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Azaleas versus rhododendrons
Azaleas are in the genus Rhododendron
. Most azaleas can be distinguished from rhodododendrons by their leaves
The best-known azalea reference book is Azaleas
, by Fred Galle, published by Timber Press. It covers all aspects of azaleas, and describes around 6000 varieties. There are a large number of other books about azaleas. A few of them are described on the books
Azaleas are long lived plants when their requirements are met. There are azaleas in Japan which are hundreds of years old, and may appear more as a small tree than a shrub, with (rarely) trunks 12 inches or more in diameter.
Azaleas are generally healthy plants when their basic cultural requirements
are met. However, they are subject to a number of problems
caused by infectious agents, insect pests, weather and nutrition deficiencies. They usually exhibit symptoms of the problem in time to correct it.
The more common causes of the complete death of an azalea are improper planting
, root problems due to poor drainage or too much watering, over-fertilizing
, or bark split due to colder weather or bigger temperature swings than it could withstand (which may not show up until warm weather sets in).
Bring your dead azalea to your county agent or a master gardener, or to an Azalea Society meeting, to get post-mortem opinions as to how to prevent similar deaths.
Azaleas are woody shrubs which keep growing all their lives. Some varieties can get quite tall, into the tens of feet, while others remain spreading groundcovers less than 12 inches in height. Upright varieties tend to also spread out with age.
The rate of growth is a good predictor of the ultimate size. It can vary from around 2 inches to 10 inches in a season, depending upon the variety, the climate and other environmental conditions, primarily water, exposure, and nutrition.
Many of the sizes listed in books and catalogs are the so-called "10 year height", which can be used as a point of comparison. For landscape use near the house, consider choosing some of the smaller varieties, say, those with a 10 year height of 3 to 5 feet, rather than continually pruning back the more vigorous varieties.
Azalea Society of America
The Azalea Society
was founded in 1979 to promote knowledge of azaleas. It has local chapters
which meet to discuss, exchange and sell azaleas, it holds an annual convention
in the spring at a different city each year, and it has an excellent quarterly journal, The Azalean
. Membership is $25 per year, which includes membership in one of the chapters, or you may join as an At-Large member. Click Join Us
to complete an online application form.
Azaleas, rhododendrons, and mountain laurel (Kalmia
) are mildly toxic when eaten by animals, and may cause abdominal and cardiovascular problems. All parts of the plants are toxic, as is honey from the flowers. See Purdue University's Indiana toxic plants
website for more details.
Many factors affect the quantity of blooms on an azalea, beginning with heredity. Some azalea varieties are normally covered with blooms, and some are normally "shy bloomers". Next year's flower buds begin to form within a few months after the plant blooms. If an azalea used to bloom well, and doesn't now, it may be due to one or more of a number of reasons:
- pruning after the buds form, which removes the buds for next year
- on some varieties, the buds are less cold-hardy than the plant, and may freeze, turn brown and not open
- lack of moisture during the late spring and summer reduces bud formation - mulch helps retain moisture and also keeps the soil cool
- less than 3 hours of sun reduces the number of buds
- a phosphorus deficiency, characterized by dull, dark green foliage with reddening underneath, reduces the number of buds
- poor plant nutrition reduces the number of buds
The bloom times
of azaleas varies quite a bit, depending on the variety and on the weather. Around Washington, DC, a few varieties bloom as early as March, most bloom in April and May, some bloom in June through September, and most of the blooms appear all at once and last about two weeks.
A given variety will bloom earlier and the bloom will be more spread out in a warmer area, and will be later and more concentrated in colder areas. In the south, some spring-blooming azaleas bloom again in the fall.
Yellowing of a leaf between dark green veins is called chlorosis and is usually caused by an iron deficiency. Many conditions can be responsible for an iron deficiency. Poor drainage, planting too deeply, heavy soil with poor aeration, insect or fungus damage in the root zone and lack of moisture all induce chlorosis. After these conditions are eliminated as possible causes, soil testing is in order. Chlorosis can be caused by malnutrition caused by alkalinity of the soil, potassium deficiency, calcium deficiency, iron deficiency, magnesium deficiency or too much phosphorus in the soil. Iron is most readily available in acidic soils between pH 4.5-6.0. When the soil pH is above 6.5, iron may be present in adequate amounts, but is in an unusable form, due to an excessive amount of calcium carbonate. This can occur when plants are placed too close to cement foundations or walkways.
Soil amendments that acidify the soil, such as iron sulfate or sulfur, are the best long term solution. For a quick but only temporary improvement in the appearance of the foliage, ferrous sulfate can be dissolved in water (1 ounce in 2 gallons of water) and sprinkled on the foliage. Some garden centers sell chelated iron that provides the same results. Follow the label recommendations for mixing and applying chelated iron. A combination of acidification with sulfur and iron supplements such as chelated iron or iron sulfate will usually treat this problem.
Chlorosis caused by magnesium deficiency is initially the same as iron, but progresses to form reddish purple blotches and marginal leaf necrosis (browning of leaf edges). Epsom salts are a good source of supplemental magnesium. Chlorosis can also be caused by nitrogen toxicity (usually caused by nitrate fertilizers) or other conditions that damage the roots such as root rot, severe cutting of the roots (perhaps caused by transplanting), root weevils or root death caused by extreme amounts of fertilizer.
Uniformly yellowish-green leaves is often just the need for more nitrogen. This will be more noticeable in the full sun. Some less sun tolerant varieties will always be light green in full sun.
range from white to purple to red, with no yellow azaleas and only a few orange azaleas.
azalea colors range from white to pink, and from yellow to orange to red.
Many azalea varieties also have two distinct colors in the same blossom, called a "bicolor", and many varieties have different color patterns
as well, sometimes with differently patterned blooms on the same plant.
Cultivar is a shortened form of the phrase "culti
iety", which refers to plants propagated vegetatively, by cuttings
, layering or grafting, rather than sexually by seed. A cultivar is identical to its parent.
Cultivar names may be up to three words set off by single quotes, and are properly preceded by the genus name and specific epithet in italics. For example, Rhododendron austrinum
The cut off end of the stem of a plant is called a cutting. By rooting
a cutting, you can make an exact copy, or "clone", of the plant. Rooting a cutting is also called "asexual reproduction" or "cloning".
There are many thousands of named varieties of azaleas. There are many more named evergreen
azaleas than deciduous
azaleas, perhaps because evergreen azaleas are easier to root in commercial quantities than deciduous azaleas.
Most of these named varieties are not available commercially. Typical garden centers and nurseries carry relatively few varieties, while the larger garden centers may carry as many as 50 or so varieties. Click Sources
for a list of mailorder azalea nurseries with more varieties. Local plant sales are another good source for less common varieties. The azaleas
database will (some day) list the known sources for particular varieties.
Leaf damage in late spring can be feeding damage from caterpillars. You can pick them off, or use a pesticide based on a recommendation from your county extension service (take in a stem for examination). The Rhododendron Looper is a caterpillar that looks exactly like a branch, stem, or even a stamen. It can align itself along the stem to hide, so look closely at the newest feeding damage, and see the frass (excrement) to find it, and squish it. They don't usually kill the plant unless you have a huge population.
Leaves are also eaten by any of several members of the weevil family (most commonly the Two-banded Japanese Weevil or the Black Vine Weevil), which are difficult to control. The leaf damage looks like notching on the outside of the leaf, all the way around the leaf, and is not very harmful to the plant. Weevils overwinter in the soil as larvae which feed on the azalea roots, which can kill the plant. Contact your local county extension service for recomended controls.
If the damage is confined to the lowest branches of small azaleas, it may be caused by rabbits. The best control is to get a cat, or to trap them. Azalea leaves and branches may also be eaten by deer.
After controlling the pests causing the damage, it may be a good idea to help the azaleas by fertilizing them (if many of the leaves are eaten off, there may not be enough left to feed the roots).
Yellow to white leaves, with black specks on the underside, may be due to azalea lacebugs. They feed from the undersides of the leaves, and suck out the chlorophyll. Check with your county extension service for recommended controls, or you can safely spray the undersides
of the leaves with dilute soapy water, preferably late in the afternoon to avoid any sunburn.
Leaves with brown tips show the plant has a problem, which may be too much water, not enough water, too much fertilizer (fertilizer "burn"), or root problems.
Too much water can be due to poor drainage, or by overwatering. If an azalea sits in water for any length of time, the roots will rot and the leaf tips will turn brown as a sign of distress. Too little water also results in brown leaf tips, usually preceded or accompanied by a wilted, limp appearance.
Root problems can also show up as browned leaves. These problems are described at Leaf drop
In general, azaleas in the landscape require little or no fertilizer. Having humus (decomposed organic matter) in the soil and maintaining an organic mulch around azaleas are more important than applying chemical fertilizers, and much safer. Decomposition of the mulch normally provides the nutrients needed for the good health of the azaleas.
If chlorosis of the leaves (yellowed, with green veins) or stunted plant growth suggest there may be nutritional deficiencies, a soil test may be useful. This can usually be arranged through your county agent at little or no cost. Soil test results will show the specific amounts of nitrogen, phosphorus, potassium and various other important elements that are present in the soil. The results may be accompanied by specific fertilizer recommendations to correct any deficiences. If not, your county agent may be able to provide specific recommendations.
Applying chemical fertilizers without knowledge of any deficiences in your soil may not help much, and may actually harm your azaleas. As a very general rule, more azaleas are killed by kindness than by neglect.
Some varieties of evergreen azaleas are grown for sale by florists in full bloom at almost any time of the year. Try to find out the variety of your gift azalea, and look it up in a reference book or in the azalea database
, to see if it is cold-hardy
in your area (most of them can't stand a frost). If it is, enjoy it inside until spring and then plant it outside in a part-sun, part-shade place in the garden (see planting azaleas
). If you want to prune it, do that soon after it blooms, to avoid cutting off the buds for next year's blooms.
While it is in the house, remove its pretty paper wrapper, and water it deeply and infrequently. A good way is to soak it in a tub of water until the bubbles stop, and then let it drain out the excess water. Do this about once a week. Exactly how often depends on its potting mix and the temperature and humidity of the room. The goal is to have moist soil, rather than having it either saturated or dry for more than a few hours at a time. Keeping it in a cool area of the house will lengthen the bloom period. Putting the pot on or near a saucer of water and gravel will raise the humidity and help it hold its leaves.
If the azalea is not cold-hardy, you can plant it outside after the last frost, still in the pot, with the rim of the pot even with the soil level, or use it as a potted plant. Remember to water it, as the roots can only get the water in the pot. Bring it back into the house during the winter as a potted plant, and put it in the coolest part of the house during the winter.
If it will be staying in the pot, fertilize it lightly every month or so through the fall, with a fertilizer low in nitrogen and high in phosporus to promote root and bud growth without promoting plant and leaf growth. Then let it rest during the winter, but don't forget to water it. Also, carefully remove it from the pot every six months or so to check the roots. If you see fine roots circling the root ball, put it into another pot, 2 to 4 inches wider than the old pot. Before repotting it, cut those circling roots by making top-to-bottom cuts every few inches, all the way around the root ball. A good potting mix is a 50/50 mixture of potting soil and fine pine bark.
Click indoor azalea care
for more (and slightly different) information.
As described in more detail at growing azaleas
, azaleas are rather easy plants to grow. If you can provide slightly acid soil with good drainage in an area with partial shade, you should be able to grow at least some of the many thousands of varieties.
Cold-hardiness is a consideration. Many varieties will grow in USDA Zone 7 to 9, with minimum winter temperatures of 0°F. Some other varieties can grow as low as Zone 4 (-30°F). Heat-hardiness is another consideration, although less is known about it. Many deciduous azalea hybrids are not tolerant of high heat for extended periods, particularly high night-time temperatures, and will slowly decline over a few years.
The leaves of some deciduous azalea varieties show attractive fall colors, ranging from yellow through crimson to purple, before dropping off in late fall.
The leaves of some evergreen azalea varieties also turn color in the fall. Some may turn and then fall off, and others retain the color through the fall and winter. They may be yellow, yellowish brown, red, dark red, or green flecked with yellowish or reddish brown.
Azaleas are either evergreen or deciduous. Deciduous azaleas drop all of their leaves in the fall. In dry weather,they may drop their leaves earlier than usual. Their leaves then grow back in the spring.In warmer climates or unusually warm winters, deciduous azaleas may retain some of their leaves through the winter..
Evergreen azaleas also drop their leaves. However, they appear to be evergreen because they grow two sets of leaves each year. Their spring leaves are thinner, larger, and grow along the stems. They drop off in the fall. Their summer leaves are thicker, smaller, grow crowded at the branch ends, and remain through the winter. They remain for several years on some varieties. In colder climates or extremely cold weather, evergreen azaleas may drop most of their leaves during the winter.
Leaves that turn brown, die and drop off during the summer usually indicate a problem with the azalea. The problem may be too little water, too much water, or too much fertilizer. If the azalea was recently planted, dig it up and check the rootball for moisture, and for the general state of the roots, which should be firm and crisp. Brown and mushy roots may indicate too much water. They may also indicate one of several different fungal infections known collectively as root rot
Azaleas can get a fungal infection called leaf gall
, which show as thickened fleshy growths. They first appear as shiny green lumps, which then become covered with white spores, and finally dry up and appear as much smaller brown dry lumps. The simplest control method is to pick them off the plant and put them into trash bags, preferably before they turn white and spread more spores which will appear as new galls the next spring.
Azaleas can get a fungal infection called petal blight
, particularly when the weather is cool and damp as they are about to bloom. It makes the petals turn brown and mushy in a day or two after they open. It is rather easily controlled with a fungicidal spray just as the buds show color.
Azaleas prefer loose, moist, well-drained soil for their roots. If the soil is heavy, mix it with as much as 50% organic matter, such as fine pine bark or rotted leaves, before using it to plant the azalea. When choosing such soil amendments, avoid materials which may be alkaline or "hot" (containing fresh manure), such as the "mushroom compost" sold for use with perennials.
If the plant is wilting, soak it in a tub of water for a few minutes, or water it slowly and thoroughly with a hose before planting it.
If the plant is in a container, remove it. Avoid pulling it by the stems, but instead turn the container upside down and lift it off the plant. Any visible roots wrapped around the rootball will strangle the plant when they grow, instead of growing out into the soil. With a sharp knife, cut these roots by making slits about 1/2" deep from the top to the bottom of the rootball, about every 2" or 3" around the rootball. Cut any matted roots off the bottom of the rootball.
In good soil, dig a hole at least a few inches wider than the rootball and just as deep as the rootball, and plant the top of the rootball even with the top of the soil. Avoid disturbing the soil at the bottom of the hole. If it is disturbed or soil must be returned to the hole, tramp it firm before planting. The goal is to avoid the azalea from sinking more deeply as the soil settles. In heavy soil, plant high, with the top of the rootball several inches above the ground level, and mound the amended soil up to the rootball. In very poorly drained soil, plant on top of the soil, or in a very shallow depression.
Put the root ball into the hole, and rotate and tip the plant to its best appearance. If it was wrapped in burlap, optionally remove it. Real burlap can safely be untied and stuffed down beside the root ball, as it will rot away in a few months. You must remove plastic burlap (usually yellow or orange), as it will not rot and will impede root growth. Add soil to fill the space under and around the root ball, tamping it firmly with your fingers, and continue until the fill soil is at the same level as the top of the root ball and the surrounding soil. The goal is to avoid any airspaces without compacting it so much that water will not enter.
Mulch the plant with 2 to 4 inches of pine straw, leaves, pine bark, wood chips or whatever is available, but leave an inch around the stems without any mulch. Then water it slowly and thoroughly. Water it again the next day, and at least once a week for several weeks. The goal is to settle the soil and remove any air spaces, and to make sure the disturbed roots have ready access to water until they can grow into the surrounding soil. Remember to watch small plants for a month or more, and large plants for a year or more, and water them deeply whenever they look wilted.
The sooner you prune the better:
- The best time to prune azaleas is in early spring, before the plant puts out new growth. Although you'll be cutting off that year's blooms, it gives the plant the full growing season to fill out, and time for the new growth to mature before winter.
- Pruning while they are in bloom is next best, and gives you some cut flowers, or pruning just after they bloom lets you enjoy the flowers on the plant.
- Since most azaleas start growing next year's flower buds soon after they bloom, pruning after mid-summer cuts off next year's bloom. Late pruning also runs the risk of the tender new growth being killed in cold climates.
Before you start, look at the plant you intend to work on, remembering that branches which are shaded out often die back and become dead wood anyway. Remove these first, as the effect of removing them may alter the way you approach pruning the rest of the branches to maintain the shape of the bush.
Use clean cutters, and keep them clean as the work progresses, using a sterilizing solution such as denatured alcohol or a 10% Chlorox solution, particularly if any cuts are in infected wood.
Older plants may have a number of tall branches which need to be eliminated. Doing that over several years reduces the shock to the plant. Remove two or three of the tallest branches, taking care to cut back to a side branch which is heading in the desired direction, and which is about 1/3rd the size of the cut branch. Cut close to that side branch, as any stubs will die back to the side branch anyway, and leave dead wood which may become infected later.
Next year take out two or three more branches using the same process, spreading the pruning over a three year cycle. This approach will result in the plant sending out new growth near the base, and lets you manage the shaping of the plant to achieve a nicely shaped bush.
A "self" flower is a flower that is all one color. If the plant has color variations, that one color is usually the same color as the variation color on the other flowers.
A "selfed" plant is a seedling from a plant crossed with itself. This type of hybridizing is usually done to reinforce some desired characteristics of the parent, which may happen with some of the seedlings from the cross.
These are completely different uses of almost the same term, and they have nothing to do with each other.
A species (singular and
plural) is a member of a group of azaleas in the wild with enough characteristics in common to indicate a common descent. Seed resulting from self-pollination of a species will be true to the parent. By contrast, seed resulting from crossing different species, or hybrid azaleas, will instead grow a variety of different azaleas with characteristics ranging from those of one of the parents to the other, anything in between, and perhaps some not seen in either parent.
North America has 16 species, sometimes referred to as native azaleas
, all deciduous. Japan and Asia have a number of species azaleas, many of them evergreen. Some species grow near other species and hybridize with each other. These natural hybrids have various mixtures of the characteristics of both parents.
The general goal when transplanting is to minimize the stress on the plant being moved. Stress is caused by leaving some of the roots behind. If possible, transplant azaleas in early fall or early spring when the weather is relatively cool, or in the winter unless the soil is frozen or soggy. If you must transplant in warm weather, choose an overcast day, or a day or so after it has rained (which cools the soil), or earlier or later in the day.
Small azaleas can be transplanted with little stress by moving a very large root ball relative to the size of the plant. That ensures that you are moving most of the roots.
For large azaleas, dig wide rather than deep to get as many roots as possible. They are generally rather shallow-rooted. The very safest approach is to dig a trench up to 12 inches deep, around the dripline of the plant. Then undercut the plant to form a cone, and start removing some of the soil an inch or so at a time, moving all around the plant, until you begin to see that you are removing roots. If possible, then get a square of burlap under the plant (tilt the plant to one side, put one edge of the burlap close to the center of the plant, wadded up so that only half of it is on the open side of the plant, then rock the plant the other way and pull the burlap through). Tie the corners of the burlap to each other across the plant. Tie the burlap as tight as possible, to keep the soil around the plant roots undisturbed. Then lift the plant by the burlap and the bottom, not by its stems.
You can reduce the stress much more by planning a year or two ahead. Moving around the plant at the drip line, cut straight down with a spade, move a few spade widths and cut again, etc., until you have cut 1/3 or 1/2 of the soil. Any cut roots will start growing on the side of the cut toward the plant. If you have the time, repeat this next year, but cut the soil you didn't cut before. The idea is that when you finally cut it loose and move it, most of the roots will have regrown inside the soil you will be moving.
Replant the azalea following the regular planting
directions. Do that as soon as possible to minimize the risk of the roots drying out. If it cannot be replanted soon, water the root ball slowly and thoroughly every day or two until you plant it.