By: Bonnie L. Grant, Certified Urban Agriculturist
Rhododendrons are so beloved they have a common nickname, Rhodies. These wonderful shrubs come in a wide array of sizes and flower colors and are easy to grow with little maintenance. Rhododendrons make excellent foundation specimens, container plants (smaller cultivars), screens or hedges, and standalone glories. It used to be that gardeners in the north couldn’t take advantage of these standout plants because they could be killed in the first hard freeze. Today, rhododendrons for zone 4 are not only possible but a reality and there are several plants from which to choose.
Rhododendrons are found native in temperate regions of the world. They are outstanding performers and landscape favorites due to their large, showy flowers. Most are evergreen and start blooming in late winter well into the summer. There are many rhododendrons for cold climates as well. New breeding techniques have developed several cultivars that can withstand zone 4 temperatures with ease. Zone 4 rhododendrons are hardy from -30 to -45 degrees Fahrenheit. (-34 to -42 C.).
Botanical scientists from the University of Minnesota, an area where much of the state is in USDA zone 4, have cracked the code on cold hardiness in Rhodies. In the 1980s, a series called Northern Lights was introduced. These are the hardiest rhododendrons ever found or produced. They can withstand temperatures in zone 4 and even possibly zone 3. The series are hybrids and crosses of Rhododendron x kosteranum and Rhododendron prinophyllum.
The specific cross resulted in F1 hybrid seedlings that produced plants of 6 feet in height with primarily pink blooms. New Northern Lights plants are continually being bred or discovered as sports. The Northern Lights series includes:
There are also several other very hardy rhododendron hybrids on the market.
One of the hardiest rhododendrons for zone 4 is PJM (stands for P. J. Mezitt, the hybridizer). It is a hybrid resulting from R. carolinianum and R. dauricum. This shrub is reliably hardy to zone 4a and has small dark green leaves and lovely lavender flowers.
Another hardy specimen is R. prinophyllum. While technically an azalea and not a true Rhodie, Rosehill azalea is hardy to -40 degrees Fahrenheit (-40 C.) and blooms in late May. The plant gets only about 3 feet tall and has exquisite rose pink flowers with heady fragrance.
R. vaseyi produces pale pink blooms in May.
Botanists are continually making inroads into increasing cold hardiness in marginal plants. Several new series seem promising as zone 4 rhododendrons but are still in trials and not widely available. Zone 4 is a tough one due to its extended and deep freezes, winds, snow and short growing season. The University of Finland has been working with hardy species to develop even hardier rhododendrons that can withstand temperatures down to -45 degrees Fahrenheit (-42 C.).
The series is called Marjatta and promises to be one of the hardiest Rhodie groups available; however, it is still in trials. The plants have deeply green, large leaves and come in a host of colors.
Even hardy rhododendrons will survive harsh winters better if they have well-draining soil, organic mulch and some protection from harsh wind, which can desiccate the plant. Choosing the right site, adding fertility to soil, checking soil pH and loosening the area well for roots to establish can mean the difference between a marginally hardy rhododendron surviving an intense winter and the other extreme, which is death.
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Planting zones are areas you can find on a growing zone map that show exactly which plants are best suited to thrive in your given area, or zone. When shopping for new plants for your garden landscape, the terms “plant hardiness zones,” “growing zones” and “planting zones” may at first seem a bit confusing. In essence, plant hardiness zones have been used by growers for years to simply identify the plants that are most likely to survive the winter in their area. Being able to understand a hardiness zone map means you’ll have a starting point for making wise planting decisions.
Planting zones are broken down into thirteen areas, also known as USDA zones, which cover the entire United States, including Hawaii, Alaska and Puerto Rico. Each agricultural zone covers a 10-degree range. Zone 1 is the coldest, with an average minimum winter temperature of -60 to -50 degrees F, while the minimum winter average temperature in Zone 13 is 60 to 70 degrees F.
Some rhododendrons reach an enormous size if not pruned. Photo: Oh Kaye, flickr.com
Question: I have a very large rhododendron (2.5 m × 2.5 m/8 ft × 8 ft) that I would like to cut back severely after it blooms. Is that possible? It’s far too big for the space available and I no longer have the energy needed to wrap up such a huge shrub for the winter. I live in hardiness zone 4b.
Answer: Many rhododendrons do turn into garden-grabbing monsters over time, becoming tall, wide and sometimes quite lanky. They far exceed the dimensions you saw on the label when you bought it and probably used when calculating where to place it. (Few gardeners seem to know that the dimensions given on a rhododendron label don’t indicate its final size, but rather its dimensions after 10 years of growth. Thus, after 20 years, if they’re never pruned, they’ll measure approximately twice that.)
Fortunately, yes, you can cut an overgrown rhododendron back quite severely and thus reduce it to a more acceptable size. However, as you already seem to be aware, after such deep pruning, don’t expect it to take on a beautiful shape and abundant bloom for 2 to 3 years.
Cutting back a rhododendron leaves little more than stubs… but don’t worry, it will grow back. Photo: properlandscaping.com
First, the ideal time to do undertake rejuvenation pruning is not after flowering (that’s when you do light pruning), but rather early in the spring, before the start of the growing season. Cut all the branches about to 60 cm (2 ft) high … and yes, you will need a saw! Some gardeners cut back even more severely, to 15 cm (6 inches) from the ground, but that sometimes backfires, the shock being so brutal that some plants don’t recover.
In just a few weeks, new growth will appear. Photo: properlandscaping.com.
In the first year, your shrub will start to regrow, producing new stems and leaves, but will probably appear very spindly and unequal at first. It will probably take 2 or even three 3 years before it really fills in and starts to look good again. And it may well not bloom abundantly until the third year either.
In the future, when your rhododendron approaches the size you want, make a habit of pruning your shrub modestly after it blooms to restrain its development, shortening one branch out of three annually. That way, you can keep it at a more acceptable size even as you enjoy an abundance of flowers each year.
Winter Protection: Don’t Waste Your Time
Winter protection isn’t necessary for rhododendrons chosen according to their hardiness zone. Photo: jardinpro.ca
You mention wrapping your rhododendron up for the winter, but that probably isn’t necessary.
The idea that rhododendrons need winter protection to survive in cold climates is highly exaggerated. Wrapping them in burlap or geotextile may make you feel useful and does keep the wind off, but won’t keep them any warmer, so is generally a waste of time. I’m not saying installing some sort of screen between the plant and the prevailing wind can’t be useful the first winter, while it’s still acclimatizing to local conditions, but from then on, no winter protection should be necessary.
Your rhododendron certainly wouldn’t reach such a large size if it weren’t well adapted! There are many rhododendrons adapted to cold climates: hardiness zone 4 and even 3 and yours undoubtedly belongs to that group. So, in winter, just let it fend for itself.
Azaleas and Rhododendrons, with their beautiful flowers, are one of the best-loved sights of spring. Producing a blaze of color, these popular ornamental shrubs are, however showy for only a relatively short season when in bloom. Their spectacular display can be extended by selecting species or cultivars with different blooming times or by planting companion plants for greater diversity.
Appealing and long-lasting planting combinations can be created with companion plants that enjoy early or late blooming periods, showy fall color, persistent showy berries or that provide attractive textural contrast with your Azaleas and Rhododendrons. These companion plants, whether shrubs, bulbs or perennials, must however be able to thrive under the same growing conditions as Azaleas and Rhododendrons.
Create appealing and long-lasting planting combinations with your Azaleas and Rhododendrons. These evergreen or deciduous shrubs are famed for their showy fall color, persistent showy berries or provide attractive textural contrast.
Here are a few tips for incorporating them into your garden:
When you have catered to the few needs of this particular hybrid then this fast growing plant will become a lovely compact mound of evergreen foliage that every year will explode with huge numbers of richly colored blooms, and it should continue doing this for around 40 years. Bees and butterflies will thank you for this bright and bold addition to your landscape whether the local climate is hot or cold. Nova Zembla is indeed a fabulous member of the rhododendron family, and we encourage you to browse other varieties like the Everred Rhododendron or the Lemon Ice Rhododendron.