QBARS - v23n2 How to Determine Shade Protection

How To Determine Shade Protection
Eldred Green, Chicago, Ill.

Winter shade is a critical factor in growing rhododendrons and other broadleaved evergreens in many parts of the country. Shade from the winter sun is necessary because the bright sun on a cold day can cause thawing of the leaves and stems. When the sun moves on the thawed leaves are exposed to the air temperature which may be way below freezing and the thawed tissues are again frozen. This is the familiar winter burn that may prove fatal.

That the sun is the causative agent can be seen on plants where the north (or shaded) side remains good while the exposed side shows severe injury. While site selection is the best way of avoiding winter injury the question arises as to how much shade a given fence, or hedge, or building will provide. The answer is that the shade cast by a given object can be predicted by using the information on the sun's angles that are used by architects, engineers, navigators, and astronomers. The sun progresses through a regular arc from the low point on December 21 to a high point on the 21st of June. From December to June the sun keeps getting higher and from June to December the direction is reversed. This also accounts for the length of the days as the angle of the sun is interrupted by the curvature of the earth.

How can the amount of shade be determined? Here are some angles for the sun at the summer and winter solstices and the vernal and autumnal equinoxes. For practical purposes the winter shade could be based on the shade cast during the equinoxes, March 21 and September 21. This allows a generous leeway. The angle of the sun is measured at noon and is given as degrees above the horizon. While most gardeners cannot be expected to have a sextant for measuring this angle, they can use the angles that have been computed and then construct a scale model to provide the desired information.

Region Dec. 21 March 21 June 21
& Sept. 21
New England, New York, upper Mich., 22 ° 45° 69 °
Minn., N. & S. Dak., Mont.
Pa., Ohio, W. Va., Ind., Ill., 25° 50° 73°
Ia., Mo., Neb., Kans., Col.
N. & S. Car., Ga.. Ark., Okla., 31° 56° 77°
N. Mex., Tenn., Ariz.

To determine the amount of shade cast by an object, like a fence, draw a line representing the ground. Use a protractor; any school supply store will have these for a dime, and mark off the angle for March and Sept. In the Chicago region this would be 50°. Now draw a line from the ground line to the angling line. If you have a four foot fence measure up from the ground line to the angling line so that the distance is some convenient multiple of four, like four inches, or four and one-half inches. Now measure from the upright line to the place where the angling line meets the ground line. This will be the shadow. Use the same scale as you did in measuring the upright line. This will be the distance.

Shadow line cast by a four ft. fence
Fig. 30.  Shadow line cast by a four ft. fence.

The diagram shows the angle for this worked out on a basis of 1 inch per foot. By marking out a grid design you can determine the safe spot for any plant during the winter. If you use the angle for June you will get the area that is constantly in complete shade. The angle for December will give you the angle of maximum winter shade. Remember the shade goes from left to right during the fall and winter months and from right to left as spring comes along.

Thus a four foot fence casts a 3½ foot shadow at March 21 in Chicago. Note that the shadow is only six inches high three feet from the fence. It is four feet high at the fence. Measure the plant you want to protect. Mark it on the diagram using the same scale, say 1 inch to the foot. Any part of the plant to the left of the sloping line would be in sun and likely to burn. Everything to the right would be safe.