common types of “azalea”.