Hardy Rhododendrons

Several things this week. The first is that the cold of this past week isn’t going to hurt perennials and bulbs. They’re dormant and underground now, covered by a good bit of snow. So you can relax on that score. The second thing to understand is that broadleaf plants such as Rhododendrons are not going to be very happy with cold weather but they’re not too badly upset by cold and cloudy/stormy weather.

What really kills them is the really cold and sunny weather.

In sunshine, the leaves absorb heat from the sun and open up their sweat pores (called stomata). The leaves lose moisture but they can’t replace it because the ground is frozen. So next spring, the leaves on that tender Rhododendron will be a clear shade of burned brown.

And that’s the reason we see “burning” on leaves. Heat absorption from clear sunny skies creates opening of the leaves of plants and the wind removes the moisture. We call it “wind burn” but it’s a bit of wind and a bit of sun and a whole bunch of moisture loss. And yes, you can prevent that if you use an anti-desiccant on your tender plants. This material (available in local garden shops) clogs up those pores and won’t let them open. Your plant is protected without having to wrap it in burlap or other really ugly covering.

The thing you want to do next spring is plan on planting hardy Rhododendrons and Azaleas. These are now both considered to be Rhododendrons by botanists but when it comes to cold gardens, there are only a few I’d recommend you search out. The first are the ‘Northern Light’ series. These plants were developed by the University of Minnesota and deserve a place in every cold garden. The bud hardiness on these plants is for -30C (and that’s cold enough for me thank you). There are a good selection to chose from including: ‘Pink Lights’, a light pink, ‘Rosy Lights’ a deep rose-pink, ‘White Lights’ with clear white flowers, ‘Spicy Lights’ is a salmon tone, ‘Orchid Lights’ is an orchid colour while my favourite ‘Golden Lights’ is a gold yellow.

For success with any Rhododendron plant shallowly. The roots of most of these plants live in the top six inches of soil. If you plant them deeply, you’ll kill them. So dig a shallow (but wide) planting hole and backfill around the plant with a 50:50 mix of peat moss and the existing soil to get the soil acidic. I like to feed with compost and use sulphur every year or two to keep the soil acidic. Other gardeners prefer to use an acid fertilizer specially designed to acidify the soil and feed this class of plant. A shallow root system also means you’ll have to mulch the plant with organic mulch (never rock) to help hold in soil moisture. And as you’ve probably guessed, this is not a plant you want to grow in a drought tolerant garden. Several deep waterings a week through the heat of the summer will keep this plant growing well.

If you’ve already collected all the Northern Light series of Rhododendron, you might want to keep an eye out for these plants. These are also considered among the hardier of the family.

A short variety that is sold locally is ‘PJM’. This hybrid cross holds it’s leaves over the winter unlike the Northern Lights that drop theirs. While it is hardy to -25/30 C, the leaves will burn if you give it winter sunshine in unprotected areas as I described above.

Rhododendron roseum is an older name but one that is often used in selling another of the hardier plants. This is a native Rhodo with flower buds that are hardy to -40C. It is a small plant of only three-feet tall but it has very fragrant white or rose pink flowers that bloom in late May. This plant does not like the heat and will suffer more in this than in cold weather. Plant it as a part shade plant out of the mid-day heat for best success. I note its real name is Rhododendron prinophyllum and it is one of the parent lines for the Northern Light series. This is a tough plant and if you can’t grow this one, you might want to consider trying other kinds of shrubs.

Rhododendron vaseyi is another of the really tough plants you should look for. It blooms in early May before any foliage comes out in a pale pink. This is a very tough plant with bud hardiness down into the -40C range. Consider planting it in naturalized settings because it does well with competition or underplanted bulbs. You’re going to love this one because it too won’t die from the cold.

On the other hand, the Exbury Hybrids are often sold by retailers (and bought by unsuspecting gardeners) because they have lovely colours and monster blooms. These plants are usually considered hardy to -15C. So the wind chill last week should have knocked these plants right back to the ground. Sorry for bringing the bad news but getting a deal on this plant isn’t really a deal.

And no, the potted azalea you’re going to get for Mother’s Day or did get for Christmas isn’t going to live outdoors over the winter. They’ll go nicely outdoors for the summer but will have to be brought in before frost if you want to see blooms again.

So that’s the deal. Pick the right variety for the local gardening climate, plant them shallowly, acidify the soil, mulch and water regularly and you too can have lovely spring blooms from this shrub.

But say goodbye to those tender plants this past week.