MSU Extension Vegetables

What caused this asparagus fern to “get kinky” and zig-zagged?  

The answer is… injury from auxin mimic herbicides, a site of action that includes dicamba, clopyralid and 2,4-D. These herbicides cause altered plant growth, because they mimic the action of a natural plant hormone called auxin.  

These symptoms are most noticeable as fern develops. Sometimes, the pattern and timing clearly support herbicide injury. For example, one grower observed this injury in parts of the field where his sprayer passes overlapped, during a postharvest application. Others have noticed injury where herbicides like dicamba were applied as a spot spray to tough weeds. In yet other cases, the injured area was the width of a sprayer pass, diminishing in the direction of sprayer travel, indicating incomplete tank cleanout. Sometimes when the injury appears growers are left scratching their heads, with injury seemingly random throughout a field. This left us doubting our diagnoses–could the zig zag growth be something else, or is it in fact herbicide injury? And do all the auxin mimics cause it, or just specific ones? 

Our team at the West Central Michigan Research and Extension Center set out to do a simple demonstration to answer this question. A single row of asparagus was mowed on June 19, 2024, around the end of commercial harvest. Herbicides were applied with a backpack sprayer on June 21. This mimicked the situation where this is a slight delay between the end of harvest and herbicide application. We included two herbicides from different site of action groups for comparison to the auxin mimics: a contact-only herbicide (i.e., carfentrazone) and a common systemic herbicide (i.e., glyphosate). 

Temperatures were around 85 degrees Fahrenheit after mowing, so some spears had emerged when herbicides were applied. Pictures were taken on July 1 (12 days after application[DAA]) and August 28 (70 DAA) and 30 (72 DAA).  

Carfentrazone 

Carfentrazone (i.e., Aim) is a PPO (Protoporphyrinogen oxidase) inhibitor with contact-only activity. By July 1, twelve days after the June 19 application, spears that were partially injured by carfentrazone grew into a shepherd’s crook, with tightly coiled tips (Figure 2). Asparagus crooks when one side of a shoot is injured, but the less injured side still grows. These partially injured shoots remained short. Next to the injured shoots, healthy shoots were growing with normal branches. This demonstrated that only the emerged spears were showing injury from the contact herbicide, while those that emerged after the application were symptom-free.  

By late August, all the shoots that had developed into tall fern appeared healthy with normal branches. Stunted, crooked shoots that were injured in the June 19 application were still visible. Some injured shoots never branched, but others developed branches at their coiled tip (Figure 3). 

Samples of injured fern were taken in August to obtain higher quality images. The shoot pictured in Figure 4 developed dense fern from branches at the coiled tip. 

A closeup shows brownish-white dead tissue injured by carfentrazone, with the shoot curling towards the injured side (Figure 5).  

The fern that did develop on the injured stem, had normal, straight branches (Figure 6). 

Glyphosate 

Glyphosate (i.e., Roundup) is a systemic 5-enolpyruvylshikimate-3-phosphate (EPSP) Synthase Inhibitor (group 9), that prevents the formation of certain amino acids. Twelve days after the postharvest application, some shoots had developed a short, bushy appearance. Numerous branches developed that were “stacked on top” of each other along the main stem. Many normal, healthy shoots were also emerging, growing and branching (Figure 7). 

In late August, all the shoots that had developed into tall fern appeared healthy with normal branches. The short, bushy, injured shoots had normal branches (Figures 8-10).  

2,4-D 

2,4-D is an auxin mimic with systemic activity. Twelve days after the postharvest application, injured shoots had twisted into a zig-zag and curly-cue appearance before they stopped growing (Figure 11). Healthy, normal shoots were also emerging, growing, and branching. 

In late August, all the shoots that had developed into tall fern appeared healthy with normal branches. Some of the misshapen shoots injured by the June 19 application were still green and visible at the base of healthy fern. They had not grown and had no or very short, limited branching (Figures 12-14). There were also some green, shorter, shoots that developed zigzagged branches (Figure 15). Other injured shoots had died and turned brown. 

Clopyralid 

Clopyralid (i.e., Spur) is an auxin mimic with systemic activity. Twelve days after the postharvest application, injured shoots had grown and branched, but the main stem and branches were twisted into zigzags (Figure 16). Healthy, normal shoots were also emerging, growing, and branching. 

By late August, some shoots developed into tall fern with normal stem-tips and branches. Some shoots developed a loop and then continued growing (Figure 17).  

More commonly, the other fern had grown medium to full height but had zigzagged tips and branches (Figures 18 & 19). 

In some cases, the branches and stem-tips had yellowed and died (Figure 20).  

Dicamba 

Dicamba (i.e., Clarity/Banvel) is an auxin mimic with systemic activity. Twelve days after the postharvest application, injured shoots had grown and branched, but the main stem branches were zigzagged (picture not shown). 

By late August, some shoots developed into tall ferns with normal stem-tips and branches. However, other fern had grown medium to full height but had zigzagged tips and branches (Figures 21-23). 

In other cases, the tips and branches grew and then died. These shoots had a short, bushy appearance (Figure 24).  

Auxin mimic herbicides can cause injury to asparagus fern that is unique compared to other herbicides, with twisted main stems and branches (Figure 25). If you see these symptoms, herbicide injury is the likely culprit.  

This does not mean auxin mimics should not be used! They can provide control of tough perennials not targeted by other herbicides. Here are some potential best practices for minimizing the risk of injury. 

When applying them to an asparagus field you can 

• Only use dicamba and clopyralid if you need to target specific weeds not controlled by other herbicides (e.g, clopyralid for Canada Thistle or Rush Skeletonweed) 
• Completely strip a field of asparagus spears at the last harvest and apply herbicide ASAP 

To minimize drift to and from asparagus you can: 

• Use low drift/low volatile formulations when available (e.g., Embed Extra) 
• Select nozzles that produce the coarsest droplet possible without sacrificing spray quality 
• Reduce sprayer speed and boom height 
• Consider drift retardants 
• Be aware of weather: high temperatures (>85 F) and low humidity increase the risk of drift 
• Be aware of inversions and windspeeds when timing applications, a little wind is good, but not too much 

There are many good resources on drift reduction, including bulletins and factsheets like Managing Pesticide Drift by the University of Florida and Avoiding Herbicide Drift by the University of Minnesota, a library of practical blog posts on all things sprayers called Sprayers 101, and this EPA webinar  on “Strategies for Managing Pesticide Drift” that delves into details.