Missouri Conservationist online

Mar. 1998 - Vol. 59 No. 3

Warm Cows & Cool Breezes

by Bruce Palmer

Too hot? Too cold? Natural windbreaks provide four-season relief.

Trees are a natural choice for moderating the extremes of our environment. They shade our homes and playgrounds, block cold winds, filter dust and pollutants from the air and make our cities nicer places to live.

Trees have been called a low-tech solution to a high-tech problem. The high-tech problem is how to reduce our nation's growing demand for energy. Americans use more and pay less for energy than any other country in the world. The simple act of planting a tree can help reduce our energy demand.

Trees can save energy in several ways: through shade, reducing the need for air conditioning; as a windbreak, lowering heating costs, and by serving as a renewable source of fuel. Foresters can document energy savings of 10 to 40 percent in homes with strategically placed trees. Whether you live in the city or on a farm, planting a few well-placed trees can reduce your energy bill.

Keeping the Cows Warm

Windbreaks usually come to mind when someone mentions planting trees for energy conservation. Landowners have planted windbreaks for years to protect homes and farmsteads from winter winds in the Great Plains and Upper Midwest. Windbreaks also protect livestock, provide wildlife habitat, shelter crop fields and screen feedlots and traffic.

A 35' windbreak will slow 35 mph open wind speeds to 10 mph at 100' and 15 mph at 200'. Buildings should be no closer than 100' to windbreak.

Trees make effective windbreaks. Their leaves and branches absorb some of the wind's energy and deflect the wind up and over a protected area. This reduces the wind velocity or wind chill effect on the downwind-or leeward-side of the windbreak. The trees' height, density and orientation determine the effectiveness of a windbreak.

Height

Windbreak height is the most important factor determining the downwind area protected by the windbreak. On the windward side of a windbreak, wind speeds are reduced upwind for a distance of two to three times the height of the windbreak. On the leeward side (the side away from the wind), the wind speed is reduced as far as 20 times the height. For example, if the tallest trees in a windbreak are 50 feet, wind speeds are reduced up to 150 feet on the windward side and 1000 feet on the leeward side.

Density

Windbreak density is the solid portion of the barrier. The number of rows, the distance between trees and species composition affect windbreak density. By adjusting windbreak density, different wind flow patterns and areas of protection are established. Windbreak densities of 25 to 35 percent are most effective for an even distribution of snow across a crop field. A density of 40 to 60 percent provides the greatest area of protection and excellent soil erosion control. High densities of 60 to 80 percent are best for protecting farmsteads and livestock areas. Surprisingly, windbreaks offer greater wind speed reductions than a solid fence.

Orientation

Windbreaks are most effective when oriented at right angles to the prevailing winds. The purpose of the windbreak will determine how it is designed. Farmsteads and feedlots typically need protection from cold winds and snow, which usually blow from the northwest in Missouri. A windbreak to protect livestock would be planted north and west of the feedlot. However, a crop field needing protection from hot summer winds would require the windbreak on the south and west, the direction of prevailing winds in summer.

Windbreaks may increase crop yields up to 20 percent. Their shelter slows hot summer winds, reducing burning and wilting of plants. Soil moisture is conserved, so the need to irrigate is decreased. With the slower winds, wind erosion is reduced, keeping the soil on your land.

A well-designed windbreak also can result in direct energy savings of 10 to 40 percent by reducing the loss of heat from homes and barns. More savings can result from livestock and crop field protection.

In the case of beef cattle, their heavy winter coat will provide protection against temperatures as low as 18 degrees. At temperatures below 18 degrees, an animal becomes stressed and requires additional feed to maintain its body temperature. With an air temperature of 0 degrees and a wind of 25 miles per hour, the wind chill is -44 degrees. The cow now needs 40 percent more feed to maintain itself, is less efficient at converting this feed into energy and is more susceptible to health problems. Although a windbreak can't raise the air temperature, it can cut the wind chill effect, resulting in warmer cows.

Windbreak Basics

Planning Your Windbreaks

Purpose	Minimum # of Rows	Fence	Tree Type & Configuration
Farmstead	2	optional
Feedlot	2	necessary
High Traffic screen	6	optional
Medium-low Traffic Screen	3	optional
Visual Screen	2	optional
Alternative version of Visual Screen	3	optional
Wildlife	5	optional

Key:

best species

option

evergreen

deciduous

• Do not place windbreaks under utility lines or use species whose mature height will interfere with the lines.
• When placing an opening through a windbreak, make openings at an angle to reduce loss of wind protection.
• Use fences to keep livestock from windbreaks.
• The most effective area of protection is two to five times the height of the tallest trees. Buildings and feedlots should be within this zone.
• Buildings should be no closer than 100 feet to a windbreak.

Choosing Your Species

For windbreaks with fewer rows:

• Two-row windbreak: use species from rows 2-3 or 1-3 or 1-4
• Three-row windbreak: use species from rows 1-2-3 or 2-3-4
• Four-row windbreak: use species from rows 1-2-3-4 or 2-3-4-5 or 1-2-3-5

Planting Your Windbreak

Stagger tree spacing so the trees in one row will be planted opposite the opening in the other row
Use these spacings within the rows:	Shrubs Evergreen Trees Deciduous Trees
Use these spacings between the rows:	Shrubs Shrubs & Trees Trees

Strategic Shade

Although city dwellers usually don't have the space to plant large windbreaks, there are still opportunities to reduce their energy consumption. The placement of trees around a home is critical to take advantage of summer shade while not blocking winter solar heat. When planning where to plant trees, remember that the sun's position in the sky changes hourly and daily. Plan for shadows that cover targeted areas during the hottest hours of the hottest weeks of summer.

Deciduous trees that provide maximum summer shade and minimum winter shade are ideal for reducing energy consumption, but they must be located properly for best year-round results.

Shade west and east windows

In Missouri, the midpoint of our maximum air conditioning use is July 25. Deciduous trees planted to shade the east and southwest to northwest sides of the house help reduce air conditioning usage. If there is space to plant only one or two trees, they should be planted on a line 75 degrees from due south. This placement provides the maximum shading during the hottest time of summer at this latitude. Select trees so that near maturity, their limbs reach within 5 feet of the walls. Be careful of planting too close-roots can damage foundations, and limbs may fall on the roof.

Avoid trees south of windows

From an energy savings perspective, the worst place to have a tree is directly south of your home. In summer when the sun is high in the sky, the shadow of the tree falls directly under the tree and doesn't shade the house at all. However, in winter the shadow of the same tree will fall on the house throughout most of the day. If you want to plant a tree on the south side of your house, it should be located at least twice its mature height away from the house.

Shade air conditioners, parking places and paved areas

Plant tall shrubs, trees or a trellis with vines to shade your air conditioner during the summer. An air conditioner runs more efficiently if it is in a cooler environment, reducing your operating cost. Likewise, less air conditioning is necessary to cool your car if it was parked in the shade. The air heats up faster over paved areas such as driveways and patios. Therefore, locate paved areas and air conditioners away from south windows and shade them with trees.

Use shrubs to shade walls

Some building materials, particularly brick and concrete, stay hot well after sunset, causing a delayed impact on air conditioners. To help shade the walls, plant tall shrubs within 4 feet of east, south and west walls. The inside edge of the shrub should reach within 1 foot of the wall after four years.

Plant a windbreak

A home on a lot of one-quarter acre should have room for a windbreak along the north and/or west side. The same principles apply as in farm windbreaks, but less space means fewer rows of trees that may be planted closer to the house. A single row of medium-sized evergreens planted close together offers protection from winter winds.

Giving evergreens plenty of sun is important so the lower branches are not shaded out.

Use solar friendly trees

For the greatest benefit, a shade tree should have a broad crown of foliage during the hot summer months. It should lose its leaves before the furnace comes on in the fall, and in winter its branches should be sparse. The amount of sun blocked by a deciduous tree in summer ranges from 60 to 90 percent. A tree's bare branches and twigs usually block 30 to 50 percent of the sun-a significant reduction of free solar energy over the winter.

The most solar friendly species have dense foliage and an open winter form. Trees with compound leaves, such as Kentucky coffeetree, ash, walnut and honeylocust, have moderately dense summer shade with sparse winter branching. Other species, such as sugar and red maple, have dense summer shade and moderately open winter branching.

For more information on selecting the right trees for your situation, consult Missouri Conservation Trees and Shrubs and Missouri Urban Trees. Both books contain tree planting tips, site requirements and other helpful information. The books are available free of charge from your nearest Conservation Department office or by writing Missouri Department of Conservation, P.O. Box 180, Jefferson City, 65102-0180.