Removing Orchards? Be Aware of Herbicide Residues

Apple growers may be planning to remove orchards, and need to consider the risk of carryover herbicides that may harm subsequent crops. Of particular interest are these commonly used apple herbicides:

• simazine (Princep, Simadex, Simazine)
• terbacil (Sinbar)
• diclobenil (Casoron)
• napropamide (Devrinol) (mostly of concern for cover or cereal crops)
• clopyralid (Lontrel) (mostly of concern for vegetables and soybeans) 

Most other soil applied herbicides (Dual II Magnum, Treflan, Sencor, Lorox, Kerb) would likely be degraded in a year, when applied at labeled rates.

Here are some general factors that affect the amount of residue:

• rate applied – lower rates cause less problems
• length of time since application – best to avoid applications in the year of removal, although some products may persist for several years
• soil pH – prefer 6.0 to 7.2 to promote herbicide degradation
• soil organic matter – higher OM encourages microbial activity and breakdown
• soil moisture – more herbicide degradation with good moisture, but saturated soil may also reduce activity
• soil management after removal (see tips below)

If you suspect your site may have herbicide residue problems, a chemical analysis may indicate if problems are present. However, the test is specific for each herbicide, and may be expensive. Contact the Pest Diagnostic Clinic about test availability and prices.

A greenhouse bioassay may give some helpful information, although the results may be difficult to interpret. Sample soil where the herbicide was applied e.g. under trees/vines and/or where overlap may have occurred. Include a sample of soil with no known herbicide residues for comparison. Grow at least four test crops – tomatoes, cucumbers, oats, lettuce or sugar beets may be sensitive. Allow at least 4-6 weeks for complete results. If injury is present, future cropping may be affected. However, this test is not a guarantee that no problems will result in future crops.

A few cultural tips to reduce problems with herbicide residues:

• Plowing may dilute the residue through a greater volume of soil – although avoid creating a zone of concentrated herbicide where future roots grow. Adjust the plough for a good shear with mixing action.
• Adjust soil pH to a more neutral level (pH of 6.0 to 7.2 is preferred).
• Improve organic matter by cover crops, mulch, manure, etc.
• Avoid application of any long-residual herbicides at least one, and preferably 2 years before removal of planting if possible.
• Note the location of tree rows to avoid in future plantings. If possible, changing row alignment (e.g. N-S to E-W) may be helpful although sunlight interception needs to be considered.
• Avoid high value crops in the year after removing. Cover crops may also be injured but can still be beneficial in renovating old orchard sites.

Related Link:

• Pest Diagnostic Clinic

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