Looking ahead to hot summer days, many gardeners are using these early days of spring to plant energy-saving shade trees. This is a good idea, because as everyone knows, shading east and west walls, rooftops, and large paved areas is essential to saving energy and staying cool. But take care that your new tree does not block desirable sunlight in winter.

During winter months, the sun’s course is relatively shorter and lower in the sky, rising south of due east and setting south of due west. Useful winter sunlight falls mainly on the south wall and south-facing roofs of houses in California. South-facing windows passively heat the interior of your house. Any tree that shades the south walls, windows and roofs of the your house in winter inhibits energy conservation, so you need to choose your trees carefully. Of course, evergreen trees are not good choices , since they shade your house year-round. Research shows that, contrary to popular belief, even some deciduous trees may block a lot of sunlight. The best choice for the south side of your house is a “solar-friendly” tree.

A solar-friendly tree has an open or thin branch density, loses its leaves early in fall, has no seed pods or catkins in winter and leafs out later in spring. It’s not true that all deciduous trees are transparent to sunlight in winter. The branches of the average, mature deciduous tree is about 40 percent dense and, depending upon light conditions, can block up to 40 percent of incoming sunlight. Also, many deciduous trees do not lose their leaves until December, well into the home heating season, and regain their leaves in early spring when you still need heat. White alder, for example, tends to lose leaves late in the season and blocks sunlight with an early and heavy set of catkins.

If you intend to plant a deciduous tree that will block sun to your south windows and walls in summer, consult your local nursery professional to help you choose the best solar-friendly tree. Good examples include certain ash specie s, Chinese hackberry, European hackberry, Chinese pistache, and sawleaf zelkova.

If a tree near your house is blocking winter sun to your south wall, you may be able to prune it to increase its “solar friendliness.” Thin the tree to remove branches without harming its overall form. If the tree is very large, you may need to contact a professional arborist to have the work expertly done. Keep in mind that the tree is to be “thinned”, not “topped.” Properly thinned, your tree will allow more sunlight to reach your house, but the tree will still look natural. Topping destroys a tree’s natural form, and forces the growth of vigorous, weakly attached upright sprouts that are subject to breakage. In a few years the topped tree will produce denser shade than the original tree, and will require expensive corrective pruning.