I can’t always tell rhododendrons and azaleas apart. Is there a way to distinguish between them?
The answer is complicated because an azalea is also a rhododendron, but not all rhododendrons are azaleas! Furthermore, some azaleas are evergreen and others are deciduous. This information from the Azalea Society of America gives a few pointers.
Excerpt:
“Some small-leaved rhododendrons look like evergreen azaleas. To tell them apart, first look at a flower—most azaleas have only 5 or 6 stamens, while most rhododendrons have 10 stamens. Then look at a leaf—azalea leaves tend to be thinner, softer and more pointed than rhododendron leaves. Azalea leaves tend to have long straight hairs parallel to the leaf surface, usually along the midrib on the underside of the leaf. Finally, using a magnifying glass, look at the underside of a leaf for tiny round structures called scales. Azalea leaves never have scales, while small-leaved rhododendron leaves are always covered with scales. (The more correct name for small-leaved rhododendrons is ‘lepidote’ rhododendrons, where lepidote means ‘covered with scales’).”
In case you are curious, evergreen azaleas were first cultivated in Japan over 400 years ago (according to Christopher Fairweather, in his book Azaleas, Globe Pequot Press, 1988). These were the evergreen azaleas. The first azaleas in European gardens (starting in the 17th century in Holland) were deciduous, and they were endemic on three continents (particularly from Turkey, the Far East, and both coasts of North America).
There is a taxonomic explanation in Fred C. Galle’s book, Azaleas (Timber Press, 1987):
“The genus Rhododendron was first recognized by Linnaeus in Species Plantarum in 1753. Linnaeus created a separate genus Azalea containing six species. In 1834, George Don […] subdivided Rhododendron into eight sections which are botanically retained today. Azalea was included under the genus Rhododendron by Don. In 1870. Dr. C. J. Maximowicz made a major contribution in the classification of Rhododendron and many oriental Azalea based on the position of leaf bud in relation to flower buds. More recent classifications reflect refinement rather than major changes. […] The question of splitting Azalea into a separate genus came up as late as 1943, but it is hoped that this classification will never be accepted.”
Why are my rhododendrons spitting water? On hot sunny days this summer, we’ve been sitting in their shade and noticing occasional tiny needle-fine droplets landing on us. Is this caused by insects (I don’t see obvious signs of them on the leaves), or is it normal?
My hunch is that the leaves are transpiring. I found an article by Bill Letcher in the Journal of the American Rhododendron Society (vol. 41, no. 2) entitled “Rhododendrons with a Drinking Problem.”
The article is about the importance of water in successful rhododendron cultivation, but also does a good job of explaining transpiration:
“The water absorbed by the root hairs, or rootlets, is drawn up in the stem of the plant and into the leaves, where it exits via small holes called stomata into the air as vapor. This process is called transpiration […] Transpiration requires a lot of energy, and this comes, as we might expect, from the sun. If all else is equal, transpiration amounts are directly proportional to the amount of radiant energy striking the leaf surface of a plant: the more energy, the greater the amount transpired. […] One factor of paramount importance to rhododendron growers which effects transpiration is the relative humidity of the air surrounding the leaves. Dry air causes appreciably more transpiration than humid air, and the presence of wind increases this dramatically by replacing the layer of air next to the leaf surfaces before it can become saturated. Ambient air temperatures will also markedly increase transpiration as they rise, sometimes with disastrous results. Our family of rhododendrons has less ability to extract moisture from the soil than, say, manzanita (which can pull all of the available water from the soil down as far as six feet!).”
If we have more hot weather, you may want to give your shrubs an occasional supplemental drink of water, since they are shallow-rooted, and our summer-dry climate (depending on how hot and dry it is) can cause stress to the plants. Keep an eye on how they are looking and do what you can to maintain their health (and preserve your shady refuge!). To quote Letcher again:
“The family of rhododendrons has either never been forced to adapt to very low available moisture conditions, or has not adapted well to them. Their roots tend to be quite shallow. The foliage is lush, dense, and of a dark, absorptive color instead of being light and reflective of the radiation. They display their displeasure with your attempts to wean them from the conditions they most enjoy, by doing exactly what a great many other plants will do: they shorten their internodal growth, reduce the number and size of their new leaves, drop some of their older leaves, and just plain sulk! Ultimately, they languish into a pitiful state of scrawnyness and pallor which is not a pleasant sight in your garden.”
This is the second year in a row that my rhododendron Blue Peter has flower buds but they are dry and somewhat dark and have no flowers at all. These buds are easy to deadhead. Can you help me salvage this rhododendron, which is very old, and beautiful when it blooms?
In order to get an accurate diagnosis you will need to take a sample of your plant (including both healthy and affected parts if possible) to a Master Gardener clinic. If they do not know what it is, ask them to send the sample to the pathology laboratory in Puyallup. It is best to go through Master Gardeners first so you will not be charged. If you send the sample yourself there will be a fee.
Meanwhile, several sources mention frost, drought, and “bud-blast” (unlikely in the Pacific Northwest) as potential causes of bud failure. Damaged flower buds and poor bud set: It is always most disappointing when fat, healthy looking flower buds either fail to open at all or only open a percentage of their buds, the rest being black and dead. Some rhododendrons regularly abort some or even all of their buds for no apparent reason. This may be due in some cases to a deficiency, perhaps magnesium, or to drought reports from various places give mixed results from applying magnesium (usually as Epsom salts)… By far the most usual cause of bud damage is frost. Flower buds are invariably less hardy than the rest of the plant so a really hard winter is sure to cause losses to flower buds. Early autumn frosts can damage buds that are not fully hardened off. This is a very annoying type of damage that may be overlooked and may not be noticed until the buds attempt to open in spring. Rhododendrons vary greatly in their ability to harden up enough to withstand early frost. In areas very prone to spring frosts, it is better to avoid growing plants that always burst into growth at the first sign of spring. Plants that frequently loose their first growth flush (and sometimes even their second) are liable to become stunted and rarely flower.
Source: The Cultivation of Rhododendrons, by P. Cox, 1993, p. 244.
A friend was told that pinching out growth buds before they begin to elongate as a means of shaping young rhododendrons would only stimulate buds further down the stems that were less than 4 years old – older than that and the growth buds would no longer be viable. I cannot find any information to suggest 4 years viability of dormant buds to be true, or untrue. Can you help?
Though pinching encourages multiple branching lower down the stem, I find no reference to it being done at a particular age.
“This practice (pinching) is recommended for most larger rhododendrons until they reach flowering size…”
(Source: A Plantsman’s Guide to Rhododendrons, by K. Cox, 1989, p. 101)
That statement indicates a younger plant, but the author then mentions several exceptions.
Here is some how-to information about pruning online:
7 Solutions to the Too-Big Rhododendron.
Should you remove old blooms from rhododendrons and, if you should, which is best: to prune or snap them off?
Here is what the American Rhododendron Society says on the subject:
“It is desirable, with the large flowered rhododendrons, to remove the withered flower clusters after the blooming season. This is fairly easily done as the central axis of the cluster, usually called a truss, will break free from the plant with a quick snap of the thumb pushing on the side, or can be cut off with a hand pruner. With the smaller flowered rhododendrons and azaleas, dead-heading is labor intensive and and generally is not required.
Dead-heading is usually done to make the bush look more attractive, to reduce the prevalence of fungus and to prevent a heavy set of seed. If it is not possible to remove the old flowers, it is usually not too detrimental, but flowering the next year may be reduced.”
I have several mature rhododendrons in my own garden, and I deadhead the parts of the shrubs which are easily reachable, leaving the other areas to their own devices. For me, it’s an aesthetic choice, and I would probably do them all if I could reach and if I didn’t get very tired of the task. (It’s hard to do well with gloves since you can’t easily feel the right place to snap off the flower head, but it’s sticky work without the gloves.) I’ve never tried pruning them off, because it seems less precise (leaves a bit of a stub), but if rhododendron experts approve (as indicated above), I may just try it this year.
Can I move my rhododendrons now, in late winter to early spring?
According to A Plantsman’s Guide to Rhododendrons by Kenneth Cox (Ward Lock Ltd., 1989), “rhododendrons are generally quite easily moved, most even in full flower and at considerable age. […] Size is really no problem, provided you have the means to do the digging and the moving. Obviously, the more rootball you can take with the plant the better, but usually you can reduce it considerably without too much harm being done. If you end up with a disproportionately small rootball, you can reduce the size of the top somewhat to compensate. The roots of a rhododendron generally extend to about 50% of the plant’s foliage diameter […] it can be far more or much less. The roots are usually less than 18 inches deep, even on a very large plant. To move a large plant, start digging […] quite far out from the stem, and continue towards it until you meet roots. Then dig all round underneath the rootball […] gently rocking the plant to ease the rootball from the soil. Watch out when lifting a plant by its main stem; it may not be strong enough to carry the weight of the rootball. The root can best be reduced by prising soil from it with a fork. […] A rhododendron can remain out of the ground for considerable periods if you keep frost and sun from the roots, and ensure that it receives regular watering. Heeling it into the ground, or covering the roots […] usually gives adequate protection. Although rhododendrons can be moved during the growing season, they will require extra watering after transplanting.”
In addition, you may find the Royal Horticultural Society’s directions on moving a mature tree or shrub helpful.
I need to know what to do with a rhododendron that has grown too big. I want to keep it, since it is a bookend to another plant. Can I cut it back, and if so, how far and when? Will it be okay and continue to bloom if I cut it back?
Could you suggest something and also suggest a really good book on care, etc., for rhodies?
The American Rhododendron Society’s page on pruning should be helpful. It describes clean-up pruning, shape pruning, and rejuvenation pruning.
Seattle author Cass Turnbull’s Guide to Pruning (Sasquatch Books, 2012). Plant Amnesty, founded by Turnbull, also has information on pruning an overgrown rhododendron.
The Rhododendron Species Foundation in Federal Way is a place you can visit to get an idea of the wide range of species available. They also have general information on rhododendron care on their website.
The Miller Library has a substantial collection of books on rhododendrons, including quite a few by Pacific Northwest authors, and you can browse them or check them out to see which species and varieties might be more suitable for your garden.
My two rhododendrons did not produce any blooms this year- they are healthy otherwise. Why?
I had the same problem with one of my rhodies this spring (all the others were fine), as did many other people in the Pacific Northwest. Following are the most likely causes:
NO FLOWERS, BUDS DO NOT OPEN. This is most likely to be caused by frost, either in mid-winter by the hardest frosts of the year, or in spring when the buds are swelling and about to open. Certain varieties have very frost-vulnerable swelling buds, while many species have buds which are easily destroyed even by quite mild winter frosts.
NO FLOWERS, NO FLOWER BUDS. There are several possibilities why rhododendrons may not flower freely:
- Too much shade. This is very common in North America where, in order to regulate sun and soil temperature, plants are placed in deep shade. This allows healthy, if straggly growth, but can inhibit flowering. The more light you can give a plant, the more likely it is to flower, so there is a trade-off between the need for shade and the need for light.
- The variety takes many years to flower (it does not sound like this is your situation).
- Kindness. Rhododendrons flower in order to reproduce. A contented, well-fed, well-watered well-shaded plant may not feel any need to reproduce, as it perceives no threat to its survival. Do not feed after mid-summer, as this encourages growth at the expense of flowers. Nurserymen cut down watering in late summer to stress plants into flowering the following year.
(Source: Rhododendrons: A Care Manual, by K. Cox, 1998, p. 73).
The above is corroborated in other sources, e.g. Success with Rhododendrons and Azaleas, by H.E. Reiley, 1992, p. 132-133.
Are azaleas poisonous to cats?
Azaleas are indeed a problem for cats and other pets. See this link from Purdue University’s Veterinary Program.
Excerpt:
“These ornamental shrubs aren’t commonly nibbled on but they can cause fatal heart problems in dogs, cats, and pet birds. Signs to watch for are similar to that of the yews and include weakness, fainting, salivation, difficulty breathing, vomiting, and diarrhea.”
According to the Handbook of Poisonous and Injurious Plants, 2nd ed., by Lewis Nelson et al. (New York Botanical Garden/Springer, 2007), all Rhododendron species, including Azaleas, contain grayanotoxins in their leaves. Honey made from the flower nectar would also be toxic.
A friend in Illinois has sent a photo this spring of a very healthy looking rhododendron – leaf buds fully elongated and beginning to unfurl, while the green, blunt flower buds remain unopened. The flower buds don’t look brown, diseased, frozen or injured, but they remain tightly closed, foliage bud growth preceding blooming. He says he has 6 plants doing the same this month. Possible reason?
Though we can’t diagnose plant problems by phone/email, early autumn frosts can inhibit flowering and not all buds are equally affected.
“Autumn frosts: These can lead to damage…if they either occur in early autumn or immediately after a late season warm spell. Continental climates with extremes of heat and cold are more likely to suffer sudden temperature changes than those with maritime climates…A sudden temperature drop will catch a plant before it has had a chance to reach maximum hardiness and it may suffer accordingly, even if normally perfectly able to withstand such a temperature in mid-winter…Speed of ripening varies considerably…There is also a variation in the hardiness of flower buds compared to foliage and growth buds. Commonly, flower buds may be as much as 10 F. less hardy than foliage…”
(Source: The Cultivation of Rhododendrons, by P. Cox, 1993, p. 119-120)