Evaluating the Site

 

Exposure

The amount of sunlight a tree receives and the time of day the sunlight is received can limit the kind of tree that can be planted. Though all-day sun is required by some plants, it can be too intense on some sites where heat and light are reflected from nearby buildings or pavement. An area with morning sun and afternoon shade often provides adequate sunlight and protection during the hottest part of the day. An area with morning shade and afternoon sun may be more suitable for trees that are tolerant of heat or drought stress. The intensity of all-day shade can vary from the open shade on the north sides of fences or buildings to the very dense shade under low tree branches.

 

Drainage

In poorly drained, compacted soils, drainage must be provided. More urban trees die from root drowning than from drying out. Before planting the tree, test the drainage of the planting hole by pouring a few gallons of water in the bottom. If the water hasn’t soaked in after an hour, there probably is a drainage problem. Near a slope, small drains may be able to run water to some lower point. On level ground, planting the tree on a slight mound may be necessary to get the root system out of the saturated soil.

 

Importance of soils

Soil conditions frequently limit planting success. Planting methods should be adjusted to fit soil types. Poorly drained, clayey soils, typical of modern urban developments, require procedures much different from the well drained, friable (crumbly) soils found in older neighborhoods.

Soil texture is the ratio of particle sizes in a soil. Sandy textured soils are composed mostly of large particles, while clayey soils contain many microscopic particles. Soil structure refers to the size and shape of soil aggregates (soil particles held together in clumps by organic compounds). Spaces between aggregates allow movement of air and water through the soil. Well-aggregated soils provide optimum air and water movement to plant roots. Poorly aggregated soils are common in city landscapes and are often the result of compaction. Thorough incorporation of organic matter (compost, peat moss, etc.) helps alleviate these problems, as does cultivation.

Soil wetness and related drainage conditions are controlled by a number of factors, including precipitation, soil texture and structure, permeability, infiltration characteristics, and landscape position. The degree to which soils drain under natural conditions can be described as follows.

  1. Somewhat excessively drained soils are characterized by rapid drainage and low available moisture during most growing seasons.
     
  2. Somewhat poorly drained soils are wet for extended periods of time and restrict the root development and growth of most plants.
     
  3. Poorly drained soils are wet throughout most of the year and restrict the root development and growth of all species of plants except wetland plants.


Both somewhat poorly and poorly drained soils may require the use of artificial drainage or special planting techniques described in the “Planting Trees” section.

Soil reaction, or pH, is a general indicator of nutrient availability. In slightly acid to neural soils (pHs between 5.5 and 7.2) most nutrients are available at optimal levels. Some nutrients, such as iron and manganese, become less available in alkaline soils (pH above 7.2) because of chemical changes caused by the alkalinity. Other nutrients become less available in highly acid soils (pH less than 5.5). Species vary in their ability to tolerate alkaline soils. Soil pH induced nutrient deficiencies can sometimes be corrected by proper soil amendment or fertilization, but selection of a species compatible with existing pH is preferable.

 

Amending the soil

If the soil has high clay content and is not friable, it should be amended with up to 25 percent organic matter (leaf, garden and mushroom compost, etc.) before the hole is backfilled. New roots will grow more rapidly in this lighter, better-drained soil mixture. However, caution is required using this technique in sites with poor drainage as it may result in “bath-tub” effect. In this case, ensure good initial drainage of the planting hole.