Sunday, November 18, 2012
The photo above is of an "urban heat sink" I walk by often at work. It is also called a microclimate. Heat sinks can be anywhere, but what are they and what causes them? First off, this heat sink often has blooming summer flowers in December (I am a teacher and don't work over winter break, so don't know how long into December actually lasts). The same flowers and plants in my garden, 8 miles north, have been zapped by late October or early November, when this urban garden is still going strong. Now, probably really tender annuals, like basil, might not survive even in this microclimate, but others do (see the petunias spilling out on the low wall?)
A heat sink is caused by the angle of the sun (the heat sink area usually needs as direct sunlight as it can get in winter) and hardscaping that soaks in heat during the day and radiates it at night. As you can see by this photo, there are plenty of heat soaking materials that surround this garden: asphalt roads, brick sidewalks, brick house, low concrete wall, and even the black metal fencing retains heat. This seems to me to be a super heat sink!
Are there heat sinks in your yard? First, look where the sun hits in winter. This area is prime for a heat sink. Brick, stone or concrete block walls (especially the latter painted in a dark color, not white) can create microclimates where you can even grow plants that are usually too cold intolerant for your USDA growing zone. Brick walks, structures, dark stones, can also do the trick.
Of course, you can also have heat sinks in summer, and this can cause colder-zone plants to fry in the hot summer sun. Again, look for where full summer sun hits and avoid planting more heat sensitive and shade loving plants there. Morning sun is weaker than afternoon sun, so this can also create a microclime in your yard.
Can you create a heat sink? Yes, if you are building a substantial wall or structure in an area that gets late winter sun, use brick, create a heat sink, and experiment with tender plants!
Happy gardening! And have a joyful Thanksgiving!