Why does it matter what time of day is hottest?

Tom Skilling answered a question Sunday from a gardener wondering what time of day is hottest. I thought, "Oh, that's a good question." And then I thought, "But why? What difference does it make?"

I guess it might make a difference if your garden is out under the open sky. Then it might be useful to know that the most intense sunlight is at about 1 p.m. and the highest temperatures are usually about 4 p.m.

But my garden is surrounded with obstructions that affect the way light and heat arrive at it. My main garden is bounded on the north by a 2-story building and a 6-foot-tall board fence and on the south by a 4-story building, with a row of trees overhead and, to the east, another 4-story building across the alley.

It gets the most light in the springtime, before the trees leaf out and when the afternoon sun is shining from a relatively low angle. In summer, there are shafts of light that reach through the tree canopy; I place plants practically to the inch to eke out a few photons from those laserlike beams.

At the far west end, with fewer trees, there's an area with enough sun to support a little patch of lawn and some part-shade plants, but everywhere else is full shade. The world at large may be hottest at 4 p.m., but when you step into our garden it's always going to be 10 or 15 degrees cooler than out in the alley.

Even in the back, the skinny patch of ground by the garbage dumpster that I like to think of as my "full sun garden" is not really in full sun. To the west and north there's that 4-story building, and there's another 2-story building to the south. That bed gets more sun than the front, but as I have come to comprehend this year, the light is limited. By 4 p.m., the sun is behind a building.

On the other hand, that bed is flanked by a parking pad, a sidewalk and a brick wall, so all the heat that was stored in those masses of brick and concrete all through the sunny day is being radiated back at my plants until late at night. It's a much more brutal environment than if the bed were flanked by lawns.

So what's the bottom line? Well, don't water in the middle of day, when the sun is most intense and much of the water will evaporate. That's true anywhere.

But apart from that, generalizations are not very helpful. Observation is all. To make smart choices of plants and design, you need to get out there and experience your garden at various times of day and various times of year. Don't just look at it; feel it. Imagine you are a platycodon in my blast-furnace rear bed. Then you'll be able to imagine why it died.

Got a garden question? I recommend you call or e-mail the Plant Clinic of The Morton Arboretum in Lisle, the Master Gardeners of the University of Illinois Extension or the Plant Information Service of the Chicago Botanic Garden in Glencoe.

All contents of this post are copyright Beth Botts. Feel free to link or share a brief excerpt with a link, but please do not reproduce photos or any other part of this blog without my express permission.