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Understanding what's below your plants will help keep them healthy

October 9, 2015
Martha Proctor

Hidden beneath the ground is one of the most important parts of a plant — its roots. As plants are unable to move to a more nutrient-dense or water-rich location, their root mass scavenges the soil in their immediate location. Roots provide the vital sustenance and support that plants require.

Most of the primary functions of roots occur in the dense network of small, fibrous, non-woody roots that are just behind the root tip. In addition to taking up water and soluble mineral nutrients from the soil, interacting with soil microbes and fungi and acclimatizing to a changing environment, roots support and anchor the plant.

In large woody plants, a big main root, the taproot, develops with a small number of lesser fibrous roots growing from it. On most woody plants the taproot is a juvenile structure. The root system of large woody plants essentially swallows the taproot as the plant grows and the lateral roots expand outwards. Sinker roots drop down from the lateral roots in soil fissures, allowing the plant to obtain water from deeper in the soil profile.

In contrast, shallow-rooted plants are more susceptible to drought but absorb surface water more quickly. The roots of a single species may differ in form from plant to plant and in different soils, but all perform the same basic functions.

The roots are the first part of the plant or tree to start growing in springtime. As the ground warms, the roots grow millions of tiny hairs that absorb water and nutrients from the soil. This is then channeled into rootlets, passed into the main roots and ultimately into the stem or trunk. As plants become well established, the root system develops laterally and usually extends far beyond the spread of the branches.

For most cultivated plants, the roots of neighboring plants meet and overlap. The greatest concentration of fibrous roots occurs in the top 18 to 24 inches of soil but many laterals grow downward to soil pockets where nutrient availability and aeration are more favorable for growth. Lateral root spread is an important indicator of the area over which plants forage below ground and has implications for individual plant fitness, and underground competition. The root biomass can far exceed the above ground portion of a plant.

Most of the nutrients taken in are absorbed by young roots; old, tough, woody roots, anchor the plant.

The depth and spread of roots are dependent on a plant’s growth characteristics and the soil’s texture and structure. Roots require adequate oxygen in soil to function. On compacted and clay soils, roots will be shallow, remaining near the surface where oxygen is available. In a drought, the root system will spread farther, mining a larger soil area for oxygen, moisture and minerals.

Plants rely on the interaction between their roots and fungi and microbes in the soil. The thin fungal threads intertwine with the plant’s root hairs. Once intertwined, fungi direct nutrients and water toward the roots of the plant. For example, mycorrhizae, specific beneficial soil fungi, expand the root’s surface-absorbing area by 100 to 1,000 times. Soil microbes increase the availability of nutrients, produce hormones that regulate plant growth, and cooperate with the biology of the root.

Though the mechanisms are still unknown, most plants are able to generate a coordinated response to changing environmental conditions. Using luminosity imaging, plant biologists hope to discover how roots interpret water and nutrient availability in soil and communicate this to the rest of the plant.

To encourage healthy roots in your garden, when planting, dig a bowl-shaped hole two to three times wider than the root ball. Cut back any circling roots. After filling in the planting hole with existing soil and compost, mulch around your plants or trees but away from the trunk with 3 to 4 inches of wood chips or mulch to promote continued root growth and light, well-drained soil. Water deeply, but only when the soil dries out.

Dig down with a trowel or use a soil probe to determine whether the top 8 to 12 inches of soil is moist out to the drip line. Excessively dry soils cause the death of small roots and reduce capacity to absorb water. Drought stress increases susceptibility to pests and disease. Overwatering deprives the soil of oxygen. Don’t wait until your plants wilt — you’re not watering plants, you’re watering their roots.

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