HO-183
LANDSCAPE ARCHITECTURE 
Using Landscape Plants to Modify Climate
LANDSCAPE ARCHITECTURE • COOPERATIVE EXTENSION SERVICE
DEPARTMENT OF HORTICULTURE • PURDUE UNIVERSITY • WEST LAFAYETTE, INDIANA
by Ada Niedenthal

If there are areas around your home that are uncomfortably hot and/or very windy, these can be turned into comfortable outdoor living spaces through good design and informed placement of woody landscape plants. This publication explains how to use trees and shrubs to create a more pleasant and practical outdoor environment around your home.
Microclimates, those small, modified climate areas around your home, can influence the amount of use and enjoyment you derive from your outdoor living area. Even minor climate modifications can be beneficial, because subtle changes in air temperature and movement, humidity, or sun exposure make a great difference in how we experience a place. Plants can improve these microclimates in a variety of ways, providing, for example, a buffer against harsh winter wind or the cooling effect of shade.
The first step in determining what opportunities you have to modify the climate around your home is to analyze your site conditions. It is important to learn, for instance, the prevailing wind direction, the amount of rainfall, and the sun and shade patterns common to your area. The site inventory checklist found in HO-161, Planning a Residential Landscape Design, will help with the analysis procedure.

Wind
Local prevailing winds are important influences on human comfort. Effective wind control demands careful analysis of the direction and strength of these seasonal winds. In Indiana the harshest winter wind usually comes from the northwest, and the most pleasant summertime breezes usually come from the southwest. There may, however, be obstacles such as buildings, fences, and walls, that alter these prevailing wind patterns and create wind tunnels or eddies. These localized wind patterns should be analyzed for their desirable or undesirable effects.

Deflecting Winter Wind
Although air movement cannot be stopped altogether, it can be deflected, intercepted, and directed by using windbreaks. The quality of
wind protection achieved depends on the height and the width of the windbreak, as well as the degree of penetrability. A solid barrier like a wall will simply block wind, causing it to flow around and over the edges. This results in an unpleasant wind acceleration as wind returns to its original course and velocity. A partially open barrier, however, allows some wind to penetrate through it. This results in a partial wind blockage and a more gradual acceleration of wind back to its original speed.
Penetrable barriers also provide effective shielding for a greater distance downwind, whereas solid barriers have a shorter zone of protection. An effective windbreak will generally reduce wind speed downwind for a distance up to 15 times the height of the windbreak. Maximum shelter is obtained downwind from 3 to 5 times the height of the windbreak (Figure 1). When planning a windbreak, start by determining where the zone of protection is needed. Then plan the height and location of the windbreak on the basis of this information.
Planting several rows of trees rather than a single row will result in the most effective protection from the wind, allowing fewer gaps through which the wind can blow. If space is a problem, use fewer rows rather than overcrowding the trees. This will result in fuller growth and healthier plants. Using plants in combination with a fence might be an effective alternative where space is limited.
Dense coniferous evergreens provide effective wind-control year around, while deciduous trees and shrubs in full leaf are effective only in the summer. Evergreen trees should be included in windbreaks whenever possible, but only those species that grow well in your area. Using a mixture of species will result in effective wind protection and may also protect against disease which could otherwise damage an entire planting.

Channelling Summer Breezes
In areas with high summer temperatures and humidity, even the slightest breeze will have a noticeably cooling influence, because it evaporates perspiration. So the best planting will tend