Manitoba gardeners might be surprised to learn that it is possible to successfully grow azaleas outdoors in our Province. Hardiness is a concern, but it is not the primary reason that many gardeners fail with this showy staple of southern landscapes. These are fussy plants, and it requires a great deal of preparation to get them to even stand a chance of growing long enough for hardiness to become an issue.
Azaleas and rhododendrons belong to the family Ericaceae, which includes blueberries, bog rosemary and bearberry. This is important to know because all plants in this family share one distinctive trait: they 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. Stick an azalea in clay soil and you have effectively committed "herbicide!"
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 the soil must be extremely well-drained. You are guaranteed to kill any member of this family by forcing it to survive even a day or two in standing water. And that includes spring melt or flash floods.
Ironically, azaleas also despise excessive dryness and are prone to wilting if their roots aren't kept constantly moist. The key 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 moist.
Azaleas are purported to do well in anything from full sun to full shade but in my experience these plants will perform best in part sun. Too much sun and they will dry out, while they simply won't flower as much in deep shade.
With this in mind, here are six critical tips for planting azaleas in Manitoba:
1. Carefully evaluate your planting location beforehand. If it collects water at any time of the year, find another spot. Find as wind-sheltered a location as possible, for example on the north or east side of a house, fence, evergreen planting or dense garden. Select a location that gets half to three-quarters sunlight each day, keeping in mind that the hot afternoon Prairie sun will desiccate these plants in just a few hours during the growing season. With azaleas, location is everything!
2. Excavate a special bed for your azaleas. It doesn't matter whether it's in native soil or in high quality imported garden soil; get out your shovel and dig. Make a hole at least 18" deep and 3' in diameter for every plant, removing the soil. 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.
3. Get out your wheelbarrow and prepare a special soil mixture for planting:
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 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 well and dump into the hole a little at a time. Tamp 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. Remove the plant from its pot, and if it is root-bound, make a few vertical slits in the roots 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 crown 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.
6. Liberally apply 2-4" of mulch to the top of the planting mound and around the plant. Pine needles or bark mulch are best because they add organic matter as they decay without suffocating the fine roots. Water thoroughly but not excessively.
After planting, water regularly, being careful not to overwater at any time. The key is to give them frequent light waterings as opposed to infrequently drenching them.
Azaleas will tolerate pruning, but I strongly recommend against it. They are naturally neat shrubs, and the flower buds for next season's spring bloom are produced in the previous summer. Unless the pruning is timed exactly, you're likely to eliminate a season's worth of flowering. Never prune in fall or winter.
There are thousands of cultivars offered around the world, but the vast majority are not hardy in Manitoba. The hardiest azaleas, and in fact the hardiest commercial varieties of this entire genus, are the "Northern Lights" series from the University of Minnesota. There are a number of selections which feature a range of sizes and colors, but they differ in individual hardiness, because while they have been marketed as a single series, they are actually different hybrids with different parents. About the only thing they share in common is that they are reliably hardy in zone 4; in colder zones, the differences are significant.
The hardiest variety of all is 'Orchid Lights' which is 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' and '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 our zone.
It is entirely possible to extend the range of almost every azalea by at least one full zone if you are prepared to provide added winter protection. As with your tender roses, 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, I would caution you to not count on this unless you are prepared to religiously repeat this process year after year. Frankly, I feel that Manitoba gardeners are better off sticking to the hardiest cultivars and enjoying them without this added hassle.