You May Have More Zones Than You Think

Some gardeners suffer from a psychosis known as zone envy. This malady is manifest in a gardener who lives in one climate zone and envies, even lusts, for the plants that can be grown in another zone that is either cooler or warmer. Many of these affected gardeners have been know to take great risks in pushing the hardiness envelope.

Locally, you may recognize a few of them by their cleverly wrapped fig trees or marginally hardy rose varieties. In southern New Jersey, they fiddle with citrus, and there is even a group in Canada that is so severely affected they are attempting to grow bananas in Toronto. Many got some very rude surprises after last winter.I will openly admit that I am likewise infected. I have tried for years to grow any number of plants in Southampton and in Westchester that normally won’t grow here and I’ve come so very, very close, but as the saying goes ... no banana. There is something of a cure, though. It’s not always effective and it’s more suited to trial and error than scientific proof. It involves the use of what gardeners have come to know as microclimates.

A microclimate is a portion of the landscape that differs significantly from the environmental conditions of the yard as a whole. Do you have a “problem area” where things just don’t seem to grow well? This may be due to the microclimate in that particular spot. For example, the corner by the sidewalk and driveway where it’s always hot and dry in the summer and the kids cut the corner so the site is compacted, is an example of a microclimate. Rather than fighting the tough conditions for growing the plants there, you’d be better off choosing plant materials adapted to that condition.

But here’s another microclimate example. We have several areas at work where wisteria is grown and for the most part they are all the same variety and the same age and yet they flower weeks apart. The first to flower are on the south side of a house. The house is painted white, but the base of the plants is surrounded by 2-inch-thick flagstone. The combination of the reflected light and the heat absorbed and retained by the stone combined with the root system that is right up against the house foundation has created a microclimate in which the soil warms early in the season and the surrounding air warms early as well.

The result is wisteria that blooms almost three weeks earlier than similar plants just 1,000 feet away that are growing in a lower, colder area. Have you ever noticed that the crocus planted close to your house on the south side will flower weeks earlier than those planted away from the house? Microclimate.

Another microclimate situation is a home that has a large roof overhang on the south side, which keeps the soil near the house hot and dry because of the sun beating on it and the rain being shed away. In this microclimate a wise gardener would do well to consider planting succulents and some of the gray foliage plants that can withstand sun and need little water.

You may have a low spot in your landscape that accumulates water. Rather than fill it in, learn to deal with this microclimate and consider sedges, Siberian iris, astilbes (they don’t need shade if given plenty of moisture) and even monardas, all of which will thrive in the wetness and sunlight.

Is there a zone 8 plant that you’ve wanted to grow for years but have been chicken to plant because you know we’re in zone 7? Do you have a south-facing wall that blocks the winter wind and absorbs the southerly sunlight? This may be your own little zone 8. Is there an alpine plant you’ve always wanted to try but you’re afraid that our warm and humid summers would do it in? How about on the north side of your house that’s shaded most of the day and yet still gets bright light and stays cool for all but a few of the doggiest days of summer?

In essence there are four conditions that create microclimates in your landscape: temperature, patterns of light, humidity distribution and air circulation. First let’s look at the temperature. When you read a thermometer and it says 90 degrees, all that means is that it’s 90 degrees at that particular location. Move around the yard and you’ll find variations of as much as 10 degrees in either direction. The soil temperature is probably 10 degrees cooler, and shadier spots with open-air circulation may be that cool as well. Near heat sinks like driveways, stone walls and the south side of the house, the temperature may actually rise 10 degrees, and remember that higher spots are generally colder than lower spots, and also that marginal plants planted on a slope are less likely to have frost damage, as frost is more likely to form on flat areas. Rule of thumb ... the more tender the plant, the higher it needs to be planted and the more sun and reflected heat it needs.

The temperature and thus the microclimate is also affected by the amount of sunlight an area receives. Areas that consistently get little or no sun tend to be cooler to colder than those that receive a great deal of sun. Remember that many factors can affect the amount of sun an area receives, including houses, walls, fences, other plants and even where you park your car.

Remember also that shady areas can also be wetter areas, as the sun doesn’t hit them and accelerate evaporation. Consider also that in the winter shadows are longer and the days are shorter, with the sun about 30 to 40 degrees lower than at this time of the year. Remember also that walls that run to the east or west reflect heat and light toward their south sides, which means they create shade on their north side. The side facing the light is the warmer side and yet both sides are ... you got it ... microclimates, with the year-round average temperature on the south side running about 10 degrees warmer than on the north side.

Water (and thus humidity) also affects the amount of heat or cold in an area. Just consider Long Island our own giant microclimate. The surrounding water keeps our summers cooler and our winters warmer than similar areas of the same latitude and elevation in Westchester and Connecticut. In fact there can even be a substantial summer and winter temperature variation of 20 degrees between Riverhead and Southampton simply due to the fact that Riverhead is surrounded by land mass and Southampton is surrounded by water. But even a small pond or a swimming pool can affect the temperature of a portion of the garden to some degree. A pond, lake or even an inlet or bay sends moisture into the air. The water vapor creates something like a miniature greenhouse effect as it traps the infrared radiation reflected from the earth. So, air that is near your pond or pool is trapping heat and the plants in this area are reacting not only to the exaggerated moisture but the added heat retention.

Lastly, air circulation. Remember that hot air rises and that the reverse is also true, cold air sinks. Look at your landscape in the late fall and see where the frost forms. These areas are most likely to be depressions in the landscape, which is going back to an earlier premise that plants that need heat belong on higher ground than those that prefer cold. Along with this is a consideration of wind. Plants that are sheltered from winter winds survive better than those that are right out there getting the brunt of the breeze. For this reason any plants that you think are tender to marginal need as much wind protection as possible.

If you are using a solid windbreak, though, remember that the velocity of the wind, and thus the cold, can be accelerated downwind at a distance equal to the height of the windbreak. Translation: A northerly wind hitting a 6-foot-high wall can be as bad or worse 6 feet south of the wall as it is on the north side of the wall.

Lots to think about? Well, if anyone ever told you that gardening was easy, you probably aren’t talking to them anymore. But if you’re smitten I know getting to know your microclimates will be a big step and of course it will help you keep growing.