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Germination Depth of Weeds (05/20/21)

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Despite the ongoing dry conditions across the state, we are seeing a significant amount of summer annual weed emergence in many areas. We have received many inquiries about the abundance of waterhemp, a small-seeded weed, despite the lack of moisture in the top 1 to 2 inches of soil in eastern ND and the Red River Valley. So we decided to do a little digging into the matter. The “loose fluffy dirt” that Dr. Dave Franzen wrote about last week made for easy excavation of many waterhemp seedlings. In general, it appears that most waterhemp that is emerged has germinated from 0.5 to 1 inches below the soil surface. This is in stark contrast to most years where waterhemp typically emerges from the top 0.5 inches of soil. So why is waterhemp emerging from deeper in the soil than usual? To quote Jeff Goldblum’s character in Jurassic Park: “Life, uh, finds a way”.

Waterhemp seedlings pictured next to a pocket knife for scale and a waterhemp seedling that germinated about 0.75" below the soil surface pictured with a tape measure for scale.
Photo Credit:
Brett Miller

Waterhemp is not the only weed emerging from deeper than expected areas in the soil. I dug up some common ragweed from our research plots near Prosper, ND and found emergence from as deep as 2 inches. Digging up other broadleaf weeds like wild buckwheat and common lambsquarters revealed germination depths from 0.5 to 2 inches for many weeds. This is not terribly surprising given the dry conditions we are facing. However, it does (and has) begged the question: what will my PRE herbicides do whenever it finally rains?

I don’t need to reinvent the wheel when it comes to writing about PRE herbicide chemistry in a dry environment. So, I will reference readers to an article by Dr. Rich Zollinger back in 2016 (https://www.ag.ndsu.edu/cpr/weeds/fate-of-soil-applied-herbicides-05-26…). What I will add to his discussion is that we need to be realistic with our expectations this year. Frankly, we will need a lot of rain to push any herbicide, let alone a root-absorbed herbicide, deep enough into the soil profile to have any significant effect on emerged weeds.

Common ragweed seedling that emerged from about 2 inches below the soil pictured with a tape measure for scale

HOWEVER, we also should realize that our PRE herbicides will be ready to help control those weeds that have not yet germinated in the top 1 to 2 inches of soil. So if and when it rains, many may be looking at a situation where it appears the PRE has “failed” because there are many emerged weeds in a field. But the reality is that it would be much worse without that herbicide application. There is still a great value to any PRE that was applied this year, but we should be aware that weeds will escape, and we need to be ready to pull the trigger on a POST application to clean up these weeds that have emerged from deeper in the soil profile.

Joe Ikley
Extension Weed Specialist