I would like to start a discussion about the Root Barrier article that has appeared on page 14 in the Aug. issue of Arbor Age magazine. I agree with most of the information on clay soils, I just am ignorant when it comes to water absorbing Tap roots and the effectiveness of root barriers.If you don't have the magazine the article can be read @ this web site www.profengineering.com What are you thoughts? What is a tap root? How many are there? I have only "heard" of one tree mentioned by Alex Shigo that had extremely long tap roots and that was in a dry mountainous region. What about Pecan trees? Thanks, Ken

Reply to post by Ken Six, on August 10, 1999 at 19:16:57:

Tap roots are the single verticle root that extends downward directly below the stump of the tree. It developes from the original root sprout from the seed. In most trees, it does not persist, or does not penetrate deeply. The genus Carya maintains the taproot, often throughout its life. I'm not sure how much water it can draw in thourgh, as it is a large wood root, not absorbing tissue. Many lateral roots may grwo from it though, and these can drawn in the water at their tips.

From "Root Characteristics of Some Important Trees of Eastern Forests: a summary of the literature", Eastern Region, USDA Forest Service, 1980:
Most of the early root growth oin pecan is directed into the taproot, which typically reaches a depth of from 1 to 3 feet during the first year. Small laterals originate throughout the length of the taproot, although a high percentage of these die back during the fall. During the second year the taproot may reach a depth of 4 feet, with the laterals making considerable horizontal growth. Mycorrhizal roots usually become abundant during the second year. By the fifth year or so the root system has attained its maximum depth, and the horizontal spread of the roots is about double that of the branches. The height of the tree now exceeds the depth of roots for the first time. By age 10 the height of the top is about four times the depth of the taproot, while the spread of branches is still only about half that of the root system. (end quote)

I don't think the taproot and its depth are critical to water absorbtion, excep as a source for lateral root development.

The article, and the diagram provided of barrier placement, does not address the tree's needs, but rather suggests a compromise that must be considered. If soil shrinking is a problem, then the roots should be kept farther from the foundation. That means more damage to the roots, though. Where shrinkage is not a concern, but root pressure is, then the barrier can be constructed closer to the wall, giving the tree more room yet preventing the damage to the wall.

Another problem I see with the diagram is that it seems to assume that the roots will grow in straight radial lines, and not make an 'end run' around the barrier. I think the barriers would be more effective if they were extended a bit farther on the ends.

Finally, the article suggests the fabric, water-permeable barriers will still allow water movement that could dry the soil. Unless the hydrology is carefully studied in each situation, a solid barrier may disrupt hydrologic flow, and actually lead to greater drying. (Perhaps the editor's scissors were busy that day.)

Reply to post by Ken Six, on August 10, 1999 at 19:16:57:

I agree that tap roots per se do not persist, and should be defined that way. Heart roots, or let's call them descending roots, go deep, if the soil and water table allow. Laterals come off of them, and easily go under root barriers. Root barriers can be efficacious if they go to the depth that the soil will allow roots to penetrate and proliferate. A 12 inch root barrier will not help much with a Douglas-fir that has roots still branching at 4 meters deep in a sandygravel soil.