2024 Post-Emergent Herbicide Reminders
Mike Basedow and Janet van Zoeren
Hopefully everyone found a good window to get pre-emergent materials on earlier this spring or last fall. Now that we are approaching summer, you may be planning to do some follow up herbicide applications to clean up what came up through your pre-emergent application, and to also prevent some of the late emerging summer annuals. In addition to possibly applying another pre-emergent material, you will likely need some post-emergent products for your remaining herbicide applications. Not surprisingly, post-emergent herbicides vary in their performance characteristics, so we put together some reminders of some of them.
For post-emergent materials, we usually first make the distinction between contact “burndown” herbicides and systemic herbicides. As the names suggest, contact materials only burn down whatever tissues they land on at the time of application, whereas systemic materials will work their way through the plant to suppress or control the entire plant.
Here are some of the post-emergent products labeled for orchard use, along with some of their key characteristics:
Contact Materials
Gramoxone and generics (Paraquat) – Group 22. A good choice for small annual broadleaves and grasses. Best control when weeds are between 1 to 6 inches in height. Larger weeds may be more difficult to control. The label recommends tank mixing with systemic products for perennial broadleaf weeds such as Canada thistle, bindweed, and dandelion, and for difficult to control annuals like giant ragweed and morning glory. Tends to work best when applied under hot conditions, and results of this application tend to be apparent shortly after application, unlike many other products. Can be used the year of planting. Do not allow spray to contact green stems (except suckers), or foliage. Use a shield when spraying around young trees. Restricted use, newer labels require additional training, and the use of a closed transfer system. No PHI listed in apple.
Rely 280 and generics (Glufosinate) – Group 10. Recommended for annual broadleaves and grasses. The label says it will help to control some of our problem perennials like bindweeds, Canada thistle, quackgrass, etc. Works best when applied under warm temperatures, high humidity, and bright sunlight. Pay close attention to the rate recommendations for weed heights. When weeds are greater than 6” in height, you’ll want to use the higher rate of 56-82 fl oz/A. This is also the rate I would be using to control those more difficult to control perennials. Avoid contact of RELY 280 solution, spray, drift or mist with green bark, stems, or foliage, as injury may occur to trees. Only trunks with callused, mature brown bark should be sprayed unless protected from spray contact by nonporous wraps, grow tubes, or waxed containers. Contact of RELY 280 with parts of trees other than mature brown bark can result in serious damage. 14 day PHI in apple.
Aim, Treevix, Venue – Group 14. These herbicides work best on small, annual broadleaves. Aim < 4 inches tall. Treevix < 6 inches for most weeds listed on the label, and Venue < 4 inches. These materials will not control grasses. Aim can be applied in the planting year, but do not allow Aim spray solution to contact green stem tissue, leaves, fruit, or blooms of trees. For Treevix, trees must be established for at least one year, and trunk shields should be used until adequate bark has formed to protect trees from potential herbicide injury (typically by 2 to 3 years after establishment). For Venue, avoid contact with green, uncallused bark of young trees established less than one year unless protected from spray contact by nonporous wraps, grow tubes, or waxed containers. Venue is labeled for controlling root suckers when they are young, prior to hardening off. Aim has a 3 day PHI in apple. Treevix, 0 day PHI in apple. Venue, 0 day PHI in apple.
Organic Contact Materials
For organic growers, there are a few herbicides as well. These are predominantly contact materials. I often see the following chemistries of organic contact herbicides.
Final-San-O – Ammoniated soap of fatty acids. The label lists control of a few key broadleaf and grass species commonly found in New York orchards. The label recommends applying to weeds < 5 inches tall, while they are young and actively growing.
Suppress, Homeplate – Caprylic acid + Capric acid. Best on weeds < 6 inches in height. Higher concentrations are recommended for larger, more difficult to control weeds, ranging from a 3-9% product solution. Homeplate is labelled for sucker control when used before suckers become woody.
Grass Systemics
Below are some of our systemic herbicides labelled for use in the orchard. Since these materials move into the plant, these tend to be the materials we recommend using to get a handle on those tough to control perennials.
Poast, Select Max, Fusilade – Group 1. These materials will control grasses, with very little to no activity on broadleaves. The best application timing will depend on the height of the grass you are trying to control. As an example, for quackgrass, the Poast label recommends applications when grasses are < 8 inches tall, and Select Max when they are between 4-12 inches tall. Poast can be applied the year of planting. Fusilade is limited to applications to non-bearing apple. Poast has a 14 day PHI in apple. Select Max, 14 day PHI in apple.
Poast suppressing quackgrass in an Eastern NY Trial.
Broadleaf Systemics
2,4-D Amine, Weedar 64, Unison (2,4-D products) – Group 4. These products will control or suppress many species of annual and perennial broadleaves. For problematic perennial weeds, the best control will be when weeds are in the early bloom to bud stage, or during the period of fall regrowth after harvest. Trees must be at least 1 year old and in vigorous condition. Do not apply to bare ground as injury may occur, and do not allow spray to drift onto or contact foliage, fruit, stems, trunks, or exposed roots, as injury may result. Do not apply under conditions favorable for drift. 14 day PHI in apple.
Stinger (Clopyralid) – Group 4. This product controls a shorter list of broadleaf weeds compared to 2,4-D products. Stinger is labeled for aster, burdock, clovers, dock, dandelions, nightshades, thistles, and some leguminous species, amongst a few others in orchard applications. Apply Stinger to clover and vetch from weed emergence up to the 5-leaf stage of growth. Apply Stinger to nightshade (black and hairy) at the 2- to 4-leaf stage of growth. For control of Canada thistle and annual sowthistle, apply Stinger from rosette up to bud stage. Can be applied to trees one year after planting and older. Avoid direct contact with foliage, fruit, or tree trunks. 30 day PHI in apple.
Stinger followed by Rely (Orange) suppressing Canada Thistle compared to Rely alone (Blue) in an Eastern NY Trial. Photos taken 3 weeks following Stinger application, and 11 days following Rely application.
Non-Selective Systemics
Glyphosate – Group 9. Glyphosate is a non-selective, systemic herbicide, and will control many annual and perennial weed species. For annual weeds, the rate is determined by the height of the weed. For perennials, timings will vary widely by the weed being controlled. We generally do not recommend using glyphosate past the first week of July in New York orchards though, as the potential for crop damage goes up the later in the season it is used. Contact of glyphosate to anything other than mature brown bark can result in serious crop damage. Extreme caution must be used to avoid contact with foliage of green bark of trunk, branches, suckers, fruit, or other parts of the tree. Avoid applications where recent pruning wounds are present. Apply with selective equipment (directed sprays, hooded sprayers, shielded sprayers, wipers) to minimize the potential for drift injury. 1 day PHI in apple.
If you’d like to discuss your particular weed management strategy, give Mike a call at 518 410 6823 or email at mrb254@cornell.edu.