Microclimates

Shade and Sunlight Exposure. Houses, walls, fences and other plants can shade the garden. Shaded areas receive less sunlight and tend to be cooler and stay wetter than unshaded areas. The amount and extent of the shade is seasonal. Shadows are longer in winter (with the noon sun at about 30 degrees up) and shorter in summer (with the noon sun at about 75 degrees up). Note: Winter days are also shorter (as little as 10 hours in San Diego County) while summer days are longer (as much as 14 hours in San Diego County). Even under full direct sunlight, some variation in sunlight intensity may occur due to angle-of-attack or reflection effects. South or southwest facing slopes receive more intense sunlight than north or northeast facing slopes. East-west running walls and fences reflect more sunlight towards their south sides while creating shadier regions on their north sides. Similarly, but to a lesser extent, north-south running walls and fences create warmer regions on their west sides and cooler regions to their east. The material the wall is made of (and painted with) can also moderate or enhance the amount of sunlight and heat that is reflected back into the garden.

Night Exposure. At night, the garden loses heat. Much of this loss is radiated upward into the atmosphere (or more accurately, toward the heat sink of space). Clouds and humidity tend to insulate the ground from this type of heat loss (the reason why desert nights are often extremely cold). In the garden, any overhead protection will insulate the area directly underneath (the reason why it's warmer at night under a tree than out in the open). It's this type of heat loss that usually determines which areas in the garden are most (or first) susceptible to frost. One way to find the frost susceptible areas is to check the garden on a cold morning (around 32 degrees F) just after dawn to see the frost pattern. Tender plants should not be planted in frost-susceptible areas.

Cold-Air Basins. Cold air can be collected and trapped by a dip or basin in the garden or by man-made barriers like houses, walls, or fences. Sloped gardens tend to be warmer (and less susceptible to frost) than their immediate surroundings since cold air is easily drained off.

Heat Sinks. The ocean is a powerful climate modifier. Even relatively small water deposits like backyard pools and ponds can create a garden microclimate that is cooler during the day and warmer at night than the rest of the garden. To a lesser extent, some garden hardscape materials which retain heat better than others have similar moderating influences.

Wind. In general, Southern California experiences westerly air currents during the day and easterly air currents at night. The wind tends to be greater near the coast and more pronounced at the base of canyons. However, the local terrain topology can produce all sorts of unusual (and sometimes severe) wind patterns. In some cases, structures and plantings can be used to control undesirable wind effects.