By By Gail Burton / horticulture columnist
Dec. 8, 2002
Root and top growth are intimately connected and totally dependent on one another.
But since roots are "out of sight," they are "out of mind." Many knowledgeable gardeners have great misconceptions about how roots function.
Roots are both fragile and tough. Anyone who has ever tried to move a large tree can attest to the toughness. The roots we generally notice when a big tree comes down are the large woody roots. These roots anchor the plant and provide support but take in little water or nutrients.
The working part of the root is the fragile thread-like root hair. Root hairs absorb almost all a plant's water and nutrients. These materials are absorbed through thin coverings that provide no protection against drying.
Under ideal conditions in the soil, most root hairs last only a day or two. When exposed to the low humidity above ground, root hairs die in just a few minutes. During transplanting many root hairs die even under the best of circumstances. Any good gardener can tell you that new transplants should be irrigated often because they are inefficient at taking in water.
Since roots are alive, they need oxygen to perform respiration. Root respiration releases the energy for growth and uptake of water and nutrients. Without air, most roots die. Roots can lack air because the soil air pores are filled with water.
Over-watered house plants often suffer from this dilemma. Extensive over-watering causes the healthy root hairs that are normally white to turn brown and soft as they suffocate.
Trees in compacted soils are often starved for oxygen as well. Compacted soil is hard like concrete and it has been packed down so that few air spaces are left.
Most of the roots, even on a large tree, are in the top 6 to 12 inches. When topsoil is added to level the ground the flow of oxygen is hindered. Soil addition, grading or extensive tilling can disturb or kill the active roots. If a large portion of the root system is killed or wounded, a section of the branches and twigs forming roughly the same portion of the canopy dies.
I've made many new beds under trees in my landscape. I dig individual holes to plant the shrubs or perennials rather than tilling. The holes I dig only disturb a small portion of the root system and do not harm the tree. I also level around trees somewhat with mulch.
The mulch seems to fill in low bare spots but is loose enough that it doesn't block air. Generally I keep mulch depth to around 3 inches to be on the safe side.
Roots grow best at soil temperatures of 50 to 60 degrees. Growth continues until the soil temperature is in the 30s. In Meridian our soil temperature generally stays in the "ideal" range almost all winter long.
So, while the roots are growing, some researchers recommend a fall fertilization. I like to schedule this task after leaf drop and while soils are still warm enough for root growth. I often use a slow release fertilizer during December. I put the fertilizer on all my tree and shrub beds but avoid the turfgrass.
Fall is also the ideal time to plant or transplant new trees, shrubs or perennials. If you purchase container plants for a new bed, remember that roots are more sensitive to cold injury without the insulating soil blanket.
Container plants are also likely to become fatally "pot bound" as roots circle the pot and tangle with one another. Try to avoid purchasing "pot bound" plants by checking the root system before purchase. If roots are only slightly tangled, cut along the sides of the root ball to prune and spread roots when planting.
As we enter the season when most roots are made, let's pause and be thankful.
Remember that it's impossible to have a healthy plant without healthy roots. Celebrate by fertilizing your roots, mulching your roots or planting a few new roots.