In the January issue of Landscape Trades, we discussed how at Ohio State University in 2003-4, we were the first to speculate that bark cracking was not solely related to cold injury, as previously accepted in the nursery/landscape trades.

After writing articles and doing presentations, I have had calls from researchers in other parts of the U.S., Canada and Europe confirming this is not simple bark splitting that is often associated with Southwest (SW) injury and cold temperatures. The scale of the problem points to something more than solely an environmental problem.

Major stock losses are occurring. Some growers have reported losses exceeding five per cent of their inventory, or 3,000 to 4,000 trees per nursery, to splitting each spring. At an average cost of $125 per tree and the number of nurseries reporting problems, the stock losses have been staggering and in excess of several million dollars.

The frequency of occurrence, lack of consistency with cold temperatures, international distribution, location on the tree (not always the SW and up into the scaffold branches), season long occurrence (not just the early spring) and lack of consistency of soil type and other factors that could identify cold injury as the primary agent all point to something more than solely cold injury.

As stated in the January article, cold may not be the causal factor, or the only reason for splitting. Butin and Shigo (1981), Caspary (1855) and Shigo (1963) indicate the actual cause of bark cracking is “preset” by a wound such as the improper removal of a basal sprout or leaving of a branch stub. Knowing this makes it possible to prevent frost cracks by minimizing wounds, and by proper and early pruning of branches and basal sprouts. By thinking bark splitting is related to cold alone, there is little that can be done to prevent it, as we are subject to the climate. However, much care should be taken not to wound trees, especially when young (Butin and Shigo, 1980). The take-home message is, “these trees may not be as tough as we all think they are and TLC can go a long way to preventing bark splitting.”

Today in 2009, the economic cost of bark cracking to the Canadian and U.S. nursery/landscape industry is estimated at $12 million annually. These cost estimates are based on a continuing pattern of strong and steady increased frequency of bark cracking since 2003. Concurrently, consumer preference for faster-working glyphosate products has driven production of various surfactants to break down the cuticle of plants and increase rate and amount of glyphosate uptake. However, the absorption of glyphosate into thin barked or pigmented-bark also increased. Exposure to an ornamental plant through green bark is considered a sub-lethal dose.

Previous researchers and current Ohio State University studies indicate sub-lethal glyphosate dosing results in glyphosate accumulation in nursery and landscape thin green-barked trees with severe consequences. The effects of glyphosate continue one and two years following single, low dose applications in terms of reduced phenolics and damaged phenolic production systems. Some growers/ landscapers are making eight applications a season, removing suckers and adventitious shoots with glyphosate products and/or applying shortly after mechanical removal of suckers.

These practices are in part causing the increased frequency of bark cracking. The role in disruption of crown dormancy and reduction in tissue hardiness with a phloem-transported herbicide is ongoing at Ohio State University.

Tippet and Shigo (1980 and 1981) explain that after wounding, the cambium forms a distinctly different tissue called the “barrier zone.” The barrier zone is a very strong protection against infection, but it is also a plane of structural weakness, because it has a different anatomical and chemical makeup from normal wood. When internal stresses caused by rapid temperature changes or wind occur near the barrier zone, the wood may separate tangentially and longitudinally along the barrier zone (McGinnes et al., 1977), because this is the “course of least resistance” for the crack to form.

Another way to explain it is, following a sudden temperature drop, the outer layers of bark and the wood cool most rapidly and are subject to appreciable tangential tension, which causes marked shrinkage and cracking to occur where the wood is structurally weakest (Larsen and Higgins, 1992). Again, however, the main point is that wounds and stubs — not frost — initiate bark splitting or frost cracks, or what are called radial shakes in forestry. However, once started, the cracks may persist for the life of the tree because stress caused by many factors including frost leads to continuation of the crack.

Link to herbicides
Generally, pre-emergents should be recommended as the backbone of a nursery or landscape weed control program. Postemergents such as Roundup can be used in certain situations, but must be always used with extreme caution and forethought. Never allow any postemergent spray in contact with green bark of ornamental materials.

Newer herbicides containing glyphosate such as Roundup-Pro, Roundup-Ultra and Round-up Ultra Max have been formulated with surfactants to increase their penetration into green tissue.

Currently we do not recommend the use of gylphosate formulations with surfactants for use in nursery production due to an increase in injury problems that we have observed, and definitely not for sucker removal.

Achieving satisfactory weed control in field-grown nursery crops generally requires repeated applications of one or more herbicides (Gilliam 1989). Four pre-emergent herbicides that have been commonly used in such programs include Treflan, Devrinol, Princep, and Gallery. Princep, however, may cause injury to certain field-grown nursery crops, and so in recent years Gallery has become more widely used. Treflan is primarily active against small-seeded annual broadleaf and grass species. Sureguard is a newer preemergent that is excellent in controlling weeds and hopefully will be registered in Ontario soon for nursery fields.

Some growers and landscapers are still removing tree suckers with broad spectrum postemergent herbicides such as Finale glufosinate, Round and Round plus glyphosate and glyphosate-containing products. This practice is not recommended! Scythe pelargonic acid is the only broad spectrum postemergent with a label for sucker removal. Glyphosate is strongly adsorbed onto soil particles, rapidly deactivated in soil, and has no residual activity. Roundup is an aromatic amino acid inhibitor and is transported in the phloem. Yellowing of new growth, death of the plant in days to weeks, leaf malformations resembling injury from phenoxy herbicides (2,4-D), and a possible purplish cast to foliage of broadleaf plants characterize glyphosate injury symptoms. Examples of glyphosate containing products are Roundup Ultra, Rodeo, Accord, Wrangler, Laredo and Sulfosate (Touchdown).

Roundup is a phloem-translocated toxic chemical that moves primarily to new leaves and roots because of translocation to the meristems. Whether taken up by the roots or shoots, these compounds are moved through the living plant cells and phloem (symplastic movement) to both the root and shoot tips. The young tissue (shoots or roots) will be discoloured or deformed and injury may persist for several sets of new leaves. Other examples of phloem-translocated toxic chemicals beyond glyphosate, whether absorbed by the roots or shoots, include the herbicides 2,4-D, dicamba, picloram, amitrole, dalapon, sethoxydim and fluazifopbutyl. These compounds move to the meristems and typically injure the youngest tissues of the plant. Roundup breaks down readily in soil but not in the plant; therefore if overspray, uptake or drift problems occur in the fall or late summer, damage may not appear until the new growth begins the following spring. This is called carry-over injury.

We speculate that the removal of suckers and adventitious shoots, especially with broad spectrum systemic postemergents (especially new formulations with increased penetration), or shortly after mechanical removal of suckers, may be one reason for the increased severity and frequency of bark splitting.

Howell and Weiser (1970) noted herbicides should be used with caution around previously injured young tissue, as re-injury will occur in the breaks. Watch when removing suckers with a spade, or adventitious sprouts with pruning shears, that you do not damage the trunk. Herbicides should be sprayed before sucker removal, not after, and use no systemic herbicides in the nursery after mid-July. After mid-July use only Scythe and again always rely on your pre-emergent program versus your postemergents. Use of glyphosate in the field at a frequency of once per month, which is commonly done, is an indication your preemergent program is not working adequately.

Current progress
Sweetbay magnolia (Magnolia virginiana) and kousa dogwood (Cornus kousa) were planted in the field the week of May 21, 2007 on Waterman Farm at Ohio State University, Columbus, Ohio. Trees were all one-year-old bare root plants. There were two fertilizer treatments: 125 pounds per acre and 250 pounds per acre.

There were five herbicide treatments: Roundup Original Max, Roundup Pro, Kleenup Pro, Tillage, and a control plot. On December 3, five of the seven sub samples were dug out of the field. Roots were measured in a volumetric flask, to obtain displacement, after washing to obtain a baseline for root growth. Trees were placed in zip-lock baggies with a moistened 50-50 sand/perlite mixture with enough to cover roots and tied with wire ties. Trees were then put into a walk-in cooler set at 5ºC ambient temperature. Wooden boxes were placed in the cooler with heating mats set at 8º, 11º, 14º, and 17ºC. Each sub sample was given a number one through five that corresponded with four bottom temperatures and one in ambient temperature. Trees were left in cooler for 70 days.

After taking out of cooler, shigometer (Osmos, Buffalo, N.Y.) readings were taken on stems (2.54 cm above root line) and roots (on bottom-most tap root or crown). Volumetric measures were taken to determine root growth. To assess cold hardiness, one-three millimetre segments of stem (new growth-where applicable) and root (older roots). Two or three root and stem segments were put into test tubes and into an ultra-low freezer (Forma Scientific, Marietta, Ohio).

A sub-lethal dose of glyphosate reduced root hardiness in Cornus kousa but not Magnolia virginiana. We also found that Magnolia virginiana roots were never dormant and produced a significant root mass during the period of shoot dormancy. In contrast, Cornus kousa roots did exhibit dormancy and even deteriorated when placed in elevated root zone temperatures of 17 C. However, the effect of glyphosate treatments was most pronounced on Cornus versus Magnolia, indicating species variability in susceptibility to glyphosate causing increased cold susceptibility via possible inhibition of root dormancy.

Dr. Hannah Mathers is an associate professor in the Department of Horticulture and Crop Science at The Ohio State University (OSU), Columbus, Ohio and working under contract with the Ontario-based Vineland Research and Innovation Cantre to help the nursery and landscape industry with research initiatives, industry projects and extension activities.