The MC can make a huge diff (1-1.5 zone diff). When it is very cold, u can be 2-5 degrees higher. I have an apricot tree by an huge water tank. Half of it froze, the rest didnt freeze. We had a very big blizzard here in Feb that killed apricot flowers before they developed. Again the small microclimate provided by my tank helped immensely. Trees can also shade from frost via late frosts.
Microclimates and how they affect plants
by Linda Strader on Jan. 27, 2011, under Desert Landscaping, Gardening problems, Starting a garden, Winter gardening
When either choosing a place to start a garden, or trying to select landscape plants for your home in the desert, understanding how microclimates work is essential. Have you ever noticed how one part of your yard is colder than another, or that a certain plant thrives in one spot of your yard, but not a few feet away? Chances are pretty good that a microclimate is influencing that plant.
Microclimates vary for every single home. They depend on a number of environmental factors including sun orientation, overall elevation, low and high spots in your yard, the proximity of drainages and washes, type of ground cover around the home (rock or plant materials) and the proximity of plants to the home, patio walls, or large boulders. Other factors include plantings underneath trees (which also vary depending on whether the tree is deciduous or evergreen) and roof overhangs.
Sun orientation affects plants not only by the amount of sunlight they receive during the day, but which orientation of sun they are receiving. This is especially important in the desert, as the west sun is much more brutal to plants that the east morning sun, and reflected heat (such as radiating off a west facing wall) can make the west sun deadly. North facing walls usually receive hot sun in the middle of the summer, but no sun in the winter, making it one of the toughest place to grow plants.
The overall elevation of your home, if you live within the city limits, probably only varies one hundred feet or so, but those low pockets in your yard can get several degrees colder in the winter since cold air flows like water-downhill. To check just how much colder, setting out several thermometers on a cold night in different areas of your yard will give you a good indication of just how much colder these areas can get; for some cold sensitive plants, this might be a problem.
If your home is adjacent to a drainage or wash, you will be much colder than homes just 100 feet away. Cold air follows drainages and it spills over into the surrounding landscape. Materials such as decomposed granite, boulders, patio walls and home walls also create microclimates. Inert materials collect heat during the day and radiate it to the atmosphere at night. Plants adjacent to such collectors will benefit from the warmer nights in the winter, but if the sun does not shine those areas will be much colder. The same holds true for summer, but in the sense that those areas will stay warmer overnight. Ground cover consisting of plants instead of gravel will be cooler in the summer. Microclimates are also found underneath tree canopies. Evergreen trees provide year-round summer cooling and winter protection for plants underneath. The eves of your house also can do the same, provide they extend far enough out to provide shade in the summer, when the sun is almost directly overhead.
Knowing how microclimates affect your yard is an important consideration when deciding on everything from planting vegetables to where the best place is to install a patio. If you are a new home owner and your yard is a blank slate, it really pays to wait before planting anything. If you can, wait a full year so you can document how the seasons and microclimates impact your yard. That way you will be able to make better decisions and avoid costly changes.