Tree Roots: the hidden support system part II

What would we do without trees? What structures could we invent and construct that would hang over our houses and offices, providing shade and cooler temperatures? Such a structure would have to hold up under 80 mph winds and heavy wet snows, and would have to retract in the winter to let in light. Trees provide these values and much more, giving off oxygen, providing housing for birds, and protection for understory plants. Thus it is very important to take good care of our trees, and the most fundamental level of that care must be directed to the roots.

In the last issue I discussed the structure and functions of tree roots, how they need oxygen and how they depend on soil micro-organisms, especially the symbiotic fungi, mycorrhizae. I explained that 80% of tree roots are in the top 8”-10” of the soil and that they can extend far beyond the dripline of the tree. And I discussed how organic matter helps tree roots by creating more space in the soil for oxygen and by supporting the mycorrhizae and other soil life. Now we need to look into the practical issues of watering, fertilizing and planting.

         There are numerous views and approaches to planting trees. In general, if many trees are to be planted and major landscaping done, it is far better to begin by adding organic matter to the entire area and digging it in. If only one or two trees are to be planted, some say it is better not to amend the soil because this encourages the roots to circle in the planting hole. Other experts like Denver consulting arborist, Gene Eyerly, believe it is better to add one third Canadian sphagnum peat to the backfill at planting. I think one-third compost works well also. Gene suggests making the planting hole a minimum of one foot bigger in diameter than the root ball. This method works well in my experience also, but recent recommendations call for digging a tapered hole three times the volume of the root ball. By making the top 6”-10” of the hole 1’-3’ wider than customary, the roots will grow faster, and the tree will develop faster.

 The majority of trees for sale are known as B&B, balled and burlap. These trees have been grown in a field. Then they are dug up and their root ball is wrapped in burlap and buried in wood chips until they are sold. Gene Eyerly says that better than 90% of the roots are left in the field, and because of that it is better to plant a smaller diameter B&B tree. He explained that in a 13-year period, he has seen a 4” diameter tree outgrow a 10” tree planted at the same time. Of course some people want a big tree fast and that is understandable, however the big B&Bs will go through a long recovery period where they will grow very slowly and be more likely to dry out and be more vulnerable to stress-related problems like diseases and borers, etc. Trees are also available that have been grown in containers. These have the advantage of having complete root systems and are quicker to establish, but they are generally much smaller, less available; and circling roots may have to be loosened or cut.

         Then there is the question of planting depth. Gene says one of the biggest problems with tree planting is planting too deep. He suggests putting the root ball 2”-4” above grade (the predominant soil level). Many experts from CSU and Cooperative Extension agree. However Alex Shigo disagrees,  “Yes, there are a lot of problems because of planting too deep, but why overreact and plant too high? Plant just right, so the soil level is where the trunk flares.” Dr. Shigo went on to explain that tree problems often start because the wrong tree is planted in the wrong soil. “The tree may not die; but it will tolerate the poor soil and have insect borers, cankers and other man-caused problems.” Thus the hardiness zone is only the first factor to consider when choosing a tree. We need to study the soil conditions where a particular species flourishes and take note of the soil types where certain trees repeatedly fail. In this case, soil qualities relate to the availability of oxygen to the roots, acidity/alkalinity, and the availability of certain nutrients and minerals.

         Watering is the next subject because once a tree is planted, the next question is “How much should I water it?” Jim Feucht, retired CSU professor of horticulture responded, “There are too many variables for one answer; it is better to check the soil, but roughly twice a week at planting, then taper off. Once a tree is established don’t water more than once a week. More trees are killed by drowning than from the lack of water.” Al Rollinger agrees. He is a Denver landscape designer and local tree explorer with a lot of experience in this realm. “Don’t love it to death with water,” he warns. “Apply long, deep and infrequent watering. Water when it’s dry. Check and see.” Al’s formula for conditions around a tree’s roots is ½ soil ¼ water and ¼ air. The logic is that if you overwater, you are driving out the oxygen so the roots suffocate. How much water to apply can be difficult to judge; trees that are overwatered have wilted leaves, the same as  trees that are experiencing drought. I asked James Feucht how that is possible. He explained that oxygen is an energy transfer element and that if the roots can’t get oxygen, they can’t take up water. Not only does low oxygen levels deter the uptake of water, but it also lessens the uptake of minerals. All these factors reduce root development, which further weakens the tree and reduces top growth. Therefore if you are not sure if a tree needs water, dig down 6”-10” and see. It can be bone dry on the surface with the roots sitting in a puddle.

Both Feucht and Rollinger recommend winter watering for conifers and broadleaf evergreens. The most drastic tree losses I see occur when evergreens are planted in the fall and then are not watered through the winter. Their roots do not have time to grow into the surrounding soil before winter, the irrigation systems are turned off, and the poor trees continue to transpire (give off water) through their leaves.

         Most people think about watering their trees but people often forget to fertilize. In places with rich soils like the Midwest, this is not a problem. However our Colorado soils are commonly deficient in nitrogen and organic matter, and a tree can’t move when it has drawn from the same soil for 40 years. But fertilization is often misunderstood. Shigo, Feucht and Eyerly all stressed that a fertilizer is not food. Fertilizers are nutrients that must be taken up to the leaves to become sugars. Fertilizers are raw materials that actually require energy in order to be used. Two of the most common mistakes that people make with fertilizing trees is to fertilize at planting time and to over-fertilize when a tree is stressed. Alex Shigo said, “To heavily fertilize roots that are starving is the kiss of death.” Likewise when a tree is newly planted it is better not to fertilize with nitrogen, because that only stimulates top growth at a time when the tree really needs root growth. Compost is fine to include at planting since it is usually very low in nitrogen. Manures can be too high in nitrogen to apply when planting.

         So how should we fertilize trees once their roots are established? James Feucht had several suggestions. He said chemical fertilizers have the advantage of getting deeper into the soil without digging, and since they are water-soluble, they are more quickly available. These can be applied by hydraulic injection which also provides some aeration, or it can be broadcast on the surface and thoroughly watered in. He believes that organic fertilizers are a little better in the long run if you can get them into the soils. In order to do this, they must be dug in, or 1½”-2” holes must be drilled 6”-10” deep in a wide strip around the drip line of the tree. I have experience with this auger-feeding method and have found it to be very effective, but our Colorado soils make this a tough job that is hard on workers and on equipment. I believe organic fertilizers can also be scatched into the surface and watered in and that earthworms will carry them deeper, especially if they are mulched as well. However this is slow. I think trees do better being fertilized slowly over a long period of time rather than being given a big jolt with no follow-up.

         In terms of damage to tree roots, here are some suggestions. Gene Eyerly says you can’t remove soil over a tree’s roots because in our Colorado clay soils, the roots run shallow and will be badly damaged. James Feucht says that if you have to dig a trench past a tree, don’t let it come closer than half the distance between the trunk and the dripline. And Alex Shigo maintains that if the roots are damaged, it is a bad idea to cut back the branches.

         On the subject of tree roots causing damage, it is rarely the tree’s fault. Most sidewalk and foundation damage is caused because trees are planted closer than 6’ from them. Our urban designers and landscapers should be giving more room for the trees. Most damage to sewer lines occurs because of the use of old tile systems that offer numerous openings for roots to enter. And suckering is often caused by planting trees in soils that are too compacted, causing the roots to come to the surface for air. So be kind to tree roots; they have a hard job in Colorado. If we take care of the needs of the tree roots, then the trunks, branches and leaves above ground will be healthier and more capable of enriching our quality of life, like nothing else can.