Native Plants and Others

It’s common knowledge that the state tree of New Hampshire is the white or paper birch (Betula papyrifera) while its state flower is the purple lilac (Syringa vulgaris.) The birch, venerated for its white bark and flexible but resilient “bend but don’t break” manner, is a hardy native, and one which stands out year round. The lilac, on the other hand, is decidedly “not from around here”, but rather an exotic species of southeastern European origin, having been brought by early settlers to Portsmouth, NH and then distributed far and wide. Despite its having existed in North America for over 300 years, it will never be a native, even though it has become naturalized (a term which is applied to both plants and people. In a way, most of us are all naturalized citizens.)  Its presence is often only revealed in spring, when its fragrant and familiar blossoms betray its position, one which once probably began in the dooryard of a colonial home long gone, its cellar hole a vestige of time past, as suggested by Robert Frost (another non-native – he was born in California) writes in his poem “Directive”:

...for the house that is no more a house,

But only a belilaced cellar hole,

Now slowly closing like a dent in dough. 

Lilacs, courtesy of April Walker

I compare the birch and lilac because they represent more than the official state flower and tree. They also illustrate the problem with declaring natives more suitable than exotics in the landscape.

Native plants (the definition of which varies, but typically used to describe those which were here before European colonization – but that is also problematic, as there were trees growing millions of years ago on this continent which are only found today in Asia) are thought to be better choices for the landscape, requiring less water and other care than exotics. There are plenty of Japanese Barberries, Norway Maples, Burning Bushes, and other plants which have escaped from cultivation and are now considered invasive species to disprove the thesis.

Let me state that I am a firm believer in the value of native plants, appreciate their beauty and status in the wild, and am a dedicated advocate for their use in managed landscapes whenever appropriate. Given the choice between a native and an exotic, I will typically opt for the former. However, as shown in the above example, exotics also have their place, so long as they don’t spread so aggressively as to displace native plants which have evolved along with native wildlife to their mutual benefit.

Drive down a country road when lilacs are in bloom, and you are likely to see lilacs growing as described by Frost, Whitman and others, an enduring and endearing shrub which has become an archetype many homeowners can’t live without in their gardens, despite its brief display. The white birch, on the other hand, is easy to spot at any time, but much harder to grow. It prefers cool, moist soil, and suffers when affected by drought. Indeed, its major nemesis is an insect called the Bronze Birch Borer which is able to detect a chemical signal emitted by birches under stress, and in which she lays her eggs which hatch and spell doom for the tree.

A similar example can be found with dogwoods: the native flowering dogwood (Cornus florida) is susceptible to another form of borer, as well as a destructive disease, when not grown under the right conditions. The exotic Kousa dogwood (Cornus kousa) is resistant to both, and is often the better choice. Hybrids between the 2 species (the so-called Rutgers types) have been produced which offer the best of both.

Clearly, the discussion of whether native plants are superior is complicated. Natives usually offer wildlife more value, and their presence in natural settings is critically important. However, it bears notice that natives may not always work well in managed landscapes, and appropriate choices may be found in plants which may not have originated here.

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.

What’s My Zone?

Gardeners and attentive news followers may know by now that the USDA has just released an update of the Plant Hardiness Zone Map, and to nobody’s surprise, New Hampshire is getting warmer, with the current “winter” adding to the accumulating evidence.

Hardiness zones are typically used to determine which plants will grow in a given area, and are displayed on the map of North America as broad colorful strokes depicting the average lowest temperature likely to be experienced each winter. Each zone is given a number, with lower digits representing colder regions, and vice versa, and the zones are in 10 degree F. increments. Each is further broken down into subranges “a” or “b” – thus, “USDA Zone 5” indicates average minimum winter temps of -10 to -20 degrees F; “5a” is -20 to -15, and “5b” being -15 to -10 degrees F. In the warmest parts of the U.S., it’s not the cold that is a limiting factor: it’s the heat, and heat zone maps have been created for those areas that will never experience a snowflake, or black ice.

The last version of the Plant Hardiness Zone Map was issued in 1990, and was based upon 12 years of meteorological data. Creators of the 2012 map were able to use the latest in computing technology to compile 30 years of readings from more measurement sites into an interactive tool that is much more detailed. When compared to the 1990 map, it is clear that warmer zones have been creeping northward, and that is also true in New Hampshire. However, some of the changes can be attributed to the additional data and computational ability, and the USDA makes no claims about the influence of planetary warming leading to climate change.


Gardeners can be a quirky bunch, often conversing in botanical Latin and using the Zone Map as part of their identity, as in, “I’m a solid Zone 5b” (meaning average minimum temperatures of -10 to -15 degrees F.) Importantly, these temperature ranges refer to ambient air and not to wind chill temperatures, which only apply to exposed human skin. I am often asked to offer plant recommendations for “my Zone 4a (-25 to -30 degrees F.) garden in Amherst”, or some other southern New Hampshire location. In fact, most of the lower half of the state lies in Zone 5a or 5b, with the extreme southeasterly portion enjoying Zone 6 (0 to -10 degrees F.) winters. That the Seacoast is milder is unsurprising, but the new map shows large scattered blocks of western New Hampshire, from south of Keene to north of New London as also being in Zone 6a. This should come as welcome news to gardeners in those areas, with a larger palette of plants they can now try.

Gardening books, plant tags and catalogs typically provide the hardiness range of a given plant, assisting the reader when deciding whether she can grow it. However, these zones, while extremely helpful, are more of a guide than a rule – there are other factors which come into play: winter sun exposure (although counterintuitive, less winter sun, especially in the afternoon, is often better for some evergreens), the amount of snow cover (more is usually better, as snow is an insulator,) plant vigor, and the microclimate – or the immediate vicinity of the plant’s intended location – is it sheltered? Does it retain the sun’s warmth into the evening? And lastly, how wet the soil is in winter – plants that do quite well in a cold yet arid climate for instance, the high desert – often perish in our icy mud.

Part of the fun in gardening is pushing the limits of what plants can be grown successfully. Northern gardeners are eager to try more tender species, while those in the south – especially snowbirds, may attempt plants that require more cold to survive. Despite our typically cold winters, we in central New England can grow an astonishing variety of plants, but our warm and humid summers often prevent success with those species happier in the more temperate climes of England or maritime Canada. Which means that we are far more likely to succeed with growing ornamental bananas and gingers rather than the holy grail of many local gardeners: stunningly perfect spires of 6-foot tall delphiniums.

Not that the bananas and gingers will make it through our winters, at least according to the new hardiness zone map.