Is there a more welcome sight in spring than an azalea in full bloom? These loosely branched, shade-loving plants with elliptical leaves can bring delightful color to all corners of the spring garden when clusters of tubular or bell-shaped flowers appear. Heirloom varieties can grow more than 20 feet tall, while dwarf varieties may stay under three feet. The range of colors is spectacular, from white to pale pink to a range of reds and purples. Most azaleas bloom in mid-spring (mid to late April), but some bloom earlier or later, and it's easy to choose a variety for any design need. Like their larger cousins, the rhododendrons, the azalea's thick leaves can are sometimes evergreen, though many varieties grown in northern zones are deciduous.
Azaleas shrubs are best planted in the spring or early fall as container-grown nursery plants. Growth rates vary by species, but most are fairly slow-growing. A one-gallon plant normally will flower in its first year but may take as much as 10 years to reach its full size.
Azalea (and rhododendrons) contain andromedotoxin and grayantoxin, substances that are quite toxic to humans and animals. Ingestion of plant parts can cause a variety of symptoms, ranging from simple digestive distress to cardiac arrest. Most fatalities involve grazing animals, such as horses.
|Botanical Name||Rhododendron spp.|
|Plant Type||Deciduous or evergreen shrub|
|Mature Size||3–20 ft. tall, wide|
|Soil Type||Light, well-drained|
|Soil pH||Acidic (4.5 to 6.0)|
|Bloom Time||Early spring to summer|
|Flower Color||White, pink, red, orange, peach|
|Hardiness Zones||6b-8a (USDA); varies by species|
|Native Areas||Asia, Europe, North America|
|Toxicity||Toxic to humans and pets|
Proper planting in the right locations is key to success with these long-lived shrubs. In a favorable location, these plants are usually quite easy to grow, but they can be very temperamental if conditions aren't exactly to their liking. Most important is for the soil to be loose, well-draining, and somewhat acidic in pH. In areas with dense soil, azaleas are often planted in artificial berms created by bringing in loose, porous soil to create elevated planting areas.
Dig a hole as deep as the container and about twice as wide, and blend in some peat moss and compost to improve the soil texture and pH. (Peat moss is a naturally acidifying amendment). It can be a good idea to take a soil sample before planting azaleas, as these species are uniquely dependent on acidic soil. Water in when planting and water daily for the first week until established.
In most regions, azaleas flourish in a partial shade location that gets a few hours of morning sun. so the flowers don't wilt if a spring day becomes too hot. In colder climates, azaleas may respond well to full sun, while gardeners in warmer climates should give their azaleas more shade.
Azaleas love acidic soil; this explains why potted nursery plants usually have mostly peat moss for the planting medium. Soil should also have good drainage and good fertility with plenty of organic matter. Azaleas do best with a natural mulch such as pine bark mulch.
Azaleas need water to bloom, and spring rain showers usually do the job. However, if the spring is dry, extra watering can be beneficial.
Temperature and Humidity
Azalea hardiness depends on the cultivar so it's important to choose your plant and your planting site carefully. Too cold, and buds may not form; too hot, and flowers may burn from over-exposure. Mildew can be a problem with azaleas; make sure they have plenty of air circulation and don't plant them too close to other large shrubs.
Azaleas also do well if given some fertilizer. Without knowing the nutrients in your soil, a basic 15-15-15 fertilizer containing equal parts nitrogen, phosphorous, and potassium is your best bet. But there are special products made just for azaleas: Espoma's "Azalea-Tone" is a good one. Acidifying fertilizers are a good choice where soils are in the neutral or alkaline range.
Fertilize in late winter or early spring, no later than the last week of March.
Types of Azalea
One can't flip through a garden catalog without seeing a new variety of azalea, many of which are bred to the special needs of particular growing regions. In almost any area of the U.S., you will be able to find dozens of azalea species and cultivars ideally suited for your climate conditions. Here are some tried and true ones:
- 'Rosy Lights': This compact (4 to 6 feet) deciduous cold-hardy shrub blooms in late spring and features a sprightly flower in a rosy red color with hues of coral and blush. This is one of the 'Northern Lights' series of azalea-rhododendron hybrids. The flowers are borne in "trusses"—not as round as traditional rhododendrons but not as diffused as traditional azaleas. It is hardy in USDA zones 3 to 7.
- 'Windbeam': With smaller leaves and a tidy habit (3 to 4 feet), this popular cultivar features luminous pale pink flowers and olive green leaves that turn bronze in autumn. Hardy in Zones 4 to 8, it may need shelter from harsh winter winds, but it is more sun and heat tolerant than other azaleas. The spring display is stunning, beginning white and then evolving to pink with tiny red freckles on the dorsal lobes.
- Golden Lights': The bright golden-orange blooms on this 'Northern Lights' cultivar offer a dramatic color for the spring landscape. Cold-hardy (USDA zones 3 to 7) and compact (3 to 6 feet) the flower trusses are flat and contain ten flowers each, in shades ranging from butter yellow to orange with salmon-pink highlights.
- 'Fragrant Star': The pure white flowers on this small, 3- to 4-foot mid-spring bloomer have a heady, spicy fragrance. The leaves are a pleasing bluish green. It is quite heat-tolerant but not as cold-hardy as others (zones 5 to 9), though can withstand cold temperatures as low as minus 20 degrees Fahrenheit if sheltered from harsh winds.
- 'Gibraltar': This popular azalea boasts frilly, faintly fragrant, bright orange flowers that emerge from crimson buds. It is of medium height (4 to 5 feet ) and relatively cold hardy (zones 5 to 8). This variety is also very mildew resistant.
Pruning is not a mandatory task with azaleas, as they show to best effect when allowed to have a slightly informal growth habit. That said, azaleas can benefit from mild pruning that will force bushier, fuller growth. Azaleas start forming buds in summer, so the best time to prune is right after the flowers drop off in late spring.
Deadheading spent flowers will help the plant look tidier and will help direct the plant's energy into green growth. However, it must be done carefully, as the buds for next year's flowers are already developing just below the current flowers. It is all too easy to pinch off the future buds if you're not careful.
Although it is a slow process, azaleas can be propagated several ways. Vegetative methods are more likely to give you plants that are exact duplicates of the parent plant. The common vegetative methods are layering, and stem cuttings. Rooting stem cuttings is generally an easier method for amateur gardeners:
- In late spring, cut five-inch-long flexible tips off the ends of the supple new stems. Pick off all but the top leaves.
- Scrape off the bark from the bottom one inch of the cutting, then dip it in rooting hormone.
- Plant the cutting in a rooting medium, such as a 50-50 mixture of peat moss and perlite; or a mixture of sand, vermiculite, and peat moss. The rooting medium should be more porous and better draining than most standard potting mixes. Use a pot at least six inches deep; a one-gallon plastic pot makes an ideal container.
- Moisten and pack the rooting mix around the cutting, then place the entire container in a large plastic bag or another clear container. Put the pot in a location that gets bright light, but not direct sunlight.
- Check periodically to see if the cutting has rooted (tug gently on the stem to see if you feel resistance). Within four to eight weeks, the cutting should develop a good network of roots.
- When roots have developed, remove the plastic and continue growing the shrub in its pot, feeding every month. Leave the new shrub in its pot through the winter and well into the following spring. It should be ready to plant when new growth is evident. For the winter, the pot should be moved to a protected location, such as a cold frame or unheated porch.
Another common method for propagating azaleas, best suited for serious amateur or professionals, is layering:
- In late spring or early summer, identify a flexible branch that can be bent to the ground, then dig a shallow trench in the soil below the branch.
- Bend the branch down into the trench so the free end extends up into the air, then pin the underground portion in place with stakes, wires, or heavy rocks. It can help to scrape off some of the bark and sprinkle rooting hormone over the wound. Fill in the trench with soil.
- Leave the layered branch in place for at least a year. During this time, the underground portion of the branch will develop roots.
- When roots have developed, sever the buried branch from the parent shrub, leaving a rooted shoot that can be carefully dug up and transplanted into a new garden location.
How to Grow Azaleas From Seed
Azaleas will grow easily from commercially purchased seeds or seeds you collect from the seed pods that are seen when the flowers drop, though it is a slow process that can require two or three years before the new shrubs flower. For this reason, seed-starting is not often attempted by amateurs. And be aware that offspring produced by seeds from hybrid shrubs may not look like the parent plant.
However, for pure azalea species, seed propagation does work predictably. Collect the seeds pods as they start to turn brown after the first fall frost. Dry the seed pods indoors until they split open, then collect the tiny seeds and clean them. In winter, fill a seed flat filled with a mixture of sand and peat moss (or commercial seed-starter mix), moisten it, then sow the seeds over the surface and cover the tray with a clear plastic lid and place it in a warm, bright location, but not in direct sunlight. The seeds typically germinate in two to six weeks, at which point they should be thinned out to a spacing of at least two inches.
When the tiny seedlings have developed two sets of true leaves, transplant them into small individual pots, then recover the pots with plastic and continue growing them in a bright location. Within a week or two, the plastic can be removed. As the weather warms, the pots can be moved outdoors to continue growing the plants in the fresh air. After this first full growing season, the plants are ready to be potted up into one-gallon pots for another year of growth—or, if they are sizable enough, they can be transplanted into the garden in the fall.
Potting and Repotting Azaleas
Smaller varieties of azaleas can be excellent choices for container culture. And growing in pots can be a good option when your garden soil isn't suitable for growing azaleas.
Use a good spacious pot at least 16 inches in diameter and 12 inches deep. Heavy materials such as ceramic or concrete are best, as they will resist tipping. The growing medium will need to be light, well-draining, and acidic in pH. A commercial peat-based potting mix blended with extra perlite or vermiculite often works well.
Feeding azaleas should be reduced after midsummer, and watering should also be somewhat reduced in early fall to toughen the plant up for winter. But after a few hard frosts, it's a good idea to water deeply to hydrate the plant, which will help prevent winter burn.
If you live in a USDA zone that is borderline for the variety you are growing, then apply a three- to five-inch layer of dry mulch over the root zone of the plant after it goes dormant. Keep the mulch slightly separated from the plant's trunk. This mulch should be removed in spring after the soil warms.
If azaleas tend to show signs of winter damage in your regions, such as split bark, you may find it necessary to protect your shrubs over winter by tenting or draping them with burlap. This is not a very attractive solution, so it's best to choose varieties known to be cold-tolerant in your region.
Common Pests and Plant Diseases
The pests and diseases most likely to affect azaleas will vary greatly depending on the species you're growing and the region you're growing it in, but overall you should be resigned to the reality that azaleas can be susceptible to quite a large number of insect and disease problems:
Various aphids, borers, lacebugs, leafhoppers, mealybugs, mites, nematodes, scale, thrips, and whiteflies can all attack azaleas. A horticultural oil such as neem oil can address these pests, though if left alone, insect pests may eventually be controlled by beneficial predatory insects.
A variety of diseases can attack azaleas, including canker, leaf spot, rust, and powdery mildew. Often these are merely cosmetic and not life-threatening, though they can be treated with fungicides. But there are also very serious root rot fungal diseases, such as Phytophthora root rot, that cause leaf wilting, stem dieback, and eventual death of the shrub. Root rot diseases occur most often in dense, wet, poorly drained soils, but if your landscape is host to these fungi you may be unable to grow some azalea cultivars. Look for varieties known to be resistant to common fungal diseases—or choose a different type of shrub altogether.
How to Get Azalea to Bloom
Most azalea varieties stay in bloom for about two weeks, barring rain or wind damage from a storm. Most are spring or early summer bloomers, though there are some that bloom later. A mature, healthy azalea in a favorable location will usually bloom with no intervention, but if yours doesn't, possible reasons include:
- Too much nitrogen is causing the shrub to add vigorous green growth but at the expense of flowers. If you feed your shrubs, use a balanced 5-5-5 or 15-15-15 fertilizer. Or, use an acidifying fertilizer designed for these plants, such as Miracid.
- Too little sun can cause azaleas to withhold blooms. These plants generally perform best in dappled shade, but with some direct sunlight in the morning.
- Root-bound plants may refuse to bloom. This is most likely to occur with potted azaleas, but if a container-grown plant is root-bound when you plant it, it may not bloom. When planting, always examine the root ball, and tease apart or slice through the roots if they are tightly wound around the inside of the container.
- Improper deadheading or pruning can cut off the early forming flower buds. If deadheading, take care to remove only the dead petals.
Common Problems With Azalea
Frequently observed cultural symptoms with azaleas include the following;
Evergreen Azalea Has Burned Leaves in Spring
This is a classic sign of winter burn, which occurs when a shrub is exposed to excessively cold winter temperatures or desiccating winds. Prune off the dead growth; the shrub usually recovers just fine. But if this is a frequent occurrence, you may find it necessary to shield the plant over the winter months. Winter burn and dieback are more often seen with evergreen varieties.
Branches Die Back, One at a Time
This is a symptom of a dreaded root rot fungus, Phytophthora. These diseases cause the roots to constrict, preventing water from reaching the branches. There is no cure for root rot, but you can reduce its likelihood by making sure the soil is well-drained. Once such a fungal disease is present in the soil, you may find it impossible to grow some types of azalea.
When the leaves on azaleas or rhododendrons show yellowing, but with visible darker green veining, you are witnessing a condition known as chlorosis. With azaleas, this is the result of alkaline soil that causes an iron deficiency in the plants. Often, feeding the shrubs with an acidifying fertilizer will help relieve the condition. Acidifying amendments, such as peat moss, pine needles, or agricultural sulfur, can also help.
Flower Petals Collapse Just After Opening
This is the result of an airborne fungus that overwinters on and around the plants. Clear away old mulch and plant debris early in the spring, then spray the plants with fungicide as the flowers are just opening.
How should I use this shrub in the landscape?
Azaleas look great planted beneath trees or at the back of a border. They can make a great specimen planting in a prominent place by an entrance or patio also. Since the flowers are so showy, ideally you want your azaleas to bloom when your garden needs a boost of spring color. Maybe after your early-blooming daffodils are fading, and before the peonies light up? There are some reblooming cultivars (the 'Encore' series) that will give you blooms from spring through late summer.
What is the difference between azaleas and rhododendrons?
In the garden industry, the names "azalea" and "rhododendron" are often used interchangeably, which can become confusing. One way to tell them apart at first glance is the size and shape of the blooms: Traditional rhododendrons have large round clusters of flowers, whereas azalea blooms are more evenly distributed on the entire shrub. Rhododendron leaves are also larger, fleshier, and darker green than azalea leaves. There are numerous cultivars, so do a bit of research to make sure you get the plant best suited for your needs. There are also many azalea-rhododendron hybrids, hardy plants that are a more manageable size and that have a wide range of flower colors.
How long does an azalea shrub live?
A healthy shrub in a favorable location can live for many decades. Lifespans of 50 years—or even a century—are quite common.
Aren't azaleas used as houseplants?
Some miniature azaleas are bred to be grown as houseplants. These are typically referred to as "greenhouse azaleas." Often grown as one-time seasonal bloomers, getting them to rebloom is difficult. Greenhouse azaleas are not amenable to being transplanted into the garden after they have bloomed indoors.
Standard garden azaleas are not well-suited for moving indoors to grow as houseplants. Azaleas typically require a winter chill period to set flower buds, and they further can be extremely prone to pest damage when moved to an indoor location.