When I first read about Hummingbird Summersweet shrubs (Clethra alnifolia 'Hummingbird'), I knew I wanted to find a place for them in my garden. These graceful three-foot mounds bloom in July with tiny flowers packed into sweet-smelling, fluffy cones that attract butterflies and hummingbirds. In autumn, their spare, elegant leaves turn bright yellow. Their open forms don't sucker and need no routine pruning.

They sounded like they would provide a lot of appeal for little effort. All they required was a moist, partly shaded location with somewhat acidic soil.

I planted five baby 'Hummingbirds' under my Jack Pine tree, which lets in a lot of light through its airy canopy and drops needles that keep the ground acidic. When I stopped watering them daily, the shrubs' leaves turned an unhealthy brown-yellow and began to drop.

After spending some time looking at the new shrubs and pondering, I noticed that the pine tree filtered the morning and midday sun, but didn't block any of the intense afternoon sun. Maybe the area was drier and hotter than I'd first thought.

I dug up the five Summersweets and moved them to a shadier area under some Amur Maples on the east side of the house, where they received a few hours of midday sun. When I watered them, all but one perked up, but as I tried to let up on watering, leaves again started falling.

I poked a finger into the soil around the little guys and it came out dry. I did the same test at different times over the next couple of days. Always dry. When I'd planted the maples a few years before, the soil had been perpetually damp. They were supposed to dry out the area to guard our basement from seepage, and I hadn't realized they were doing such a fine job.

I transplanted the four remaining Summersweets to the back of the house. The sun only shone there during the middle of summer, and I suspected the soil was slightly alkaline, but it was the wettest place in the yard, a low spot with thick clay soil.

The shrubs slowly made new leaves and even grew a bit during the next couple of years.

That's how I learned that I didn't know my garden's microclimates, not well enough to choose the right spot for a plant on the first try.

A microclimate is an area of your property with unique environmental characteristics. What makes one microclimate different from another? Three key factors are soil moisture, light, and soil acidity.

• Soil composition (clay, sand, loam): Soil moisture is determined by the proportions of clay, sand, and loam in your soil. Clay holds the most moisture and sand the least, but the air pockets in loam make moisture accessible to plants and make it easier for dry soil to re-absorb moisture.


• Precipitation: Total annual precipitation affects soil moisture, but so does the pattern of precipitation—does it all come during a rainy season? are there periods of drought? is there regular rain at least every week?


• Local water sources: Local sources of moisture can affect your soil. How deep is your water table? Is there groundwater or surface water nearby?


• Drainage pattern of land: Is the site elevated or in a hollow? Does runoff drain into or away from it?


• Competition from surrounding plants: Are the surrounding plants water hogs? Do they have shallow or deep root systems? How much will they compete with new plants for water? Knowing the answers to these questions will help you choose good companions for your existing plants.


• Overhead foliage or obstacles: Foliage can block a significant amount of precipitation. So can overhanging eaves or other structures. These create drier microclimates under them.


• Exposure: Both wind and sun can dry out an area that might otherwise retain moisture. Is the site sheltered by shrubs or lattice, or is it located in a wind tunnel? Does it receive dappled sun or direct sun?

• Direct, intense sunlight all day: From sunrise to sunset, not a shadow marks the area.


• Dappled shade: A pattern of shade and sunlight covers the ground during most of the day.


• Indirect light, but no direct sunlight: A site in full shade can still receive a lot of indirect light from reflections off nearby surfaces. This allows many plants enough light to thrive where they wouldn't if conditions were less open.


• Direct light during part of the day and indirect or dappled light during part of the day, or all three types of light in one day: Sites near buildings or walls are often cut off from the sun until it clears the obstruction, at which time it resumes one of the above patterns.


• Full shade with little indirect light: A site that remains dark most of the time, such as the area under a deck, offers little reflected light and no direct sunlight. Fewer plants can adapt to such conditions.

• Elevation changes the intensity of the sun. Higher ground catches sun first and so will warm up quicker than surrounding areas. This is part of the reason why raised beds give plants a longer season of growth.


• Light exposure can vary tremendously by season. A site can be sunny all summer but shaded all winter if it's located on the north side of a building. Conversely, it can be sunny all winter but shaded all summer if it's under dense deciduous trees such as maples.

Soil acidity is measured on the pH scale. A lower number equals higher acidity, and because the scale is logarithmic, each number is ten times more acidic than the number above it. Neutral pH is 7, and plants vary in their tastes and tolerances. Many traditional garden plants, including lawn, are happiest with a soil pH near 6.5.

Soil acidity is influenced by local sources of acidity (like many evergreens) or alkalinity (such as limestone). Though there are both manufactured and naturally occurring substances that you can use to change your soil's acidity, you can avoid a never-ending battle against your site's personality by choosing plants that will enjoy the soil your site offers.

If you garden for more than a year or two, you may find yourself taking up plants and moving them to areas you think they'll like better. Experimenting with and observing your plants will teach you about your land's unique variations in soil moisture, light, and soil acidity. You can use this knowledge to assemple communities of plants that are well adapted to your garden's microclimates.