|
Hannah Mathers, Ohio State University Extension, Horticulture and Crop Science |
Herbicides are classified as selective or nonselective. Selective herbicides control weeds without injury to the ornamental plants in the same application area. The tolerant ornamental plants will appear on the label. The nonselective herbicides will kill any plant or plant part they are applied to and are recommended for non-crop areas or as directed sprays only.
An example of a selective ornamental herbicide is Fusilade. It is a selective postemergent and kills grasses but does not control broadleaf weeds. Fusilade can generally be applied over-the-top in non-grassy ornamental landscape beds and/or ground covers; however, the label does indicate certain ornamental plants, such as Juniperus horizontalis, that will be affected by over-the-top sprays. An examples of a nonselective herbicide is Roundup. Nonselective herbicides should never be applied over-the-top of ornamental plantings.
With increased use of herbicides, in some situations, nursery growers are concerned about herbicide persistence or residues affecting subsequent crops. Some preemergents persist in the soil, and some soil sterilants used along railroad embankments or under power lines are extremely persistent. Soils can be chemically analyzed for herbicide residues, but this is expensive, complicated, and can be done only in specialized laboratories. Moreover, the results of the analysis do not indicate the effects on the next crop.
An inexpensive and fairly reliable way to determine herbicide carryover is to do a crop biological assay, or a bioassay. A bioassay involves planting a sensitive indicator plant into soil collected from a site, then evaluating whether symptoms of herbicide injury develop. Bioassays should also be done if you are not certain of the previous cropping history of the field. Soil sampling for a crop bioassay is similar to sampling for fertilizer levels. Samples should be gathered from several areas of the field. Remember that the assay is only as reliable as the sample collected.
An example of a persistent preemergent is Casoron, which is tied up by organic matter and is slowly decomposed by soil microbes. Casoron residue damage generally shows up as one-directional rooting. Other injury symptoms of Casoron include leaf yellowing or veinal, interveinal, marginal, or overall chlorosis. At low concentrations, a halo effect, or marginal chlorosis, is evident. Injury appears on the new growth.
Simazine or Princep can build up in soils with repeated applications over several years. Simazine injury appears as yellowing or veinal, interveinal, marginal, or overall chlorosis. Injury appears first in the new growth, since the chemical is translocated to the growing point. Plants may outgrow injury, which is the result of low concentrations. The whole leaf may become chlorotic at high concentrations. Atrazine residues are more serious in northern climates as inactivation is very slow below 75F.