Factors Affecting Root Growth in the Landscape

Published: January 1, 2012

The root environment for a landscape plant is often far different from that found in the native habitat of that plant. There is usually considerable disturbance of the soil during building construction, leading to loss of topsoil, compaction and grade changes, causing poor soil drainage. As a result, conditions for growth of roots and beneficial microorganisms in the soil are often far from ideal, leading to poor performance and pest susceptibility of plants in the landscape. Although it is rarely possible to maintain or create an ideal root zone environment in a landscape, it is often possible, though soil conservation and cultural practices, to create conditions in which roots can survive and grow reasonably well. Keeping in mind some general guidelines on what roots need in order to grow can help with decision making on establishment and maintenance practices.

In general roots grow best in a moist soil with adequate air filled pore space to allow good internal drainage and gas exchange. Air filled pore space is primarily dependent on the texture, structure and level of compaction of a soil. The texture of landscape soil often starts with a high clay content, since topsoil may not be saved and replaced after construction. Compaction by equipment during construction can reduce pore space of a tight soil even further. Attempts should be made during construction to limit compaction by restricting travel routes of equipment and laying down thick layers of wood chips to distribute the weight. Organic matter (OM) can play a key role in maintaining a porous structure. As soil microbes break down soil OM, they exude waxy substances which cement clay particles together into water stable aggregates. Mulch applied to the soil surface around landscape plants can help to maintain a reasonable OM content in the soil. This generally works best where groups of trees and shrubs are mulched together in large, turf-free areas. This approach can also minimize compaction from foot traffic over the root zones of trees and shrubs.

What do roots need?

Roots are dynamic and their growth is strongly affected by the environmental conditions found in the root zone. To survive and grow, they need Air (gas exchange), Water, Non-limiting Temperature, Carbohydrates, Minerals, Space, Low Soil Density, Microbial Associations and Non-toxic soil chemistry. Some parameters to shoot for might be a minimum 25% air filled pore space which contains no less than 10% Oxygen one day after a soaking rain (free air is 21%), bulk density less than 1.3 grams per cubic centimeter, a temperature between 40 and 90 degrees F and a pH between 4.5 and 7.5. In a compacted soil, CO2 can accumulate to toxic levels due to poor gas exchange. Under chronic waterlogged conditions, there may also be an accumulation of toxic metabolites from anaerobic respiration and toxic forms of iron and manganese (reduced from +3 to +2 ) may accumulate.

Cultural Practices Promoting Root Growth

If the quality of the soil in the root zone of a landscape starts out reasonably high, certain cultural practices can help to maintain an environment conducive to root growth of ornamental plants. Good planting technique can have long term benefits. Pulverize the soil in a wide, but shallow area and never work the soil deeper than the root system. Trees should be planted so that a root flair is evident at the soil surface. Planting too deep can lead to root suffocation and development of stem girdling roots. Research by Dr. Gary Watson at the Morton Arboretum has shown that roots of fescue give off allelopathic chemicals which retard the growth of other plants. Mulching groups of plants together in turf-free zones greatly enhances establishment and growth of trees and shrubs relative to that of plants with small mulch rings. This practice can also help to maintain a high organic matter content, leading to improved soil structure and increased microbial activity. In addition to mycorrhizae, many fungi and bacteria have beneficial effects on plant root growth.

Irrigation practices can have a major impact on the quality of the root zone environment for ornamentals in a landscape. In general the irrigation required to keep turf green in mid-summer is greater than that required to keep established ornamentals healthy. Irrigation applied by a sprinkler tends to compact the soil surface and reduce infiltration rate and gas exchange. It can also promote disease by creating a moisture film on leaves. If possible, it is best to irrigate ornamental beds using drip lines under the mulch set on a different schedule that that of the turf sprinklers.

Excessive fertilization with nitrogen can retard root growth by depleting woody plants of carbohydrates. Most woody ornamentals in a landscape do not require more than about 3 pounds of actual nitrogen per 1000 square feet per year to keep them healthy. Remember that plants get some of their nitrogen from the breakdown of organic matter in the soil, so maintaining OM with good mulching practices is important. Whenever possible, split nitrogen into 2 or 3 applications and use a slow release nitrogen source. Always fertilize according to soil test results and do not apply phosphorus or potassium unless they are called for. Yearly fertilization with a "complete" fertilizer without soil testing may lead to imbalances that can interfere with uptake of certain minerals like iron and manganese.