Across the state, pumpkin plantings are hitting their stride with varying degrees of success.
The grower phone calls I’ve received lately revolve around options for managing emerged weeds. Broadcast options are limited to grass-selective herbicides such as sethoxydim, clethodim, and halosulfuron (Sandea®) for small broadleaf weeds and sedge suppression. All of these products can result in some minor crop injury—even from the adjuvants used to improve their activity.
Another less popular option is the use of post-directed or hooded applications of non-selective herbicides between rows. In 2022 and 2023, we compared the crop safety of post-directed applications of glyphosate (Roundup® and others), carfentrazone (Aim®), and glufosinate (Rely® and others). Glyphosate and carfentrazone are labeled for post-directed use in pumpkins, and we were hoping to document that glufosinate was a fit as well.
Here’s what we did
We planted ‘Bayhorse Gold’ jack o’lantern pumpkin seeds in June and allowed them to grow for 5 weeks. At 5 weeks, we turned all the vines in one direction and used a backpack sprayer to make a “sloppy” post-directed application, contacting approximately 10% of each vine starting at the tip (Figure 1). We applied the products listed in Table 1 plus 0.25% non-ionic surfactant by volume. We rated for visible pumpkin plant injury 1, 2, 4, and 6 weeks after treatment on a scale of 0% (no injury) to 100% (dead pumpkin plants). At harvest, we weighed each fruit and graded them into orange or green based on rind color.
Figure 1. Celia Corado makes a post-directed herbicide application to pumpkins at the Pinney Purdue Agriculture Center in 2023 (Photo by S.L. Meyers).
Table 1. Herbicides applied in post-directed research trials in pumpkin in 2022 and 2023.
What did we find?
Between 1 and 4 weeks after treatment, injury was greatest from the glyphosate treatment: 32% at 1 week after treatment, 21% by 2 weeks after treatment, but less than 10% by 4 weeks after treatment (Figure 2). Carfentrazone (Aim®) and glufosinate (Liberty®) resulted in less injury than glyphosate and were below 10% injury by 2 weeks after treatment. Injury from carfentrazone and glufosinate were localized necrosis- dead spots and patches of leaf and stem tissue (Figure 3). In some plants, the primary growing point was killed by carfentrazone and glufosinate, but secondary branches emerged. Pumpkins treated with glyphosate exhibited yellow and white leaves at the growing points. These results are expected because carfentrazone and glufosinate are generally contact herbicides and glyphosate is systemic.
Figure 2. Effect of post-directed herbicide application on pumpkin crop injury 1, 2, 4, and 6 weeks after treatment (WAT).
Figure 3. Leaf injury symptoms following post-directed herbicide treatments (. Photos by C. Corado).
When it came to yield, there were no statistical differences for orange or total pumpkin number or weight for any treatment (Figures 4 and 5).
Figure 4. Effect of post-directed herbicide application on pumpkin fruit/plot. One plot is 12 pumpkin plants.
Figure 5. Effect of post-directed herbicide application on pumpkin yield (kg)/plot. One plot is 12 pumpkin plants.
Take-home message
Row middle applications are not for everyone, but they can be an effective piece of the weed management puzzle. Data from our small plot research suggests that their benefits may outweigh their risks. As with any change in your production system, the use of post-directed herbicides between pumpkin rows should be done on a trial basis first. Given the lack of postemergence weed control options in pumpkins, it is important to emphasize the need for relying on multiple control measures that may include cover crops, cultivation, and the use of soil-applied herbicides at planting.