How can so many colorful annuals and perennials be so surprisingly happy in the many large pots on the sidewalks in front of downtown shops? Downtown does not seem like it should be so comfortable to them, with all the concrete, glass, stucco and all sorts of other reflective surfaces to enhance the harsh exposure. Yet, the buildings and street trees instead provide a bit of shade to soften exposure and hold in a bit of ambient humidity. Some areas are even too dark for many plants.
Exposure is as important to selection of appropriate plants as climate and soil (or potting media) are. Plants that need full sun exposure will not perform like they should in partial shade. Those that want partial shade may get roasted by too much sunlight.
Walls, fences, and windows to the north can enhance the harshness of exposure by reflecting glare. To the south, they can conversely limit exposure with shade. Such structures to the west increase morning warmth but provide shelter from afternoon heat. To the east, these features are not so advantageous, since they shade morning sun, but enhance afternoon heat. Pavement adds to glare without providing any shade.
As if this is not confusing enough, there are many more factors to consider. Coarsely textured or darkly colored surfaces do not reflect glare as severely as smooth or lightly colored surfaces do. Broad eaves, like those that adorn classic ranch style architecture, not only add more shade, but also limit reflected glare from walls below. Taller buildings, like two story Victorian houses, create more shade than single story Spanish colonial houses with flat roofs. Even shade trees are variable, ranging from the dark evergreen shade of Southern magnolia, to the light deciduous (absent through winter) shade of silk tree.
Most plants do well within a significant range of exposure, between full sun and partial shade. A few, like agaves, yuccas, oleanders and most eucalyptus, are resilient to harsh exposure, but would not survive significant shade, and would look tired in even partial shade. Even fewer, like ferns, fuchsias, impatiens and Japanese aralia, tolerate darker shade (although none survive in complete shade), but would get roasted if too exposed. Shade, reflected glare, and anything that might affect exposure must be considered in the selection of appropriate plants for each particular application.
Modern urban lifestyles are becoming less conducive to gardening all the time. Bigger and taller homes cast larger shadows over smaller garden spaces. The taller fences between these homes do not help. The densely evergreen trees employed to obscure the views of other larger and taller homes also obscure sunlight. Not much sunlight reaches the ground where shorter plants need it.
Not many plants are sensitive to mere heat alone. Actually, many plants prefer warm weather. The difficulty that some plants have with heat locally is that it typically accompanies aridity, and often accompanies afternoon breezes. As appealing as breezes and minimal humidity are to us while the weather is warm, they promote and accelerate desiccation of exposed sensitive foliage.
Tony Tomeo 