When I was young my father would sing—occasionally. My Dad’s songs were mainly about cowboys, who worked hard, died young, their favourite horse or girl and usually a loss in some fateful misadventure. My favourite song was, “When the work’s all done this fall.” I loved the story of it, and the part where the hero of the song is fatally injured (of course not so fatally that he can’t get off another 20 or so verses):

“…That night, this cowboy
Went out to stand his guard.
The night was dark and cloudy
And storming very hard.
The cattle they got frightened
And rushed in wild stampede,
That cowboy tried to herd them
While riding at full speed …
His saddle horse did stumble
And on him it did fall.
Now, poor boy won’t see his Mother
When the work’s all done this fall…”

In this song the roundup goes all wrong and takes a tragic turn. In the world of ornamental weed control, I have also seen the use of Roundup (glyphosate), when not used with discretion and extreme care, ending in tragedy for some ornamental plants — who “won’t see their Mothers when the work’s all done this fall!”

Generally, preemergence herbicides should be recommended as the “backbone” of a nursery or landscape weed control program. Achieving satisfactory weed control generally requires repeated applications of one or more herbicides (Gilliam 1989). Two applications, one in the spring (March) and one in the fall (August) of preemergence herbicides are recommended. Using less than two will result in more frequent use of postemergence herbicides and potential injury to ornamental plants. Postemergence herbicides are like the “big-guns” in your weed control program, and limiting their use to rescue treatments and perimeters is best. Postemergence spray contacting with green bark on ornamental plant materials should always be avoided! Newer herbicides containing glyphosate have been formulated with surfactants to increase their penetration into green tissue. Currently we do not recommend the use of gylphosate formulations with increased surfactants for use in nursery production, due to an increase in injury problems that we have observed, and definitely not for sucker removal.

Herbicides should be sprayed before sucker removal, not after, and the use of systemic herbicides in the nursery after mid-July is not recommended. Many people believe you should remove the sucker and then apply post-emergent, as the presence of the sucker increases the uptake of the herbicide into the plant. However, applying post-emergent systemic herbicides after removing the sucker is similar to conducting a basal stump application, since you are applying it to a cut surface. Sucker removal after systemic applications become more important as the season progresses. Fall is the time when plants are accumulating stores in the roots and systemic postemergence applications are recommended for killing stumps and perennial root systems. Therefore, after mid-July, use only your preemergence versus postemergence herbicides for weed control. Again, use of glyphosate in the field at a frequency of once per month (or more), which is common in some nurseries, is an indication your preemergence program is not working adequately. If you need help with your preemergence program email me, mathers.7@osu.edu.

Sucker removal
The best way to do sucker removal is with a sprout inhibitor, or formulations containing naphthaleneacetic acid (NAA), to inhibit the development of adventitious shoots. These products replace the apical dominance of the tree, which is why the tree is suckering, i.e. due to disruption of the normal apical dominance of the plant. Mechanical removal of suckers, which is another common practice, has been shown to increase sprouting, as this creates a wound response in the plant and initiation of more adventious shoots. Paint or spray the NAA products onto pruning cuts after sprouts are removed. A one per cent solution applied to run off, to the lower 30 inches of trunk in March or April when growth has just begun and all leaves are removed in the area to be sprayed is recommended.

Confusing but not better
Roundup (glyphosate) has been labeled for use in ornamental plantings for over 30 years. It has been utilized in the nursery and landscape industries extensively due to the numerous attributes that it possesses. Some of these attributes are limited soil activity, non-volatile, broad-spectrum, systemic, low environmental impact, ability to translocate throughout the plant, ease of use, and low mammalian toxicity. Roundup went off patent in 2000, thus opening the market to many generic brands of glyphosate, each utilizing various surfactants with varying doses. Consumers tend to want a faster-working product, thus the makers of glyphosate products have incorporated many surfactants to break down the cuticle of the plant.

These products have been sold as providing better weed control — which has not been shown in research — and worse, if contact is made with nursery or landscape plants, injury may result. Research is currently in place at Ohio State University to study different glyphosate-containing products with varying surfactants and we have recently found a significant reduction in trunk cold hardiness with the use of newer (increased surfactant) formulations.

Trying to find a product like the “original” Roundup is like entering a maze! Roundup originally was sold with isopropylamine salt. However, today you can find glyphosate containing products labeled with four salts, Isopropylamine (ipa), trimethylsulfuronium, diammonium (NH4)2 or potassium . There are glyphosate products with three different kinds of surfactants and full partial or no load, nonionic, cationic (polyoxyethylene tallow amine) and transorb surfactant system. Concentrations are also different. Products are labeled using acid equivalents or as active ingredient — #/ac 3 to 5 plus salt 4 to 6.7 making it very difficult to compare product to product for activity. In short — buying glyphosate has become one of those mystifying experiences, where you begin to wonder if Amelia Earhart and other incidences in the Bermuda Triangle are not somehow linked to glyphosate. Listed below are various labels for glyphosate in today’s market:

1.3 lb. ae/gal. ipa salt with full, partial or no adjuvant load
2. 3 lb. ae/gal. (NH4)2 salt with full adjuvant load
3. 3.7 lb. ae/gal. ipa salt with full adjuvant load
4. 4 lb. ae/gal. ipa salt without adjuvants
5. 65% SG NH4 salt with full adjuvant load

Sub-lethal dosing As indicated above, glyphosate has been utilized in the nursery and landscape industries for years due to its numerous attributes, including readily breaking down by microbes within the soil. However, although glyphosate is broken down within the soil, once in the phloem of a plant, it may take years to break down. Exposure to an ornamental plant through green bark is considered a sub-lethal dose and thus can cause injury to the plant. Absorption of glyphosate is possible with thin bark or bark with pigment (Kuhns 1992). Also, it is suspected glyphosate may accumulate in mulch and contribute further to glyphosate injury. Roundup can have carry-over if applied to peat (John Ahrens, personal communications, 2007) so accumulation in mulch is a possibility. Injury symptoms from sub-lethal doses of glyphosate may not be present up to two years after absorption of the glyphosate occurred (Kuhns 1992).

There are several symptoms that will occur when a woody plant is exposed to sub-lethal doses of glyphosate. These include witches broom, stunting, bark cracking or splitting, loss of apical dominance, individual dead limbs (Mathers 2006), chlorosis, and/or death (Kuhns 1992), (Ferrell et al. 2006). We speculate that the removal of suckers and adventious shoots, especially with broad spectrum systemic postemergents (especially new formulations of glyphosate) 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. Glyphosate injury is difficult to diagnose for two main reasons: the amount of time the herbicide is contained within the plant and the numerous symptoms that may occur. There are several ways to minimize the risk of glyphosate injury. These include calibrating spray equipment, applying the correct amount of glyphosate, making two passes in rows to prevent overspray and taking extra caution when spraying around thin barked trees (Kuhns 1992).

Summary Around ornamental plants use a glyphosate product that has no adjuvant load, meaning a product that indicates you need to add an adjuvant — but don’t. The products that have a full load of adjuvant are the worst around ornamental plants with green bark, as these already have a surfactant (you have no idea what) that will increase their uptake into your plants. Elude the tragedy of roundup time. Use caution with these and all “big-gun” postemergence herbicides and rely on your “back-bone” products.

References Ferrell, J.A. et al. 2005. Diagnosing Herbicide Injury. University of Florida Cooperative Extension Service. SS-AGR-15. 1-28. Kuhns, L.J. 1992. Glyphosate Applications to the Bark of Nine Tree Species. Proc. Northeast Weed Sci. Soc. 46:23-26 Mathers, Hannah. 2006. Herbicide Injury. NMPro.

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