Weed Management in Fruit Trees Starts with Fall Herbicide Application

By Sushila Chaudhari

Fall is a perfect time to apply residual and foliar-active herbicides in established tree fruit, especially to manage hard-to-control perennials such as woody perennial vines and trees, Canada thistle, dandelion, and quackgrass, and winter annuals such as horseweed or marestail, white campion and yellow rocket. Fall herbicide application should be made after fruit trees are dormant. Under those conditions, there is little chance for crop injury, so herbicides can be sprayed against the trunk and stems of the crop and into the center of bushes. This allows for coverage of 100% of the area in the tree line. In spring, new growth often appears and bark softens before herbicides are applied. This can result in tree injury.

Herbicide application in fall appears to be counter-intuitive in that the intended activity is five to six months away. However, experience has shown that herbicides applied in fall outperform the same treatments applied in the spring. Evidently, cold soil temperature reduces herbicide degradation, and rain moves the herbicide into the upper layers of soil where it is active as soon as weeds begin to germinate in spring. Little herbicide is lost by runoff or wind erosion during winter.

Most of the long-residual herbicides labeled for fruit trees may be applied in fall (Table 1) or spring. Growers normally should use at least three residual herbicides with different modes of action each year. Each time a residual herbicide is applied, a foliar active herbicide should be included in the tank-mix. A tank mix with post-emergent herbicides helps to clear the ground under the trees, which is critical for preemergence herbicides to reach the soil surface. It is also advisable to mow or use a hedge trimmer to remove the large or dead vegetation under trees before applying any pre-emergence herbicides because this will help the residual herbicide penetrate the soil that is essential for herbicide activation and control of emerging weed seedlings. In fall and early spring, glyphosate or an auxin disruptor should be used.

When there is new growth on trees, paraquat (Gramoxone), carfentrazone (Aim) or glufosinate (Rely) should be used to kill weeds but cause no long-term injury to crop plants. Inclusion of pyraflufen-ethyl (Venue) with these post-emergence applications improves burndown and broadens the weed control spectrum.

Some herbicides are more effective when applied in fall. Pronamide (Kerb) is most effective against quackgrass in fall. Dichlobenil (Casoron) granules traditionally have been applied in fall, which allowed the herbicide to disperse into the soil over winter, and provide three to four months of perennial weed control next season. The liquid formulation of Casoron appears to be equally effective in fall. However, several uses are only on the granule label.

Post-emergence herbicides, such as glyphosate (Roundup), 2,4-D, clopyralid (Stinger) and fluroxypyr (Starane), are often more effective against annual and perennial weeds when applied in fall. For example, glyphosate applied in fall provides very good horseweed control into next summer if the population is not glyphosate-resistant. Evidently, horseweed seeds germinate in fall and the small seedlings remain alive near the soil surface. They are killed by glyphosate fall application. Glyphosate applied in the same area in early spring is less effective than when it is applied in fall. In case of glyphosate-resistant horseweed population, use other herbicide options such as Saflufenacil (Treevix), Terbacil (Sinbar), indaziflam (Alion) and Oxyfluorfen (GoalTender).

Fall is a good time to take action for control of woody perennials such as poison ivy, Virginia creeper, wild grape and tree seedlings (poplar, maple). At this time, perennial plants translocate carbohydrates towards underground plant parts such as crowns, rhizomes and fleshy roots to reserve food for starting growth during the following season. The application of systemic products such as glyphosate is appropriate as it will translocate to kill below-ground parts and inhibit growth during the upcoming season. For effective results, these perennials should be treated before leaves senesce. But extreme care should be made at the time of application so that glyphosate does not come in contact with fruit trees. In most cases, these woody perennials may need to be removed manually from the orchards.

An effective weed control plan might include pronamide (Kerb) plus norflurazon (Solicam) plus glyphosate in fall, followed by simazine (Princep) plus flumioxazin (Chateau) in the spring. Glufosinate (Rely, Lifeline, Reckon) or paraquat (Gramoxone) may be applied in June or July to kill emerged weeds. In fall, indaziflam (Alion) may be applied with glyphosate for preemergence control the following year. In the second spring, apply diuron (Karmex) plus oryzalin (Surflan) plus paraquat, followed in mid-season by Aim plus Venue plus fluazifop (Fusilade). Saflufenacil (Treevix) may be applied during the growing season to kill emerged broadleaves, including horseweed. It may be applied four times with zero days before harvest.

By applying herbicides several times in a season, fruit growers will improve overall weed control and reduce potential for weed resistance. They also will reduce potential of crop injury from over-use of some chemicals.

Michigan State University Extension bulletin E0154, “Michigan Fruit Management Guide,” Herbicide section, contains lists of all currently labeled herbicides for each crop and the preferred time of year of application. The Herbicide section includes a table that lists the mode of action, solubility in water and the soil half-life for each herbicide. Use this table (along with herbicide label) and the accompanying herbicide effectiveness table against specific weeds to select herbicides of different modes of action for various applications during the year.