You know the saying that moss always grows on the north side of trees? That’s because the north-facing side of tree trunks is usually slightly cooler, shadier, and more moist than their other sides. Moss likes cool, shady, moist spots, so it often grows on the north-facing sides of trees — at least in North America.
Such mossy spots exist because of differences in microclimate. A microclimate is the climate of a small area that differs from its surrounding area. It may be hotter, cooler, wetter, windier — but something makes that spot slightly different from its surroundings.
In the southeastern Piedmont, north-facing steep slopes adjacent to creeks and rivers create microclimates that favor some of my favorite plants: Beech trees, spring ephemeral wildflowers like Hepatica and Blood Root, and Pawpaw trees to name a few.
Even the smallest suburban yard has microclimates. The south side of your house differs from the north side. A hilltop will be drier and windier than the bottom of a hill.
If you pay attention, soon you’ll know which parts of your yard get frost soonest, where the snow melts first — or last. These are clues to microclimates in your yard.
One of the most striking microclimates in my yard is the south-facing wall of my garage. Even during a record 20-inch snowfall, less than an inch piled up within two feet of that wall. The birds figured it out, huddling together for warmth on the one spot of remaining bare ground. I think it may be a full zone higher there. I’m mostly 7B, but I’m pretty sure I could safely grow Zone 8 plants behind my garage. I’ll get around to trying eventually.
Last week, two Winterhazel (Corylopsis sp.) shrubs growing within 20 feet of each other reminded me of the importance of microclimate differences. I wrote about these non-native early spring bloomers here. The one in the photo at the top of this entry shows off its lovely yellow autumn color. Probably because it’s not native, it is slower to color up in fall, which makes it more susceptible to early freezes.
The Winterhazel in the above photo grows a bit uphill from an identical shrub that I planted just above a little pond near our creek. Both shrubs are the same size. The one in the top photo often blooms a few days before the other, but I hadn’t realized the significance of that difference until last week when our low hit 24 degrees Fahrenheit.
The shrub above was unaffected by the cold. But its sister shrub by the pond, which had only just begun to show fall color, was zapped hard. The leaves died instantly, turning pale and falling to the ground. Here’s a photo of the zapped shrub shot the same day as the one above. You may need to click on it to see the few dead leaves still clinging to the branches.
Although unsightly, the zapped Winterhazel is not damaged. It will still bloom next spring. But the striking difference between the appearance of these identical shrubs caused me to take a hard look at their microclimates.
The still-beautiful Winterhazel is a bit higher up the hill, which may mean the cold air doesn’t collect on top of it quite as quickly. But I think the big difference is my house. The pretty Winterhazel is completely protected from north winds by my house. The zapped Winterhazel is not only lower down the hill, it is just far enough to be out of the wind shadow of my home. North winds have a direct line from the north side of my yard, past my house, to the south-facing floodplain where the zapped shrub resides.
The difference between the two microclimates was probably just a degree or two, but combined with a biting north wind, the zapped shrub surrendered to the cold.
These two shrubs reminded me of the importance of paying attention to microclimates whenever I’m planting new additions or relocating established plants. As in so much of life, the smallest differences can have the largest impacts.
For an excellent discussion of microclimates, check out this site.