Rhododendrons & Kin

Rhododendrons & Kin

Perhaps more difficult to grow than roses, rhododendrons and related species are a challenge for those of us who garden in our Southern Great Lakes Region. It can be done if their requirements are met, with appropriate siting, and appropriate selection of varieties.

Rhododendrons and their Kissin' Cousins, the azaleas, are of the family Ericaceae. This family includes the heathers and heaths, and all are acid loving plants. In areas of ample rainfall, loose acidic soils, and moderate temperatures, rhododendrons and azaleas not only survive, but thrive. Our conditions have several strikes against growing rhodies and kin, but these can be corrected and worked around.

The most common problem encountered when growing these plants involves soil acidity. Rhodies and azaleas require a pH of 5.5 to survive. Here in the southern Great Lakes region, there are some areas of acidic soil, to which any blueberry grower can attest. Most of the region, however, has soils at or above pH 7.0. This is because a large portion of our area lies over limestone substrate. The limestone is alkaline, and this leaches upwards to sweeten our soils. Before successfully growing rhododendrons or azaleas, most of us are going to have to make adjustments in our soil's pH. This will be discussed later.

Along with the problem of less than ideal pH, many gardeners in our area have a clay-based loam. The greater the distance from the lakes, the heavier the soil becomes. In their natural environment, rhododendrons and azaleas are found in areas of light, loamy soil. They will rot if their roots are kept wet for too long. The soil must drain well, and at the same time, be moisture-retentive. Now, this might sound like a paradox, but it really isn't. Loose loam can drain well, which means it will not have any pockets of moisture or "bogginess". It can stay moist and light. Thus, whatever medium these plants grow in must have the ability to hold moisture without becoming soggy.

One of the other common mistakes people make when growing these plants regards where they site their rhodies and azaleas. Rhododendrons and azaleas do not take to an exposed position, and they do not take to long periods in the sun. In nature, they are understory trees. They cannot take the wind whipping them around, or the sun baking them. Because their roots are close to the soil surface, they cannot take the sun heating up their "feet", and do not tolerate drying out, which will happen when they are in an open, exposed position. In the Winter, exposed sites can cause the leaves to dry out from the dehydrating effects of the wind. Sun scald can also occur when tissues heat up and then refreeze.

The last major problem in with the choice of cultivars. There are over 900 species and countless hybrids of rhododendrons and azaleas. New varieties are constantly being introduced. A vast number of them are tender plants, at least by our growing standards. Many of these are more suited to the Pacific Northwest or the Piedmont Region than to our region.

Now, if you are not entirely put-off by all this discouraging news, rhodies and azaleas can be grown successfully if we tackle these problems before rushing out and buying plants.

Look around your yard and see which areas are sheltered from the wind and sun. Logical spots would be the north and eastern exposures of your yard. The main thing to remember is not to plant them under eaves and overhangs or directly next to your foundation. The eaves and overhangs keep moisture from reaching your plants, and foundations can leach alkali into the soil. Site the plants at least three feet away from the walls of your buildings.

Now that you have looked around and found good planting spots, let's look at the pH and the soil structure. There are various good soil testing kits that can be purchased at local hardware stores or garden centers. If you do purchase them, follow the directions exactly. If you would rather have someone else test your soil, there are many labs and agencies that will test your soil for you for a nominal fee. Not only will you learn about your soil pH, these tests will indicate the mineral content and most reports will tell you what trace elements are lacking, and many will make recommendations for soil amendments. Your county extension agent can point you in the right direction for soil testing.

Once you have determined your native pH, you will need to adjust the pH so that it reaches the 5.5 level. There are many ways to do this, but one way I will tell you how not to do this is by adding aluminum sulfate. Aluminum sulfate is toxic to plants, in particular rhododendrons. Unless you are very experienced in what you are doing, you can kill a planting of rhodies and azaleas in a matter of a week or two by applying this incorrectly onto the beds. When there is ample organic matter in the soil, aluminum sulfate is bound-up, and made insoluble, thus unable to be taken up in the plant. Clay loams do not have enough organic material in them to begin with, and all that aluminum goes straight to the heart of the plant, so to speak, poisoning it. Aluminum sulfate also leaves heavy metal traces in the soil. Why stores continue to sell this is beyond me!

A far better way to adjust pH and improve the oil is organically. Incorporate lots of shredded or composted leaves, partially rotted shredded bark mulch, and if you can get them, finely shredded oak leaves to the soil. If you can get your hands on pine needles, add copious amounts of them to the soil, too. To this mix, add Canadian sphagnum peat moss. The goal is to make a light, yet moisture retentive soil with an acid base. The peat-to-soil ratio should be about 50/50. Mix the peat in thoroughly, so that it does not crust on top of the soil, which might prevent moisture from entering the soil. After the soil is starting to feel 'fluffy', add about a third again portion of sand (unless your soil is sandy to begin with), and remix the whole bed. If the soil beneath this mix still tends to drain poorly, you will need to berm the soil up or make this a raised planting bed so the roots will not rot.

Recently there has been a lot of progress in the hybridization of cold-hardy rhododendrons and azaleas. In the not too distant past, the varieties were slim pickins' for those of us who live in the colder portions of the country.

Our zone in this region is generally zone 5 with a few pockets of zone 6 thrown in around some narrow bands of the southern and eastern shores of the lakes. To break it down even further, our region is generally in Zones 5a (-15 to -20 degrees average lowest temperatures), 5b (-10 to -15 average lowest temperatures), and 6a (-5 to -10 degrees average lowest temperatures). Keep that in mind when shopping for these plants.

When looking at varieties, notice that there are evergreen and deciduous varieties of both rhodies and azaleas. Just what is the difference?

All azaleas are rhododendrons. Rhododendrons are not azaleas. Most rhododendrons are evergreen, and only a few are deciduous. Azaleas can be either. Some people claim that rhododendrons are classed as such because their leaves are thicker and larger than those of the azaleas, which are generally not as thick or large. More accurately, rhododendrons have at least ten, sometimes more, stamens. Azaleas have five stamens on average. That aside, let's look at some good varieties to grow in our region:

If you want a sure bet, start with one of the species rhododendrons, R.prinophyllum, or 'Roseshell Azalea'. The flower buds are hardy down to about -40 degrees F. They are small in stature, grow about three feet tall, and have fragrant white to rose colored flowers that bloom in May. They will need light shade as they are heat sensitive. Add about 3 inches of shredded bark mulch around these plants to keep their feet cool.

R. catawbiense is a good species rhodie with lush evergreen leaves. 'Nova Zembla' is a variety with red flowers. Roseum Elegans have a pinkish-lavender color. English Roseum is a light rose, and Album has white flowers. They are hardy to about -25 degrees F. Again, they need to be in a protected spot.

R. arborescens, or 'Sweet Azalea', is a deciduous species plant that will do well in our area, hardy to about -25 degrees F. It blooms in late June to July, and is pleasingly fragrant. This variety can grow 8 to 10 feet tall.

Any of the "Northern Lights' series of hybrid deciduous azaleas should do quite well in our area. These plants are phenomenally hard, in the -40 degree F range, and quite a few of them are fragrant. Although they can stand more sun than most, they still need to be protected from too hot of sun and from too much wind. Mulch to keep their feet cool.

Do you want more evergreen varieties? Well, here are a few hybrids to choose from:

'Elvira', developed in Finland, a small growing evergreen, two feet tall and spreading in habit, hardy to about -29 degrees F.

'Haaga', also from Finland, a four foot plant, white, reportedly hardy down to -35 degrees F.

'Joe Paterno', a four foot plant, white, reportedly hardy down to -20 degrees F.

'Jenny Talbot', about a four foot plant with slightly smaller leaves then the above mentioned evergreen rhodies. Has yellow flowers, hardy to about -20 degrees F.
We could go on and on, but you want to plant now that you have bought a few of these beauties!

Well, all these plants have shallow roots. So, you are only going to plant them about a third deep. First, you are going to save any soil that falls out when you remove these plants from their pots, and you are going to gently tease their roots apart. Scoop out about 1/3 of the top of your prepared beds. In other words if the plants' soil and root ball are about 18 inches high, only scoop out about 6 inches of soil. Place your plants in that small depression, and mix the back fill with the rest of the soil that fell away from the plants. Add some old, partially rotted and shredded wood mulch to the backfill, and mound it the rest of the way up the root ball of your plants. Water them in, and mulch with about 3 inches of shredded wood mulch, keeping it away from the main stems. Do not fertilize, and that should do it.

If the leaves start to yellow, check the pH before adding other products such as sulfur, because the yellowing might not be related to pH but rather to poor drainage or to an exposed site, or other stresses. If the yellowing is related to pH, an emergency application of chealeted iron is needed. Also apply the recommended rate of soil sulfur for long range benefits.

After the first year, apply an organic, acidic fertilizer such as cottonseed meal in the early Spring, and once more in late May to early June, and no later than the 10th of June. Apply about 1/2 the rate as for other ornamentals. Gently scratch it into the top inch or so of soil. Remember these are shallow rooted and you don't want to damage the surface feeding roots! For additional acidifying benefits, pine needles, shredded oak leaves, and coffee grounds can be blended in with the mulch. As always, follow recommendations for the fertilizers and acidifiers.

If you have chosen wisely and sited your plants well, you should not have too much problem with sun or wind damage. It is still a good idea in the late Fall to spray an anti-desiccant on the leaves of these plants to keep the evergreen varieties from sustaining damage and dehydration. Some people build windbreaks for the first year or so after planting.

There you have it! With a little prep work and study, you should have a head start on successfully growing rhododendrons and azaleas here in the southern Great Lakes region. It takes a little time and effort, but the rewards are so great!

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