Marin Master Gardeners
University of California
Marin Master Gardeners

Marin IJ Articles

Be kind to to your plants' roots so they can thrive

May 14, 2011
Martha Proctor

WHAT CAN YOU as a gardener do to support the plant roots in your garden? Plant roots perform some of the most vital functions on our planet by providing sustenance and support for most plants and thus, secondarily, for much of human life and activity.

Roots are considered to be part of a plant, usually below the ground, that lacks nodes, shoots and leaves, holds the plant in position, draws water and nourishment from the soil, and stores food. Up to 90 percent of a tree's roots can be found in the top 18 inches of soil beneath and around the trunk of the tree. Nutrients, oxygen and water are more readily available in that top level of soil. Surprisingly, the mass of a tree's roots can be up to five times greater than that of the foliage above ground. Roots are critical as they are the source of all nutrient exchange between the plant and its surrounding soil.

The tip of the root, called the root cap, has the task of forcing its way into near-solid earth. Behind the cap are root hairs, thin hairlike outgrowths of root cells, which excrete organic compounds into the narrow region of soil in close proximity to the roots, the rhizosphere.

The rhizosphere contains many bacteria that feed on sloughed-off plant cells and the proteins and sugars released by roots. To access the available nutrients, plants produce thousands of root hairs per square centimeter. These root hairs greatly increase the surface area of the root system, allowing the roots to absorb water and minerals more efficiently and more extensively.

For example, the mass of root hairs increase a tree's "feeding" area by two to 10 times. Root hairs are short-lived and are only produced on growing tissue, not on woody root material.

As the upper horizon of the soil is the area where most feeding roots, nutrients and most soil organisms coexist, top dressing the soil with compost and green manure helps to promote a healthy population of microbes and produces the best-looking and healthiest plants. If the soil is healthy, the plant is more likely to develop a vigorous lateral root system. From these lateral roots, vertical roots, called sinkers, can develop anywhere along the length of the laterals, increasing the plant's access to nutrients and its stability.

It is no surprise to gardeners in Marin County that roots have a harder time creating pathways through clay or compacted soils. Roots can suffocate with overwatering or in compacted soil. Weakened roots become more susceptible to pests and soil borne diseases.

The addition of compost/organic matter improves the soil structure, which in turn, improves pore space, allowing easy and deep root growth. Adding a 2-inch layer of mulch such as straw, newspaper and cardboard assists the root system as it helps to prevent moisture loss and discourages weeds. The weed roots would otherwise compete for the available nutrients. Avoid piling mulch around the trunk of a tree/shrub as it  encourages root or crown rot.

To facilitate their search for food and nutrients, the roots of approximately 80 percent of all vascular plant species enter into a symbiosis with certain fungi. The word "mycorrhiza" describes this mutualistic association between fungi (myco) and the roots (rhizae) of plants. Although only a small part of the plant root is symbiotic with the mycorrhizal fungus, this association is advantageous for both organisms.

The plant gains increased exploration of the soil (rhizosphere) and thus a wider sphere from which to draw in water and nutrients. The fungus uses the carbon provided by the plant for its physiological functions, growth and development.

Mycorrhizal fungi are thought to improve water absorption, increase drought resistance, reduce transplant shock and exude substances that reduce infections caused by soil pathogens. Thus, mycorrhizae are especially vital in soils with low nutrient availability, poor structure or
low water-holding capacity. A mycorrhizal relationship also occurs in plants
that do not have roots, such as mosses and liverworts.

Together the roots, mycorrhizae and the microbes in the soil promote the decomposition of various minerals. Use caution when using soluble fertilizers, herbicides and fungicides as these can diminish fungal populations.

Before buying a plant, if possible, carefully slip the root and soil mass slightly out of the container to check the condition of the roots. The roots of root-bound plants grow to the bottom of the container and then begin to encircle it. Even after being transplanted, they will not branch out but continue to encircle the planting hole. To avoid this, soak the roots for up to an hour in water and then separate and trim them back before replanting. A plant in a small 4-inch pot will more readily adapt to its
new environment than a larger, more developed and possibly more root-bound plant. Healthy, appropriately grown, container plants successfully adapt when properly transplanted.

So when the soil warms enough for planting this spring, remember how important the roots are to the health and vitality of your plants.

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