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Wire Baskets: A Further Look
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Research sheds new light on the wire-basket controversy
Trees in the Nursery - Field Production
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Dr. Glen P. Lumis
Copyright American Nurseryman, August 15, 1990, Vol. 172, No. 4
Reprinted with permission.
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In the past 20 years, tree spades have revolutionized transplanting techniques in most of North America. Since their introduction, these machines have largely replaced such traditional nursery tasks as hand digging, string balling and drum lacing. However, nurserymen using tree spades soon realized that the tapered, cone-shaped root balls the spades produce require more support during transplanting than conventional root balls.
As tree spades came on the scene in the early 1970’s, manufacturers developed wire baskets to accommodate the new shape of spade-dug root balls. Today, wire baskets, made of either galvanized or non-galvanized wire, are available to fit root balls of almost any size or shape. While these baskets were created primarily to provide extra support to spade-dug root balls, they also have several other functions.
For example, root balls handled and transported in wire baskets are less likely to crack or break than string balls. Wire baskets can also be used in lifting and maneuvering trees during shipping and at planting sites.
In addition, wire baskets enable nurserymen to use sandy soils – traditionally avoided in tree production – to grow large trees. Burlap liners in wire baskets help contain root balls with sandy soil.
However, one critical question has remained for nurserymen who use these containers: Should wire baskets be removed when trees are transplanted?
Some nurserymen argue that wire baskets girdle tree roots. This can lead to disease and insect problems, tree instability, premature decline and even death. Certain tree-planting specifications require landscapers to remove wire baskets before planting (2).
Others contend that the baskets’ large openings make girdling unlikely, and that roots will grow around wire with no adverse effects. Yet there is little scientific information about how wire baskets affect root growth.
In 1983, Sellers published photos implicating wire in the decline of a sugar maple 11 years after it was transplanted (7). However, adjacent trees planted in intact baskets at the same time were not affected.
More recently, Feucht indicated that problems can arise as soon as four years after planting (3). Feucht said tree decline is subtle, and root tissue does not rejoin after growing around wire.
Urban warned that large, flaring butt roots girdled by wire would not support a large tree (8). Potential root problems associated with wire baskets were also illustrated by Feucht and Butler (4).
In contrast, Carpenter stated that there are many thousands of trees in wire baskets, and so far, the number of reported problems is very small (1). In 1988, Struger and I showed photographic evidence of tissue bridging and growth of intact bark and wood over wire (6).
I have heard both sides of the argument over whether planted wire baskets inhibit growth from growers, arborists, and landscape architects and contractors alike. But this question deserves considerably more attention than it has received. With little information to go on, I decided to study trees that were planted in wire baskets between four and 15 years ago.
During the past several years, my research team at the University of Guelph carefully hand excavated 34 trees of seven species at 10 municipal parks in southern Ontario. The trees ranged in size from 3- to 12-inch-caliper.
My objectives were to excavate the root systems of trees established in wire baskets, to locate roots that had grown around wire, to determine the integrity of the root tissue and to examine the wire.
All but two of the trees we excavated were healthy and growing well. They had good leaf color, adequate shoot growth and no dead branches or dieback. (The two exceptions were Acer saccharum trees, which were growing on a site previously altered by construction.)
I brought many of the root samples to the lab, where I let them air dry. The roots were then cut with a table saw fitted with a carbide blade. From the cut roots I found that, in every instance, the root tissues joined or bridged across the wire, meaning there was intact bark and wood over the wire.
This is in contrast to Feucht’s findings (3), in which sketches illustrated that root tissues do not rejoin after growing around wire.
I also found that some roots displayed external swelling around embedded wire. Yet in such cases, the root bark, or periderm, resumed normal growth after the root enclosed the wire.
The wire-bridging process was clearly evident in the roots of Salix alba var. tristis (golden weeping willow), which was transplanted in a 28-inch-diameter wire basket seven years earlier. As the root began to grow over the wire, slight infolding occurred. However, when the root tissues met, the tissue bridged. As the wire became deeply embedded, the surrounding root tissues were completely intact, including the bark and vascular tissues.
Some may argue that, as I once heard a contractor say, “a willow will grow over anything.” But what about species that grow slower than willow? Two examples from my study provide the answer to this query.
The first involved Acer platanoides ‘Columnare’ (columnar Norway maple) – a specimen that had been growing in the same site in a wire basket for 11 years. A root tissue bridge formed in the second year after the root had touched the wire. After this initial bridging, all root tissues were intact.
A second example is an Acer saccharum (sugar maple), which had been growing in a wire basket for 14 years. In one of the sugar maple’s roots, an infolded bark ridge formed just above the wire, probably resulting from slow growth. However, after the initial infolding, the root tissue bridged and resumed active growth.
Microscopic examinations (under white and ultraviolet light) of this sections of this sugar maple root revealed intact periderm, phloem and xylem.
I also compared root xylem vessels in the tissue bridge directly above the embedded wire to those from another part of the root. Vessels directly above the wire appeared shorter, had thicker walls and were skewed at an angle. However, normally sized vessels that formed close to the root bark several years later were nearly similar to unaffected vessels.
Although I did not determine the water-conducting ability of wire-affected vessels, vessels formed several years after wire was embedded were so similar to unaffected vessels that I suspect water movement is not impeded.
During excavation, I also prepared some roots – both with and without embedded wire – for dye flow tests. I made a wedge-shaped slit in each root, which I then surrounded with a silicone reservoir. I added acid fuchsine dye (0.5 percent solution) to the reservoirs and let it stand for 24 hours.
I then removed these injected roots and returned them to the lab. These sections revealed that the dye moved across the tissue bridges, indicating that water movement was not totally impeded. (A colleague at the University of Guelph has recently completed further studies to determine the water flow and transpiration rates in wire affected roots of two-year-old seedlings (5).)
Another major concern with the use of wire baskets is the longevity of wire in soil. I spoke to several manufacturers about this, but none had specific information on deterioration rates, and industry opinions regarding the effect of soil moisture and pH on wire corrosion vary greatly. Obviously, there seemed a need to find out more about wire characteristics.
I retained wire samples from many of the excavation sites. A hydraulic testing machine was used to determine the wire tensile strengths of these samples.
At one site, several trees had been planted in wire baskets 11 years earlier in sandy loam soil with a pH of 6. From this particular site, the wire near the soil’s surface was not discolored and appeared like new. Near the bottom of the root ball, the wire was only faintly discolored and displayed slight corrosion.
Samples of this 9-gauge, galvanized wire had an average tensile strength of 1,597 foot-pounds. Values of that magnitude were among the highest I measured for either new or planted wire baskets. Obviously, these wire baskets were still strong and were not deteriorating even after 11 years in the ground.
At another site, I tested the tensile strength of wire samples from both galvanized and non-galvanized baskets made of 9-gauge wire, which had been planted for four years in clay loam soil with a pH of 8.0.
Over the 19-inch-deep galvanized baskets, wire appearance changed from slightly discolored at the top of the basket to slightly corroded at the bottom. Tensile strength decreased an average of only 5 percent from top to bottom in these baskets. Based on the strength of new galvanized wire before basket construction, it should remain intact for many years.
Non-galvanized baskets were moderately corroded over the complete depth, yet wire tensile strength decreased an average of only 3 percent from top to bottom.
Two other samples of non-galvanized wire, which had been in soil for seven and 10 years, were extensively corroded and were flaking. However, the wire from these baskets was not easy to bend or break. These two samples had tensile strengths of 798 and 952 foot-pounds respectively.
These results indicate that neither galvanized nor non-galvanized wire corroded significantly as a result of either less oxygen or more moisture at various soil depths.
After several years of field and laboratory study, I have made the following conclusions about wire baskets and their effects on root growth.
- Roots grow around basket wire, forming a complete union of bark and wood tissue; roots are not permanently girdled.
- Root tissue formed after growing over wire permits translocation.
- There appears to be no injury to, or break in, the root periderm that would allow pathogens to enter a tree.
- Basket wire remains intact in soil for many years, and wire strength diminishes slowly.
- Removing wire baskets at planting time is not necessary to assure growth and survival of large tree roots. However, you should remove any rope across the top of the ball, and bend back or remove basket loops.
- Using a correctly sized basket for each root ball is imperative. The top horizontal wire should be at least several inches above the top of the soil ball.
Literature Cited
1. Carpenter, P.L. 1987. “Are There Long-Term Problems with Wire Baskets?” Tech Notes, National Landscape Assoc. July/Aug.
2. Close, N. 1985. “M.T.C. Makes Significant Changes to Landscape Contract Specs.” Hort Review 3(2):12.
3. Feucht, J.R. 1986. “Wire Baskets Can Be Slow Killers of Trees.” Amer. Nurs. 163 (6):156-159.
4. Feucht, J.R. and J. D. Butler. 1988. Landscape Maintenance. Van Nostrand Reinhold Co., NY.
5. Goodwin, C 1989. “embedded Wire in Tree Roots: Implications for Tree Growth and Root Function.” M.Sc.
Thesis, University of Guelph.
6. Lumis, G.P. and S. Struger. 1988. “Root Tissue Development Around Wire-basket Transplant Containers.” HortScience. 23:401.
7. Sellers, A.G. 1983. “Tree Roots and Wire Baskets, Are They Compatible?” Ontario Shade Tree Coun. Nwsl. 2:4.
8. Urban, J. 1989. “New Techniques in Urban Tree Plantings.” J. Arbor. 15:281-284.
Dr. Glen P. Lumis is professor of landscape horticulture in the department of horticultural science, University of Guelph, Guelph, Ontario. Former research technician Steven Struger assisted with the research reported in this article.
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