By: Teo Spengler
Zone 4 is not as cold as it gets in the continental USA, but it’s still pretty cold. That means that plants requiring warm climates need not apply for positions in zone 4 perennial gardens. What about azaleas, those foundation shrubs of so many flowering gardens? You’ll find more than a few varieties of cold hardy azaleas that would thrive in zone 4. Read on for tips about growing azaleas in cold climates.
Azaleas are beloved by gardeners for their showy, colorful flowers. They belong to the genus Rhododendron, one of the largest genera of woody plants. Although azaleas are most often associated with mild climates, you can start growing azaleas in cold climates if you select cold hardy azaleas. Many azaleas for zone 4 belong to the sub-genus Pentanthera.
One of the most important series of hybrid azaleas available in commerce is the Northern Lights Series. It was developed and released by the University of Minnesota Landscape Arboretum. Every one of the cold hardy azaleas in this series will survive down to temperatures of -45 degrees F. (-42 C.). That means that these hybrids can all be characterized as zone 4 azalea bushes.
If you want zone 4 azalea bushes that stand six to eight feet tall, take a look at Northern Lights F1 hybrid seedlings. These cold hardy azaleas are extremely prolific when it comes to flowers, and, come May, your bushes will be laden with fragrant pink flowers.
For light pink flowers with a sweet smell, consider the “Pink Lights” selection. The shrubs grow to eight feet tall. If you prefer your azaleas a deep rosy pink, go for “Rosy Lights” azalea. These bushes are also about eight feet tall and wide.
“White Lights” is a type of cold hardy azaleas offering white flowers, hardy to -35 degrees Fahrenheit (-37 C.). The buds start out a delicate pale pink shade, but the mature flowers are white. Bushes grow to five feet tall. “Golden Lights” are similar zone 4 azalea bushes but offer golden blossoms.
You can find azaleas for zone 4 that were not developed by Northern Lights too. For example, Roseshell azalea (Rhododendron prinophyllum) is native to the northeastern segment of the country, but can be found growing in the wild as far west as Missouri.
If you are ready to start growing azaleas in cold climates, these are hardy to -40 degrees Fahrenheit (-40 C.). The bushes only get to three feet tall. The fragrant flowers range from white to rose pink flowers.
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The Spruce / Evgeniya Vlasova
In many parts of the United States, especially the Southeast, it just wouldn’t be springtime without flowering azalea shrubs (Rhododendron spp.). They put on an unsurpassed show of color in woodland gardens in late March and early April.
Because azaleas have been selectively bred for centuries, there are thousands of cultivars, derived from hundreds of species found throughout the Northern Hemisphere. Many of the crucial parent species are natives of western China. Extensive hybridization means that new varieties appear every year, including many that are now suitable for northern climates.
The Rhododendron genus comprises both azalea and rhododendron species. The two classes of flowering shrubs are very similar, with the technical differences found in the structure of the flowers. Rhododendron flowers have 10 stamens, while azaleas have five. Beyond that, azaleas generally have smaller leaves and branches and are more often deciduous (i.e., they shed their leaves annually), while rhododendrons have larger, leathery leaves and are usually evergreen. For the most part, azaleas bloom earlier than rhododendrons.
Here are 10 popular types of azaleas to grow in your garden.
Like rhododendrons, azaleas prefer rich, acidic soil, with a pH of 4.5–6.0. This often can be accomplished simply by mulching regularly with pine needs or another acid-rich organic material. Regular feeding with an acidifying fertilizer will also do the trick.
Thanks to modern plant breeders, azaleas aren't just for southern and coastal gardeners any more. Gardeners across the United States can enjoy beautiful azaleas by selecting the right plants from the start. In choosing the best azaleas for your garden, keep the following considerations in mind:
It happens to all gardeners eventually: A favorite perennial or shrub is in the wrong place, and needs to be moved. Transplanting azaleas is no different.
If it’s an azalea, you’re in luck because azaleas have shallow roots, are easy to dig up, and recover quickly from the stress of moving. Even mature azaleas can be moved if you are careful to minimize damage to the root systems. Read on if you are interested in learning how to transplant azalea bushes.
The best time to move a shrub – any shrub – is early in the morning or late in the afternoon on a cool, cloudy day in early spring or fall.
Avoid transplanting azaleas or any shrub during extended dry periods, or when daytime temperatures are above 80°F.
(If you’re careful, you can move an azalea successfully without prior root-pruning, but it will recover quicker if you plan ahead and prune those roots before transplanting.)
Dig a donut-like trench all around the azalea 8-12 inches out from the trunk. This will sever all the shallow outward growing roots. Make the trench about six inches wide and about a foot deep. Fill the trench back in with the same, now loosened, soil. Soon, new fibrous roots and root hairs will grow into the soil within the trench.
Most of an azalea’s roots are shallow, not deep. When it comes to digging one up, wider trumps deeper. The wider, the better.
When you are ready to move the azalea (ideally a year or so after root-pruning), prune the top back by about one-third. Read more about Basic Pruning for Shrubs in this blog.
Dig the new hole. Free the azalea by slicing a circle just outside the original root-prune trench, and 12-18 inches deep. (If you didn’t root-prune, make the cut 12-18 inches out from the azalea’s trunk.) Water thoroughly.
Force the shovel all around and up under the root mass and lift it up, keeping as much soil as possible in contact with the roots. (Any of the fibrous, hairlike roots that get dry will die.) Slide a tarp under the root ball and wrap it up the sides before moving it.
The root ball must be kept constantly moist and the shrub shaded until it is transplanted into its new hole.
The new planting hole should be a foot or two wider than the root ball and about the same depth. Do not put fertilizer in the hole as this can damage tender roots. Use the tarp to slide the azalea to its new location.
Drop the root ball into the hole and refill with the same native soil, stopping to water a few times to make sure the soil settles in around the roots and there are no air pockets.
Be sure the shrub is set no deeper in the new hole than it was in its original location.
Water thoroughly to settle the soil in around the roots. Build up a dike around the trunk a few feet out that will hold water over the root zone so it percolates into the soil rather than running off.
Cover the ground around the trunk out to a foot or so beyond where the roots ended with an organic mulch 3-6 inches deep. Keep the mulch or organic matter an inch or two away from the trunk. You can use hay, straw, pine needles, dry leaves, bark chips, or well rotted compost. Azaleas are acid lovers, so don’t use a mulch that has a basic pH. You can always amend your native soil with our Acidic Potting Soil if your soil is not acidic enough after performing a pH test.
Once your azalea is in its new position, it will need to be babied for several months. If it doesn’t get an inch of rain per week, you will want to supply the difference. Water thoroughly and deeply once a week or when the soil gets almost (but not completely) dry. Do not keep the soil around the roots constantly wet, because that will prevent air from reaching the roots and lead to rot. Transplanted shrubs are more at risk for dying of dehydration or disease so watch your azalea plant closely.
When it comes to landscaping with azaleas, you CAN change your mind and transplant at the right time of year.
Check out the Azalea Grow Guide for more information on growing these beautiful flowering shrubs and having them flourish for many years!
There are thousands of varieties, hybrids, and wild species of azaleas and rhododendrons. Gardeners can choose from low growing ground cover azaleas, to treelike rhododendrons over 25 feet tall from the early flowers that bloom in early spring, to repeat bloomers like ‘Autumn Embers‘ that flower through late summer from deciduous rhododendrons that are cold hardy to United States department of agriculture plant hardiness Zone 4, to evergreen azalea flower plants like the popular ‘ Formosa ’ variety that almost defines Deep South gardens. All types of azalea shrubs are noteworthy for their brightly colored, long lasting flowers and their ease of cultivation.
All types of azalea shrubs are noteworthy for their brightly colored blooms, long lasting flowers and their ease of cultivation. They can grow in a variety of shapes and sizes. Wondering how big do azaleas get? The Formosa and George Tabor are our largest at 6-8 feet tall while Red Ruffles stays dwarf and as a small rounded bush. The larger shrubs make great azalea hedges. Different varieties have different growth rates for how fast azaleas grow.
Actually, the names are interchangeable since all are in the genus Rhododendron. In recent years the common name, “rhododendron”, has come to be used for most of the Rhododendron species that have large, leathery, evergreen leaves and ten stamens per flower and “azalea” is used for most of the ones with smaller (deciduous or evergreen) green leaves, and five stamens. But, there are exceptions including deciduous azaleas that lose their leaves over winter. Here we refer to all members of the genus Rhododendron as azaleas.
Azalea bushes are easier to transplant and easier to maintain than most shrubs. Just follow a few simple guidelines…
Wondering when to plant azaleas? The best time to plant out azaleas is during the moderate weather of late spring or early autumn. Azaleas can be damaged by strong winds and should be planted where they get some shelter from the wind, such as near a building or a strand of evergreen shrubs or trees.
For evergreen azaleas, select a position in the landscape that will provide some shade during the mid-day summer heat. Dappled shade is ideal. Deciduous azaleas usually flower more abundantly in full sun, but still do quite well in partly shady places. The hotter the summers, the more shade azaleas should get, both the evergreen and the deciduous types. However, full shade all day long is to be avoided except in tropical (Zones 10-11) climates.
Azaleas need a soil that is well drained and rich in organics, although sandy soil that is low in organics is also suitable as long as it gets regular top mulching. Rocky or heavy clay soils are not suitable. If your soil remains soggy for long periods, don’t plant azaleas. To test if it is well drained soil, dig a hole six inches deep and six inches across and fill it with water. If the water has not drained from the hole within three hours, the soil is too poorly drained for azaleas.
Azaleas are acid loving plants, preferring a pH between 4.5 and 6.0. Perform a soil test to check the pH level in your soil levels. If your alkaline soil is calcareous, limey, or has a pH higher than 7, you should forget about growing azaleas in the ground. If your soil pH is between 6.0 and 7.0, you can lower it by adding granular sulfur or iron sulfate to the soil. Work the sulfur into the acid soil about six inches deep. See the accompanying table for amounts to use. If possible, you should get the soil pH corrected at least nine months before planting azalea bushes. Our Acidic Potting Soil will amend your native soil or potting soil nicely.
To lower the pH of a typical loamy soil to 5.5:
6” into the soil
6” into the soil
|7.0||3.5 lbs/100 sq ft||21 lbs/100 sq ft|
|6.5||2.5 lbs/100 sq ft||14 lbs/100 sq ft|
|6.0||1.0 lb/100 sq ft||6 lbs/100 sq ft|
Before planting, consider the mature height of the azaleas you will be planting. Space them about half that far apart. Dig a hole with a diameter about twice that of the pot the azalea came in, and about the same depth. Thoroughly water the soil in the plant’s pot before starting, then place the pot on its side and slide the top of the root ball out. If the plant is stuck, you can slip a long-bladed knife around the inside edge to loosen it. Gently loosen some of the roots along the sides and bottom of the root mass, and pull them outward so they are not encircling the root mass.
It shouldn’t be necessary to prune any of the roots unless they are wound around the circumference of the pot. In that case the offending roots should be shortened so that when they are in the ground they will grow outward and not continue growing in a circle.
Mound up some soil in the middle of the hole and place the center of the azalea’s root mass on top of the mound, spreading roots out all around the center. Backfill until the shrub’s stem is at the same level it was in the original pot, never lower. You may have to pull the plant up as you backfill. Too shallow is better than too deep. When the hole is half filled, give it and the roots a good soaking of water. When the water has drained, readjust the depth of the stem if necessary and finish filling the hole. Gently tamp the soil down with your hands.
Build a 3-6 inch high dike of soil around the outside of the root zone. This will help impound water over the roots while it sinks into the soil. Water your azaleas thoroughly. Spread organic matter 3-6 inches deep over the root zone and beyond to help keep soil moist. You can use hay, straw, leaves, pine needles, pine bark or wood chips, grass clippings or compost. Do not use mushroom compost as this contains lime and can raise the pH. Do not fertilize or use plant food.
Azaleas can be planted in containers too as well as raised beds.
Keep your azaleas well watered during their first growing season. If planted in the fall or winter, you can water once every week or two. Planted during the growing season, they should get watered every day or two for three or four months. If you’re having a dry spell, or your soil is very sandy, you should water every day for the first three or four months. This can also cause yellow leaves. The most common reason for a newly planted azalea to die is lack of enough water.
Once established (after a year of growth), azaleas can tolerate moderate dry spells and should not need any supplemental watering unless it hasn’t rained in three weeks or more.
Azaleas have shallow root systems, so they should be well mulched to prevent the soil around those roots from drying out or getting too hot. Use an organic mulch or peat moss that will decompose and add nutrients to the soil. You shouldn’t have to provide additional fertilizer for azaleas that are well mulched. Try to maintain a mulch layer 2-5 inches deep around the azaleas at all times, but keep it a couple inches away from the trunk itself.
Pick off dead flowers to encourage more growth next year
If you remove spent flowers right away, the plants will put their energy into vegetative growth rather than fruit and seed production. Evergreen rhododendron plants especially should be deadheaded as soon as flowering is complete, otherwise they will have fewer flowers next year. (This is not usually the case for most deciduous azaleas.) Cut or break off only the spent flower cluster, being careful you don’t remove next year’s buds which are already clustered at the base of the old flower cluster.
Azaleas generally do not need any pruning, but if necessary to maintain shape or size, prune azaleas right after flowering. Azaleas start forming their flower buds for the next season very soon after blooming, and you don’t want to reduce next year’s gorgeous display. You can even prune your plant into an azalea tree.
To rejuvenate an old spindly plant, cut it back by a third or more. The cut branches will re-sprout regardless of their proximity to other branches.
Once established, azaleas are very low maintenance shrubs, but you will need to water them during prolonged periods of drought or when soil is dry. Check the soil regularly for consistent moisture.
If you live in the South, the spring azalea extravaganza is so bright, it’s almost gaudy. Azaleas are one of my favorite plants – they make fantastic foundation shrubs, the blooms are to die for, and if you plant them carefully, they’ll give you decades of low-maintenance enjoyment.
Here’s what you need to know to grow azaleas in your garden.
Azaleas (Rhododendron sp.) are one of the hallmarks of Southern gardens, with thousands of cultivated hybrids and varieties available. In general, azaleas are divided into two groups:
When choosing azaleas for your garden, keep these factors in mind:
Fall is a great time to plant azaleas, although you can generally plant them any time during the growing season. When planting azaleas, follow these tips:
For more detailed planting information, check out our article on How To Plant Container-Grown Shrubs.
If properly planted, azaleas are low-maintenance plants. Follow these tips:
Azaleas (Rhododendron spp.) typically bloom flowers every spring in a variety of colors, such as pink, white, purple or red. Arizona consists of United States Department of Agriculture plant hardiness zones 5 to 10, which have average winter low temperatures from minus 20 degrees to plus 35 degrees Fahrenheit. This vast difference in climates makes choosing the right azalea essential if you want it to thrive. Fortunately, there are many azalea varieties and they differ in plant hardiness, usually ranging from zone 4 to 9, making most of Arizona a suitable climate to grow azaleas. Once a variety is chosen, most azaleas are planted in a similar manner.
Select a planting location that receives morning sun and afternoon shade. Do not plant near sunny sidewalks and driveways, or near west-facing walls because the heat emitted from these surfaces is more likely to cause the azaleas to dry out during the summer months, especially in the hotter zones of Arizona. In addition, do not plant under trees since trees and the azaleas will compete for the same water and nutrients. Avoid planting in areas where water tends to puddle, such as near a downspout.
Test the soil’s pH level and adjust the levels. Take a soil sample to your local county extension office to be tested or purchase pH soil testing strips at your local nursery. Ideally, the soil should have a pH level between 4.5 and 6.0. Based on the results, if necessary, adjust the pH level by adding sulfur to lower the acidic level or lime to raise it. Dosing will be based on your initial results, so follow the dosing instructions on the label.
Dig a hole that is the same depth, but two to three times as wide as the plant’s container. Plant multiple azaleas to accommodate their full mature width, which differs among varieties.
Mix in organic matter, such as compost or leaf mold, to the soil you just dug out until the new soil is composed of one-third to one-half organic matter. Then, fill the hole back in, packing it down lightly as you go.
Water the azalea before planting it and take it out of its container. Tapping on the sides with a hammer can sometimes help get it out. If the roots are circling the soil, take a knife and cut four even vertical slits that are 2 inches deep and then loosen the roots with your hands.
Plant your azalea in the center of the amended soil. Dig a hole deep enough so that the top of the root ball when planted is 1 inch above ground level. However, some Arizona soils have high amounts of clay particles. If your soil is normally high in clay, then dig a hole so that the top of the root ball is 2 to 4 inches above ground level when planted.
Back fill the amended soil to fill in around the root ball and pack it down lightly. Form a gradual slope from the top of the root ball to the outside perimeter of the original hole you dug out in step 3.
Water the azalea with 2 to 3 inches of water and then lay 4 inches of organic mulch, such as bark, wood chips, pine straw or shredded leaves. This will help maintain water and help the soil from baking, especially in the hotter Arizona zones. Mulch also keeps the soil warmer during the winter in the cooler zones. Mulch should be sloped like the soil to help with water drainage.