What's the hardiness zone rating for my city? It depends!

On Hardiness And Mesoclimates
Separating Myths From Facts
by Jim Kohut, Staff Writer

Hardiness is a funny thing, to put it mildly. The principal objective of the concept is simple - try to give gardeners a simple way to predict what woody or perennial plants will grow where based on their ability to survive the winter. After all, since we can say that there are minimum winter temperatures above which a plant will pretty much never fail to pull through and come again the following spring, and there are also winter lows below which a plant is certain to die, then does it not follow that there is a temperature somewhere in between that we can label as the turning point? A temperature above which a plant is sure to survive, and below which it isn’t?

As I said, it’s a noble goal, but far from achievable in the real world. For you see, winter plant survival is a complex combination of low temperature injury point, required growing season, heating degree units, susceptibility to late spring and early fall frosts, and response to spring cues to start growing and fall cues to shut down and harden plant tissues, all in a calculation which is quite unique to each species and even variety. And that’s just from the plants’ perspective!

Climates, Microclimates and Mesoclimates

If you think it’s a challenge to pin down a plant to one simple rating for winter survival, try pinning down the particular “climatic conditions” of a given location. For one thing, we all know that weather is wildly variable from year to year - we get warm years and cold years, and in fact the only thing it never seems to be is “average”.

	Because most of us live in suburbs, we experience a moderated climate

We are intuitively aware of climatic effects on a continental scale. We know that it tends to get warmer as we move south, and that the two coasts have much more moderate climates than the continental interior. Cold arctic air slides down from the north, and warm breezes blow up from the south. We know that mountain ranges squeeze out water on the windward side, and leave a relative drought on the leeward side. While it might be difficult to quantify the particular climate of a location in absolute terms, it is quite easy to compare one region to another when the differences are significant.

We also know that there are significant variations in the local “climate” on our own human scale. The south side of a house is considerably warmer than the north side. We can feel the heat of black soil baking in the hot summer sun, and the relative coolness of a grassy green lawn. We find we are able to grow more varieties of plants in areas closely sheltered by buildings, trees and dense gardens than in the middle of an open field or yard. These are examples of microclimates, which can affect the survivability of a particular plant tremendously, and which can deviate from the particular hardiness zone “rating” of a region by a whole zone or more.

But in between these two extremes of climate lies a real gray area, one which is poorly understood by both gardeners and climatologists alike, and yet which has a significant effect on the winter survivability of plants and the fundamental underpinnings of the whole hardiness concept itself. It’s called the “mesoclimate”, and it is a consequence of climatic effects that are still constrained by the bounds of a regional climate, but which extend beyond the reach of a single human being to influence.

The Urban Heat Island Effect

The concept of a mesoclimate is most easily illustrated by the way our cities and towns modify the regional climate. Most gardeners intuitively know that a “marginal” plant for their region is more likely to survive in the middle of a dense garden in a big suburban development inside the city than it is in the middle of an open field outside of town. This is a result of the “urban heat island” effect, an empirical observation that cities and towns are almost always warmer than the surrounding outlying regions.

Cars, asphalt and factories all contribute to the urban heat island effect

The reasons for this are complicated, and involve modifications to wind patterns by buildings and urban greenspaces, greater absorption of sunlight by asphalt and other manufactured surfaces, the greenhouse effects of smog and urban pollution, and direct heating factors such as heated buildings, car exhaust and factory smokestacks. However, the results are quite easy to measure. They are most evident in cities where the airport is located outside of the urban island; temperature readings inside the city are often many degrees warmer than the outlying airport readings, particularly late at night when winter lows are usually reached.

In fact, the urban heat island effect is of great consequence to the validity of hardiness zone ratings on a “meso-scale” such as cities, towns and neighborhoods, and can lead to false conclusions. Obviously, if the climatic data for a particular city are derived from readings taken at an exposed rural airport, they will tend to under-report the winter lows and thus give a hardiness rating that is up to half a zone lower than the urban and suburban neighborhoods. On the other hand, downtown readings may lead to a falsely optimistic hardiness zone rating that is in fact greater than the average homeowner might experience in a suburban yard.

Exposure And The Shelterbelt Effect

A large part of the relative “warmth” of a city or town compared to its outlying areas is a result of the cumulative shelter afforded by buildings and mature trees, an effect which can be (and is) mimicked outside of an urban environment with the use of shelterbelts. The basis behind this has to do with the control of wind and how that affects ambient temperature.

	This poor tree feels the full brunt of the prevailing winds

When it comes to air temperature, wind is the great equalizer. It mixes and mingles air masses of different temperatures and blends them to produce a resulting temperature at equilibrium. In the absence of the movement of wind, air tends to act as an insulator. In fact it is a rather good insulator; the insulation in your house works by trapping air and preventing it from moving and mixing.

The daytime sun will heat all the darker objects on the surface of the earth, such as asphalt, homes, trees, grass, soil, water, etc., to a temperature above that of ambient. In the absence of wind, the still air will act as a trap, allowing the surface temperature to heat above the temperature of the greater air mass. Then, after the sun has set, it will act as a blanket, trapping the retained heat of the day.

The net effect is that in the absence of wind, the net temperature of a location will be higher than that of another location immediately adjacent but which is fully exposed, both daytime and nighttime temperatures. To plants, this means more heating degree units in the daytime, and warmer temperatures at night. Both of these factors, and specifically the nighttime heat retention, have the effect of “raising” the hardiness zone that a plant will experience. This is essentially how a shelterbelt works, and how the urban heat island effect works.

Forests are another example of climatic modifiers. Like buildings and shelterbelts, they will interfere with the movement of air masses near the ground, and thus afford a high degree of shelter. This is especially true underneath the forest canopy, where little if any wind penetrates, which is why some tender plants such as rhododendrons and azaleas grow almost exclusively as understory plants. The trade-off, however, is a serious lack of sunlight in the growing season and competition from tree roots.

Mesoclimates And Topography

There are other factors besides exposure and shelter which can affect a mesoclimate. The most common is the topography or terrain of a region. While a general climatic area of a few square miles or more may exhibit a certain average hardiness zone rating, there may be regions within where elevation modifies both the temperature and the exposure, and thus the “equivalent” hardiness zone rating.

Hilltops are more exposed than the surrounding valleys

Hills and valleys are extreme examples of this effect. Generally speaking, hilltops are more exposed to the wind and climatic effects than the average terrain, while valleys are more protected. As a result, fewer plants can be successfully grown atop hills than in the valleys below. This effect is further exacerbated by the “adiabatic lapse rate”, a fancy term for the way that temperature drops as altitude increases. It explains why no trees grow on the tops of high mountains, and why the snow caps are always at the top.

Having said this, hills and valleys have a second contrary dynamic that comes into play in terms of plants. Because cold air is heavier than warm air and thus settles downwards, valleys tend to form what are known as “frost pockets”, pools where colder air accumulates as it sinks down the hills. For plants susceptible to frosts, particularly fruit trees, it is actually better to plant them along the slopes of the hills as opposed to in the valleys, so that the frosts slip on by and collect in the valleys at night.

Mesoclimate And The Hardiness Zone In Your Yard

So what does all this mean in regards to the particular “hardiness zone” in your yard? Before even attempting to answer such a question, it’s important to recognize that hardiness zone ratings are a regional effect, not a local effect. On the whole, when various reporting stations are used in both urban and rural environments, an averaged “picture” emerges which suggests the overall hardiness of the region. However, it says little about the “hardiness” zone in your yard.

	A favorable microclimate can have a bigger impact on what can grow there

It is “regionally” possible to say an area is zone 4, but that is only a macroscopic statement. In all likelihood, there will be locations within that region which are equivalent to zone 5, and those that are equivalent to zone 3 when it comes to the plants that can be grown there without additional protection. But it very much stretches the capacity of hardiness zones to try and imply a specific hardiness zone on a spot as small as a yard (small, of course, in a climatic context). This is extremely true in a microclimatic sense, but is equally as true on the scale of a city or other mesoclimatic element.

The good news is that you can use mesoclimatic modification to your advantage. If you live on an open prairie, plant a shelterbelt to increase the range of hardy plants you will be able to grow. If you live in a town, you’ve already got a leg up on your rural neighbors. If you live in a big city, you should be able to grow an even wider variety of plants. And if you live in the downtown core, you should be able to grow some rather “exotic” plants, although you might be a little pressed for open soil and clean air in which to do this!

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