KEEPING WARMTH AT BAY

MODERATING WINTER’S COLD

I’m banking, for instance, on the slightly warmer temperatures near the wall of my house to get my stewartia tree, which is borderline cold-hardy around here, through my winters, when temperatures often drop to minus 20 degrees Fahrenheit.

Proximity to concrete, stone or any other material that retains the sun’s heat also provides a slightly warmer microclimate. I’m expecting spring to arrive early, with a colorful blaze of tulips, in the bed pressed up against the south side of my brick house.

Even sooner, in late winter, winter aconites will be spreading their small, yellow blossoms in the slightly raised bed surrounding my terrace. Those aconites warm more quickly thanks to the double benefit of raised beds and a concrete wall surrounding them. (Actually, the benefit is more for me, getting to enjoy their blooms extra early.)

Microclimates can also be useful for keeping plants cooler. By training my hardy kiwi vines right up against the shaded, north sides of their hefty supports, I keep direct winter sunlight from warming their trunks. This avoids the splitting that occurs when their trunks are alternately warmed during winter days, then rapidly cooled as the winter sun drops below the horizon.

Diluted white latex paint on the trunks of young trees keeps them cold through winter days and nights to likewise prevent damage.

By planting the coveted blue poppy in a bed on the east side of my house, I hoped to give the plant the summer coolness it demands. Even that east bed was still too sultry, however; the plants collapsed.

Microclimates are important when growing fruiting plants that blossom early in the season because frozen blossoms do not go on to become fruits. Earlyseason bloomers need microclimates that are slow to warm up. The north side of a house or other building stays cool because it’s shaded from winter sun.

SLOPES PLAY A ROLE, TOO

South-facing slopes stare directly at the sun, so they warm up early in spring and are warmer both in summer and winter. Therefore, a south-facing slope can be used to hasten fruit ripening on a plant like persimmon, whose late blossoms are rarely threatened by frost but which does need a long season to ripen when grown near its northern limit.

Sunlight glances off north slopes, delaying their warming in spring and keeping them cool in summer. Such a microclimate is ideal for an early-blooming fruit tree like apricot or peach, and for plants such as sweet peas that enjoy cool summer weather.

If a slope has some elevation, the air is going to cool it by about 1 degree Fahrenheit for every 200 feet of elevation. Avoid planting at the very top, though, because it will be windy.

Oddly enough, valleys might also provide a cooler microclimate. On clear, windless nights, heat that the ground absorbed during the day is re-radiated back to the sky. Cold air, then at ground level, is denser than warmer air, so if there is any slope, the cold air runs downhill as water would. The cold air collects in valleys.

You might have noticed a temperature change even from slight differences in elevation as you ride a bicycle or drive a car on a clear summer night.

It’s a bit chilly this morning (55 degrees Fahrenheit.) I’m going to find a warmer microclimate in which to sit.