Azalea or Rhododendron?

Forsythias, those cheerful harbingers of spring (but certainly not the first woody plants to bloom) are completing their annual gaudy display, and as they fade into their seasonal greenness, our eyes fall upon the bright mauve-purple blossoms seemingly as common as Forsythia – the PJMs. These rugged evergreens with smallish oval-shaped leaves are thought of by many as azaleas, but they are not. They are Rhododendrons.

It is easy to understand the confusion, because they are related. All azaleas are Rhododendrons, but not all Rhododendrons are azaleas (they are botanically differentiated by the number of stamens each flower carries: azaleas have five, while Rhododendrons bear ten or more.) And azaleas are grouped within the genus Rhododendron, and part of the larger heath family which also includes heathers, blueberries and cranberries, mountain laurels, and more – all prefer cool, moist acidic soil, so mulches are always helpful (see my previous article.) Rhododendrons (not azaleas) can be divided into 2 categories: large-leaved and small-leaved types, and the PJM belongs to the latter group. Generally speaking, those with large leaves prefer some shade, although they flower better if given at least 1/2 day of sun, and the small-leaved Rhododendrons perform best in full sun, but tolerate some shade.

Those familiar with azaleas know them to bear bright red, pink, purple or white flowers on spreading bushes that remain semi-evergreen in the winter. These types perform better in more southerly locations, but can be found locally each spring when in bloom. They generally need careful siting to prevent the crisping of their foliage in our cold and dry winter air. Deciduous azaleas are far more reliable, and have a much greater range of flower color – from white to yellow, orange to red, but no blue or purple. And rather than having the narrow bloom times of the semi-evergreen varieties, by choosing different deciduous types, one can have flowers from April to August.

Rhododendron ‘PJM’ was hybridized in the 1930’s by the late Ed Mezitt of Weston Nurseries in Massachusetts, and he named it after the initial letters of his father, who founded the nursery. This fortuitous marriage of an Asian species and an American native has become the benchmark of hardy rhododendrons, and it is very hardy. It is also quite easy to grow, thriving in full sun to part shade. Its only downfall is soggy soil, as it leads to root rot. Indeed, its ease of growth has made it as common as lilac and forsythia in the northeast, and combines well with some Magnolias which bloom at the same time.

Because its purply color can be a bit jarring in the landscape, Ed Mezitt continued to breed rhododendrons in search of calmer pinks and whites, but also a true red (which he never achieved in his small-leaved hybrids, but came close with ‘Landmark’, one of my favorites.) The varieties ‘Olga Mezitt’ and ‘Aglo’ (Olga spelled backwards) are nearly as popular as the PJM, and bloom about a week or so later, nearly in concert with crabtrees. Their showy flowers are a clearer pink, but their habit is a bit more “open” than the denser ‘PJM’. These varieties, as well as many others including ‘PJM’ have deep green foliage that is aromatic when brushed against, and which turns dark bronze to mahogany when the colder weather arrives in the fall.

Mezitt’s breeding work also included azaleas, and he used the rich palette of deciduous species native to New England and the Northeast in his work, as well as a hardy semi-evergreen species from Korea. The deciduous hybrids are outstanding, because most bloom in early to mid-summer. (A future article will cover these superb “Summer Blooming Azaleas.”) Further reasons to grow these types: they are often sweetly fragrant, and many have vivid fall foliage, extending their season of beauty. Although his semi-evergreen introductions are less well-known (and hence not easy to find,) they generally perform better in central New England than those more commonly available, because that is where they were bred and tested.

Large-leaved Rhododendrons will soon be blooming in May. Although there are hundreds of varieties hardy for our area, most nurseries and garden centers stock relatively few. Because they are frequently used as “foundation plants”, it’s best to look for varieties which remain naturally compact, rather than dealing with green monsters which can quickly swallow up a window. Look for a discussion in an upcoming issue of some of the lesser-known varieties which I consider to be better in many respects from the traditional offerings.

Much Too Much Mulch

It’s officially Spring (after the winter that never was.) Gardeners and landscapers alike have been spreading mulches of shredded bark and/or wood chips around trees and shrubs and over emerging perennials, an annual event as regular as the swallows returning to Capistrano. Unfortunately, eruptions of mulch “volcanoes” are more predictable than the real things, as they are found an nearly every street and in every neighborhood. More about these in just a moment.

The widespread us of mulches is a relatively new phenomenon – shredded bark, which is removed from logs prior to their being milled or processed into pulp, was considered waste until the late ‘60’s or early ‘70’s, and it accumulated in massive piles surrounding lumber mills. If gardeners mulched at all, it was usually with peat moss, a less than perfect option for many reasons. Sometimes grass clippings would be used, or perhaps composted leaves.

Eventually though, a use for those piles of slowly decomposing bark was found, and a huge market was created – bark mulch. Because today’s log de-barking machines are even more efficient, it’s inevitable that some wood ends up in the mix, so bark mulch is less pure than it used to be. Somewhere along the way, an entrepreneur decided that inferior wood, old pallets, construction debris and the like (with no bark at all) could be chipped and ground up into small pieces, colored with dyes (brown, black, or red) and then sold as “bark” mulch. This product is inferior to the real thing, and should actually be called “colored wood chips”.

Using organic mulches (I’m leaving aside stone mulches for the moment) have many benefits, if properly applied. They help to modulate soil temperatures in cold or warm weather and slow down the evaporation of soil moisture. They help prevent soil-borne weed seeds from sprouting (but serve as a good incubator for those that land on the surface.) By offering themselves up as a food source, they help foster a healthy community of microbes which break them down by composting them into less complex substances that are released to the soil, increasing its nutrient and organic content as well as its tilth. Earthworms and other organisms are good recyclers of soil organic matter, which benefit plants even more. Further, microbes also become intimately associated with root hairs in the soil, and form an immense extension of a plant’s root system. Many plants cannot exist without these microbial populations.

However, if improperly applied, mulch can lead to plant distress or death. Over the past few years I have come to find that overmulching is a leading cause of plant death – especially of perennials. If you are applying more mulch yearly than is being broken down by the natural decomposition process, you are applying too much. Try to maintain an even layer about 2- or 3-inches thick from year to year. When mulches approach 4-inches in depth or more, they can actually repel water and reverse one of the benefits of mulch. If the surface needs to be refreshed, a very thin layer (sufficient to barely cover the ground and no more) may be spread, and never over the tops of perennials, for it can smother them over time.

Back to mulch volcanoes, those tall mounds of mulch around the trunks of trees and even shrubs. Where they originated is anybody’s guess, but the practice continues to be spread by uninformed landscapers as well as homeowners. Why are they bad? Bark respires by exchanging oxygen and carbon dioxide, and as mulches tend to hold excess moisture, they interfere with this respiration if piled against the trunk. Another consequence is the development of tree diseases which are favored in moist conditions. Follow nature’s model: walk through any forest, and you will not see the decaying leaf litter up against tree trunks. The natural flare of the trunk is clearly visible, and should be around your own trees as well.

As to what sort of mulch to use, I prefer the true aged (or partially composted) darker mulches, which have a softer and finer texture than more freshly-ground bark. I also like the dark color, as it resembles the color of soil and shows plants off well against it. I avoid the use of any mulches which are basically chipped and colored wood chunks – and especially those that are dyed red or orange to resemble true hemlock mulch (which, when fresh, is reddish, and is hardly ever available anymore.) I find this color disturbing in the landscape, but that is a personal preference.

I also like to use pine needles, shredded leaves and even compost in the garden whenever available. These are freely available and beneficial to the soil (and pine needles do not increase its acidity, despite popular belief.) “Weed barrier” fabrics do nothing to control the spread of weeds that originate from wind-blown seeds, and inhibit the growth and spread of desirable plants, particularly ground covers.

In any case, mulches should not be a dominant garden theme, but a helpful solution to make gardening easier while improving the soil at the same time. Vast barrens of bark mulch are often created and maintained, but do nothing to enhance the landscape. Even a meager lawn can look better, with scarcely more time needed to manage it. Dense plantings of shrubs, perennials and ground covers, topped with a light layer of organic mulch every other year or so, will require little mulch and maintenance over time – the best combination of beauty and function for today’s gardeners of whatever ability.