Strange things are always occurring in the garden. Or we think they are strange at first until we apply a little bit of logic to the situation! Over Thanksgiving I was visiting my in-laws. When we pulled up into the driveway I noticed something right away…the irises were blooming! Here in Tennessee we’ve had several hard frosts at this point already and even though these are re-blooming irises they should have gone dormant by now. The reason they did not go to sleep for the winter yet is because of the micro-climate that was created around them.
A micro-climate is a smaller area of a garden where the climate is different from the rest of the region. A number of factors can create a micro-climate including elevation, amount of sun/shade, wind exposure, and rocky material (these are just a few possible factors of many). The two factors that have effected the irises are the sun exposure and the proximity to rocky material. Stone and rock are heat sinks which means in this situation that they absorb heat and release it at night. The driveway and sidewalk, where the irises are, create a heat zone in the direct sun. As the days are sunny they warm up the stonework area then release the absorbed heat through the night which raises the nearby temperatures giving the irises a micro-climate to continue their blooming.
Gardeners can use the micro-climate effect to their advantage. A heat sink placed in the vegetable garden will help plants continue to grow longer into the fall and winter. It’s a good way to keep those greens actively growing for a few more weeks in fall or into winter. Paving stones, bricks, gravel, natural rock and stone, and concrete blocks can all be used to either make stone walkways or raised beds that can be heat sinks.
Wind can be a major factor in the garden. There are air currents in my yard that can cause the temperature to be significantly different just by moving a few feet away. Windbreaks can be created by using evergreen trees, fences planted with vine plants, trellises, or walls can all alter the flow of the air and change micro-climates. Deciduous trees can alter micro-climates by adding 3 seasons of shade or by breaking wind (go ahead you can laugh, trees do break wind).
Experienced gardeners will often try to do what we call “Zone Cheating”. This is where we challenge a plant to thrive in a heat zone that is at least one zone removed from where all the books say it should be! Gardeners will take a zone 6 plant and grow it in a zone 5. Or a zone 8 and grow it in a 7. Sometimes the plants do fine without help but by adding in a few micro-climate factors like stone work or wind breaks the gardener can improve the chance of success. (Here’s the most recent U.S. Plant Hardiness Zone Map)
So they next time you think there’s something strange going on in the garden examine the situation and use that micro-climate to your advantage!