Oregon Strawberry Commission
Research Progress Report

1994 - 1995

Weed Control In Strawberries with Clopyralid (Stinger)

Ed Peachey and Garvin Crabtree
Horticulture Department, OSU


The cost of controlling weeds and the consequences of poor weed control represent a significant portion of the enterprise budget in strawberry production. Effective management of weeds in this crop has been dependent on the use of several chemical and cultural measures. Even with this integrated approach, certain weed species such as Canada thistle continue to be a problem.

Previous research has demonstrated the potential for selective weed control with clopyralid in strawberries and provided the basis for submitting to EPA a request for registration. More use data are needed in Oregon to make this an effective weed control program, however. A knowledge of the best timing of clopyralid applications for maximum weed control and crop selectivity is critical to best use of this herbicide. Significant yield reductions were seen following spring applications of 0.25 lbs ai/A, indicating that a split application of clopyralid may be essential to limit crop injury and achieve adequate thistle control. Also, fall applications in mint appeared to improve Canada thistle control and improve crop tolerance

Crop tolerance with repeated use of clopyralid was evaluated from 1990-1993 over a range of fall application dates with or without a spring application. Although strawberry yields were not significantly affected by clopyralid applications in these trials, there was evidence that clopyralid applied at greater than 0.25 lbs ai/acre during flower bud initiation decreased strawberry yield. Moreover, there was evidence that the effect was more severe on older plantings. Therefore, the objectives of this research were to again assess: 1)single and split application rates (mid-October application coupled with May application) for strawberry tolerance to clopyralid in a 'third year' planting and 2) thistle control with the same application timing.


An experiment was established in the fall of 1993 with plots 25' long by 10' wide (40 inch row spacing) and treatments applied in a randomized complete block design with 4 replications. Clopyralid was applied at 3 rates on October 19, 1993 to both 'single fall' and 'fall plus spring' herbicide treatments (see Table 1 for treatment description). Spring treatments were applied on April 30, 30 days before predicted first harvest. Growth response of strawberries to clopyralid was evaluated on November 11, 1993 and May 20, 1994. Strawberries were harvested on June 6 and June 12, 1994 from two 7.5 foot sections of the two middle rows. Percent irregular berries and average berry weight were determined from a 25 berry sample. Six rows on the east side of this plot were Benton variety while the remainder were Totem. Yield data were adjusted to account for slight differences in yields of the control plots between these two varieties.

In addition, a second plot was established in the spring of 1993 at the Lewis Brown Horticulture Research farm near Corvallis, OR in an area infested with Canada thistle. Treatments were applied to the Canada thistle on the same dates and rates as at the Mt. Angel site (See Table 1 and 2). Emerged thistle shoots were counted on April 30, 1994, and thistle control estimated visually on a biomass basis on April 30, 1994 and November 18,1994.

Results and discussion

Total strawberry yield was unaffected by the application of clopyralid at the timing and rates listed in Table 1. Average berry weight may have been reduced in some treatments but the results are inconsistent. The high rate of clopyralid (0.25 lbs ai/ace) applied in the spring (Treatment 6) increased the percent of irregular fruit. However, there was no effect on number of irregular fruit in the fall/spring split application at the highest rates (Treatment 4), indicating that the increase in irregular fruit may not be related to clopyralid.

Thistle control after the winter of 93/94 was best for the fall application of clopyralid (See Table 2). However there was still a strong residual effect from the spring application of one year before. At the November 94 evaluation, all treatments with clopyralid performed well with the exception of the low fall rate. Both fall and spring applications were effective at controlling Canada thistle.

These results confirm previous experience and indicate that the split application of clopyralid with a total of 0.375 lbs ai/acre will provide excellent control of thistle with little chance of damage to the strawberries. However, this field of strawberries was exceptionally vigorous for the third year of production, and possibly did not fully test the effect of clopyralid on fields in serious decline. This trial will be kept in place for one more year during the fourth year of production.