Weed Control in No-Till Pumpkin – UKREC - 2005
Joseph Masabni, Joe Williams, Department of Horticulture, UKREC

Pumpkin is a crop that has seen recent increase in the planted acreage in Kentucky. More growers also benefit from double cropping their fields by planting pumpkins after winter wheat. Not only they gain 2 crops in one season, but they also benefit from the wheat stubble left after harvest as a natural weed control barrier. This is becoming more popular with growers that some are sacrificing a fall-planted rye or wheat with an herbicide kill, pushing the dead straw down and no-till planting pumpkins on the heavy straw stubble.

One obvious advantage is that expensive or selective herbicide have to be used only in the planting strip instead of the whole field.

In order to evaluate this system, modified to adopt transplanted pumpkins instead of seeded pumpkins, an experiment was started in fall 2004 with the drill seeding of winter wheat at about 90 lb/A. Wheat was burned down with Gramoxone on 1 June 2005, which was later rolled down to provide a thick cover.

Herbicides were applied using a CO2-pressurized backpack sprayer with a 4-nozzle boom calibrated to spray a 5 ft band at 30 psi and 3 mph walking speed. The 8002-nozzles were set at 17" above ground level to obtain good spray overlap and complete weed coverage.

Plots were 10 ft x 35 ft long. The experimental design consisted of a randomized complete block with 3 replications. Each plot was sprayed with 2 passes of the boom to cover the 10 ft plot width.

Herbicide treatments were applied on 17 June 2005, one week after transplanting pumpkins. Two varieties of small-fruited pumpkins were planted in each plot; cotton candy (white) and hybrid Pam (orange). These varieties were used instead of the traditional Jack-O-Lantern pumpkins because of increasing interest in smaller fruits suitable to carry by kids. All treatments were applied early in the morning with average wind speed of 2.5 mph.

Visual weed control ratings were collected at various dates. The scale used in these ratings were on a 1-10 scale, with 1 = no control and 10 = complete kill or no weeds present. A rating of 7 (70-75% control) or more is considered a commercially acceptable value.

Few grasses and broadleaves were present in the field. Therefore, no specific evaluation of each individual species was conducted. Instead, weeds were pooled as either broadleaves (BL) or grasses (GRASS) during the visual control ratings.

Treatments 1 (T1) and 2 (T2) are currently labeled herbicides on pumpkins. T1 was sprayed at 2X rate to determine if any injury is possible with high Sandea (labeled rate is 0.5-0.67 oz, with 2 oz max. per crop).

Treatments 3-10 are not labeled herbicides and must not be used by growers under any situation. I remind growers that use of non-labeled herbicides is against the law. The herbicides are listed here so that growers can observe the severe injury possible with these herbicides.

A final but important point to consider when reviewing the results, is that the treatments were applied 1 week post-transplanting (POST-TR) the 2 pumpkin cultivars. Plants were about 4" tall with 1-3 true leaves at time of transplanting. This experiment was designed specifically to determine the worst-case scenario when testing potential new herbicides for use on pumpkins. The logic behind this design, is that if a new or non-labeled herbicide was safe when applied POST-TR, then chances are high that it would be safe if applied preemergence (PRE).

When visual injury ratings were taken 11 days after herbicide application (Table 1), Spartan (T5+T6), Chateau, and Matrix had excellent grass and broadleaf weed control, but significantly injured pumpkin growth of both cultivars, with 60-90% injury range. By 30 days after treatment (Table 2), the overall weed rating for all treatments ranged between 58-95%. The weediest plots were observed with T3 and the cleaned with T6. In general, weed control improved between 11 and 30 days for most treatments, except for a slight drop for T1 (Sandea) and a significant drop for T10 (Matrix).

Cotton Candy (white pumpkin) was observed to be more sensitive overall to herbicide injury than Hybrid Pam (orange pumpkin). Treatments 1,6,9,10 significantly reduced both number and weight of fruits per plot. Outlook and Define, at both low and high rates, didn’t show any statistical decrease in yield data. However, actual yield values decreased by 13% and 18% for Outlook T3 and T4, respectively and by 25% and 30% for Define T7 and T8. Therefore, even if Outlook and Define appear to be safe on the 2 pumpkin cultivars tested, a considerable yield decrease was observed, which may not be economically suitable for growers. This was a dry year in Kentucky, growers are encouraged to consider the potential injury had we had the normal or possibly excessive rainfall.

Fruit weight did not differ significantly among all treatments, except for Chateau that seriously injured both plant growth, vigor and yield. With Cotton Candy cultivar, Chateau, Spartan (high rate), and Matrix had significant injury both on plant growth and on fruit set and size. The same plots were the cleanest in terms of weed pressure 1 month after application with 70-100% weed control.

On Hybrid Pam, only Spartan (high rate) statistically reduced both number and weight per plot. Sandea 2X rate (T1) didn’t appear to negatively affect Hybrid Pam or reduce its yield performance. Plots treated with Spartan (low rate, T5) and Define (high rate, T8) had higher number and yield of pumpkins, but absolute values were not statistically different from T2 control. No significant differences were observed on fruit size with any treatment. Hybrid Pam grew out of Spartan (low rate, T5) initial injury (11 days after treatment). However, high rate of Spartan (T6) resulted in significant season-long injury.