Time to remove flowering weeds

Marissa Schuh, IPM Extension Educator. Adapted from a 2021 article.

After a rainy growing season, many growers have probably wished they could turn off the rain.  There is one type of rain that growers can partially control-- weed seed rain.  

Every time a weed goes to seed, dozens to thousands of seeds enter our soil, creating hours of future weeding. As we enter the part of the growing season where many weeds are flowering and getting ready to set seed, this is a key time for us to pull and thwack weeds. 

Marestail  seeds move around with the help of white, wind-catching filaments. Photo: Enrico Blasutto, CC BY-SA 3.0, via Wikimedia Commons

Weeds on our farm comes from our weed seedbank
The soil on each farm contains its own mix of weed seeds -- this will reflect the history of the land and how it has been managed over time. When these weed seeds are in the top two inches of soil and receive the right combination of water, temperature, light, and nutrition, weeds from this seedbank germinate and become problems in our field.

The weed seeds can potentially lay in wait for decades. Every 20 years, a long-running experiment in Michigan digs up bottles of seeds buried in 1879. Seeds of moth mullein, dug up and planted last year, were able to germinate after 140 years.

This year's weeds are ready to add to the seedbank
While we are stuck with our own weed seedbank and will have to manage those weed seeds as they germinate, we can work to stop weeds from adding new seeds to the seedbank.  When we control weeds this time of year in our spring planted crops, the weeds have already dinged yield if they were growing alongside our crops when the crops were young (the critical weed free period).  When we manage weeds now, we are trying to help our future selves by reducing the number of weed seeds in our farm's soil.

Common ragweed is a common weed, who causes a lot of season allergies in the process of making many short-lived seeds. Chris Evans, University of Illinois, Bugwood.org

The table below includes common summer annual weeds, how many seeds the plant produces on average, and how long it takes for half of the seeds in the soil to die out. Note that these are just averages, and what happens in your growing space will depend on weather, insects, soil bacteria, and more.

Weed	Average seed production per plant	Time to 50% depletion of seeds in soil
Common ragweed	3,500	1.5 years
Common lambsquarters	72,500	12 years
Eastern black nightshade	10,000	5 to 8 years
Velvetleaf	7,800	8 years
Giant ragweed	10,300	Less than 2 years
Marestail/Horseweed	200,000	Unknown

With this type of weed seed deposition and longevity, pulling or thwacking large weeds as you scout or check crop readiness is saving future you from time weeding by hand or with a tractor.

This velvetleaf plant is getting ready to flower, meaning this is a great time to remove it. Photo: Robert Flogaus-Faust, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

It looks like some weeds have already set seed, what do I do now?
Avoid letting weed seeds get buried in the soil. The longer they sit on the surface of the ground, the more likely they are to get eaten by birds, rodents, or insects.

Sources

Renner, K.A. 2000. Weed pest ecology and management in Michigan Field Crop Pest Ecology and Management. Michigan State University Extension Bulletin E-2704, January 2000.

Michigan State University Extension. 2005. Integrated Weed Management: One year’s seeding, February 2005, Michigan State University Extension Bulletin E-2931.

Michigan State University Extension. (n.d.). Michigan's Worst Weeds. https://www.canr.msu.edu/weeds/extension/michigan-s-worst-weeds

Charles L. Mohler, John R. Teasdale, Antonio DiTommaso. 2021. Manage Weeds On Your Farm. SARE,

Moncada, K. M., & Sheaffer, C. C. 2010. Risk management guide for organic producers.

Weed Science Society of America. 2009, April 6. Want to keep your compost weed-free? https://wssa.net/2009/04/want-to-keep-your-compost-weed-free/