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Of all the colorful flowering shrubs that northern gardeners and homeowners bemoan not having available for their yards and gardens, none is more often lamented than that most colorful of spring bloomers, the rhododendron, and its close cousin the azalea.
These landscape staples of homes in more temperate regions are simply breathtaking to behold in full bloom, rivaling pretty well any other shrub for color and flower impact. Sadly, most northern gardeners are trapped in the past, back in a time when azaleas and rhodos were truly the realms of warmer climates. But this is no longer the case!
You see, there was a time when roses were also thought to be the domain of warm southern climes and just beyond the grasp of us despondent northerners.
And yet, thanks to some hard work, genuine innovation, and a little luck, the finest of the northern plant breeders put their minds to it, took up our cause, and in time returned to us the first roses specifically designed to survive and thrive in our cold climate.
Hundreds of varieties have since appeared, in a myriad of flower colors and improvements in disease tolerance, habit, and form. Today, only the most uninformed of northern gardeners would lament the lack of a hardy rose for almost every situation.
But this is a story about azaleas and rhododendrons. And as with the rose, so too have the pioneering northern plant developers put their minds to the task, and the results have been stunning.
Where once rhododendrons and azaleas would turn up their roots at the mere mention of winter temperatures below -20°C (-4°F), there are now numerous varieties fully hardy in zone 4, with select cultivars definitely worth a try in zone 3! Times have changed, and now more than ever it’s possible and even desirable to welcome these performers into your northern landscape.
Unlike roses, however, azaleas and rhododendrons are not carefree in any way, even for those “fortunate” gardeners in the warmer zones of our continent. There’s far more to successfully growing these garden beauties than just coaxing them through the winter.
Put even more accurately, there are many more factors that determine their likelihood to survive a northern winter than their genetic ability to tolerate low temperatures alone.
That’s why if you intend to bring these plants into your yards and gardens, you must pay close attention to everything described in this article; anything less will result in certain disappointment.
Using Azaleas and Rhododendrons in the Landscape
To know how to plant and grow azaleas and rhododendrons, you need to know a little about this rather unique genus of plants. While at one time these were two separate genera, they have since been combined into the single genus “Rhododendron”.
The actual taxonomic differences between the two are subtle and mostly involve flower characteristics. For northerners, though, it is easier to distinguish between the two by loosely stating that most rhododendrons are broadleaf evergreens, while most azaleas are deciduous, losing their leaves each fall like many other of our northern shrubs.
Other than these differences, both are small to medium-sized shrubs. Their primary attribute is their flowering performance, and for this, they owe no apology – they are absolutely stunning in bloom. No comparably sized shrub makes as bold a statement in the landscape when in bloom.
Because azaleas are deciduous and bloom before the leaves appear, the entire shrub seems to turn the color of the bloom, one large mass of rich color set against the soft green of spring. The evergreen Rhodos hold their bold flowers over their deep green foliage, not quite as overwhelming but still highly effective.
Most of the species of rhododendrons (at least in colder climates) tend to bloom in shades of purple, lilac, mauve, or pink, and so these colors seem to dominate the selection of popular cultivars.
However, thanks to creative breeding efforts which have crossed these with some of the diverse rhododendron species from warmer regions, hardy rhododendron and azalea hybrids are now available in shades of yellow, orange, red and white, and the list continues to grow.
Rhodos and azaleas are the consummate garden shrubs, fitting exceptionally well into almost any garden planting. They make excellent foundation plants or features in border gardens, and will even stand alone in the lawn as accents where hardy enough.
Their flowering impact is so powerful that one plant can be enough to color an entire spring garden, but they also group exceptionally well.
When designing a planting around a rhododendron or azalea, keep in mind that while they are in your garden primarily for their colorful bloom, they will gracefully recede into the background of any garden the rest of the year. A few select varieties even have good fall colors, but this is the exception rather than the rule.
They are purported to do well in anything from full sun to full shade, but my extensive experience with these plants suggests that they will perform best in half to three-quarters sun. Too much sun and they will dry out, while they simply won’t flower as much in deep shade.
Evergreen Rhodos need reliable winter shade, too, because their foliage can suffer from winter burn. These plants are very slow growing but very long-lived, so consider them a long-term landscape investment. However, most flowers profusely from a young age, so even when they’re small, they will still contribute to your garden composition.
The Key to Success is in the Soil
Rhododendrons and azaleas belong to the Ericaceous family of plants, a moniker derived from the botanical name for spring heath, a family which includes blueberries, bog rosemary, bearberry, Scottish heather, and mountain laurel, along with too many subtropical species to possibly list here.
This is a most important fact to know because all the plants in this family share one very distinctive trait in common; they all have an affinity for acidic soil. In fact, with only a few exceptions, all members of this family will languish and eventually die in alkaline or even neutral garden soil!
There is an interesting story behind this characteristic. All the plants of the earth have certain “specialties”, certain growing preferences that enable them to thrive and dominate in a particular region, out-competing the other vegetation.
Where almost all other plants on earth would despise growing in highly acidic soil (acidic soil is often referred to as “sour”), this one genus has carved out its niche in the plant world by actually thriving in acidic soil, growing where few other plants will dare.
The downside of this magical adaptation is that these plants are not well adapted to tolerate the other end of the pH spectrum, alkaline soils.
So stick an azalea in clay soil, and you have effectively committed “herbicide”. Stick it in your good old four-way garden soil, and you have condemned it to years of suffering followed by its ultimate demise.
Yes, this is putting it quite harshly, but if you can’t comprehend just how important this fact is to the success of these special plants, you shouldn’t even consider trying them in your yard.
But the soil story doesn’t end there. All members of this family also require highly organic soil, a trait they carry over from their native preference for growing in the decaying organic material of the forest floor and peat bogs, as opposed to black soil or clay. And the soil must be supremely well-drained.
You are pretty well guaranteed to kill any member of this family by forcing it to survive even 2-3 days of standing water, and that includes spring melt or flash floods. These plants have very fine roots which require a consistent supply of oxygen, and standing water will quickly drown them, leading to fatal root rot.
Ironically, while they abhor standing water, azaleas and rhododendrons also despise excessive dryness and drought, and are prone to wilting if their roots aren’t kept constantly moist. It’s all part of their adaptation to survive in their natural habitats, and so it’s a biosystem you must recreate in your garden to keep them happy.
The key here is to really understand the difference between moist soil and standing water. To achieve this important balance, you must provide them with optimal drainage, and then water them regularly to keep the soil constantly moistened.
Sound fussy? I told you so! But if you’ve ever seen a mature rhododendron in full bloom, you’ll understand why people go through such efforts to bring them into their gardens and landscapes.
Planting and Care Tips
Since we’ve devoted an entire section to the importance of soil, you can see what’s coming next. The single most important step in welcoming azaleas and rhododendrons into your landscape is proper soil preparation before planting.
Unless you live in a few select locations in the north, for example in a peat bog or a native pine forest, be prepared to amend your soil or forget about ever trying to grow these in your yard.
Here are the critical steps for planting azaleas and rhododendrons in your garden. It is critical that you follow these steps for planting preparation and soil amendment precisely!
1. Carefully and objectively evaluate your planting location beforehand. If it collects water at any time of the year, don’t even bother; find another spot. Try to select a location that gets half to three-quarters of sunlight each day, less if you’re planting an evergreen rhododendron.
If there is any question at all about the hardiness of this cultivar for your area, try to make the location as sheltered as possible, for example on the north or east side of a house, fence, evergreen planting, or dense garden.
You want to find a location with minimal winter wind and in the case of evergreen rhododendrons, minimal winter sun.
2. Excavate a special area for every rhododendron or azalea you intend to plant. It doesn’t matter whether this is in native soil or in high-quality imported garden soil; get out your shovel and dig.
You’ll want to dig a hole at least 18″ deep and 3′ in diameter for every plant. If you’re planting a few together, dig the entire area 18″ deep and at least 18″ away from every plant.
If you’re digging into dense clay, I would go even wider, gradually sloping the walls of the hole. Get rid of the native soil; you won’t be using any of it for these babies.
3. Get out your wheelbarrow and prepare a special soil mixture to fill into the hole. Here’s the recipe;
- 2 parts peat moss
- 2 parts fallen pine needles (decayed or fresh)
- 1 part non-limestone sand
- 1 part top-quality black garden soil
Pine needles aren’t easy to find, but they are far and away the best organic material for these special plants; they are highly acidic and well-aerated. You might also try a fine bark mulch or leaf compost but don’t use sawdust or wood chips as these will deplete the nitrogen in the soil.
Mix this all together well, and dump it into the hole. Tamp it down well with your hands to pack it in, and water well. Keep adding and tamping until you form a gradual mound that rises about 2-3″ above grade level in the center, gradually sloping to grade level at the edges.
4. Form a hole in the center of each mound with your hands (this mixture is very easy to work with) the diameter and depth of the root ball of the plant. Remove the plant from its pot, and if the plant is root-bound at all, make a few vertical slits in the roots along the side of the root ball with a sharp knife.
Place the plant in the hole, making sure that the top of the root ball (the crown of the plant) is 2-3″ above grade (i.e. level with the center of the raised mound). It is critical that the plant sits substantially above grade to ensure optimal drainage.
5. Use your hands to fill the soil back against the root ball and to smooth out the transition with the mound. The top of the root ball should be level with the mound, neither sitting above it nor below it.
6. Liberally apply 2-4″ of mulch to the top of the planting mound and around the plant. Again, pine needles or bark mulch is best, because either of these will add organic matter as they decay without packing down and suffocating the fine roots. Water thoroughly but not excessively.
Once the azaleas and rhododendrons have been planted, water them regularly, being careful not to overwater them at any time. The key is to give them lighter waterings frequently as opposed to infrequent drenchings.
Rhododendrons and azaleas will tolerate pruning, but I strongly recommend against it. First of all, they are neat shrubs without human intervention, so pruning should be completely unnecessary.
Secondly, the flower buds for next season’s spring bloom are produced in the previous summer, so unless the pruning is timed exactly, you’re likely to eliminate a season’s worth of flowering. And never, ever prune in fall or winter!
Hardy Varieties for Northern Gardens
There are literally thousands of rhododendron and azalea cultivars offered around the world, but the vast majority are not hardy in the North. If you want to grow these plants in your northern gardens, you need to seek out specific hardy cultivars, and not ones from the bargain basement store or the reject bin either.
While I would by no means discourage Rhodo experts from experimenting with marginally hardy varieties or some of the newer introductions, I would strongly encourage the novices in the crowd to stick to one of the following proven series and varieties, as appropriate to your zone.
Once you’re successful with any of these, you’ll have a starting point from which to diversify your interests.
The hardiest azaleas, and in fact the hardiest commercial varieties of this entire genus, are the “Northern Lights” series from the University of Minnesota. These plants were introduced starting in the 1980s, with new varieties continuing to be released even today.
There are a number of varieties that feature a range of sizes and colors, but the most important thing to note about this series is that they ALL differ in individual hardiness.
This is because while the University has labeled them as a single series, they are actually different hybrids with different parents. The only thing they share in common is that they are reliably hardy in zone 4, but in colder zones, the differences are significant.
I can say unequivocally that the hardiest cultivar of this series, and by corollary the hardiest of all commercial varieties, is ‘Orchid Lights’ (Rhododendron ‘Orchid Lights’). This variety is definitely hardy in zone 3b and is a reliable performer in zone 3a with proper care and siting.
I have never had a year when mine didn’t bloom at least some, and I provide them with no additional winter protection. The next hardiest are ‘Rosy Lights’ (Rhododendron ‘Rosy Lights’) and ‘Northern Lights’ (Rhododendron ‘Northern Lights’), which are significantly taller.
These two are slightly less reliable in zone 3, but still worth a shot in sheltered locations. All the other “Lights” series (e.g. ‘Lemon Lights’, ‘Mandarin Lights’, ‘Spicy Lights’, ‘White Lights’, etc.) are hardy to zone 4a, and should only be tried by the truly adventurous in zone 3.
The hardiest evergreen rhododendron is ‘P.J.M.’ (Rhododendron ‘P.J.M.’), an introduction of Peter J. Mezzitt of Weston Nurseries in Massachusetts (hence the name).
This Rhodo is vegetatively hardy in a shady zone 3a location but isn’t a reliable bloomer in this zone; I would say that unless you’re willing to provide extra protection or have an extremely sheltered location with a favorable microclimate, it is best in zone 3b and warmer.
All the other rhodos in the ‘P.J.M.’ series (‘Elite’, ‘Victor’, Compact’, etc.) are best tried in zone 4a and warmer.
There is a new series of evergreen rhododendrons from the University of Finland known as the “Marjatta” series which are purportedly hardy to zone 4. These are relatively new releases, and while early testing seems to back up this claim, I would caution novices to try one of the others before venturing into this series.
Despite this caution, these are beautiful rhododendrons with large leaves, reminiscent of the more subtropical varieties. I’m optimistic that they will prove their hardiness in Zone 4, and I’m even trying a couple in my Zone 3a garden; I’ll let you all know what happens.
For the avid gardeners in the crowd, it is entirely possible to extend the range of almost every rhododendron and azalea by at least one full zone if you are prepared to provide added winter protection.
As with your tender roses, you’ll want to wrap the plants in late fall with a thick cover of straw or peat mulch to act as insulation. While this method is tried and true and works very well, I would caution the hobby gardener or average homeowner not to count on this unless you are prepared to religiously repeat this process each and every year.
Frankly, I feel that homeowners in zones 3-5 are better off sticking to the hardest cultivars and enjoying them without this added hassle.