Contributed by Albert T. Adjesiwor, Assistant Professor & Extension Weed Management Specialist, University of Idaho
Most of us have probably asked this question before: Why do weeds keep coming back? The short answer is: weeds save and invest a lot in what we call the weed seedbank.
The short definition of weed seedbank is “accumulated weed seeds in the soil.” Weed seedbanks represent the single most important contributor to annual weed persistence in agricultural fields.
Here’s an analogy: if I have a job in a company that could fire me at any time, and I make $500,0000 a year (I wish), the logical thing to do is save and invest as much money as possible from every paycheck. This is exactly what most of our problematic weeds do. They produce a lot of seeds every single year. However, because they grow in very unpredictable environments and may not be able to produce enough seeds every single time, they “save” a lot of the seeds they produce in the only bank they have: the weed seedbank.
Thus, our ability to successfully manage problematic agricultural weeds over the long term (especially annual weeds) depends almost entirely on our understanding of and ability to manage the seedbank. This is easy, right? Why haven’t we been studying this? We have!
A search for the term “weed seed bank” in Google Scholar returned 374,000 results in 0.11 seconds. This demonstrates that weed scientists have devoted a lot of time and effort to studying weed seedbanks. The reality is that most weeds produce seeds that choose when they come up. A significant proportion of the seeds produced by most annual weeds are dormant, meaning the seeds will not germinate even when conditions are optimal (Figure 1). This means even if we manage to kill 100% of all emerged weeds at a particular location, it will not cause local extinction. This is the primary reason weeds keep coming back.
Figure 1. Depiction of the major inputs and outputs to the weed seed bank.
One of the longest-running weed seed longevity studies (Beal Seed Experiment at Michigan State University) has shown that some weed seeds may survive in the soil for more than 140 years (Figure 2)! That’s right, one-hundred-and-forty years. The experiment is still ongoing with the next batch of seeds scheduled to be unearthed in 2040 (stay tuned!). To read more about the Beal Seed Experiment, please visit their website.
The non-dormant weed seeds will germinate and emerge every year. Some of the seedlings may die through various means: competition, tillage, and herbicide application, among others. Some may survive and produce seeds to add to the weed seedbank (Figure 1). The cycle continues every single season.
You might be thinking, why don’t we just kill all the seeds in the soil? If it was that simple, I (and many weed scientists) probably won’t have a job. The closest we have come to killing weed seeds in the field is through the use of preemergence herbicides. However, preemergence herbicides only kill germinating (non-dormant) weed seeds.
Yes, there’s fumigation, but it is not very effective and not commonly used in most cropping systems because of toxicity concerns. Put differently, we currently have no practical way of controlling dormant weed seeds in the soil. This leaves us with nature–we rely on seed-eating bugs, rodents, and pathogens to do this for us.
All hope is not lost. As part of the Pacific Northwest Herbicide Resistance Initiative, we are devoting a lot of resources to studying the weed seedbank and how we can better manage these in our cropping systems. I will leave you with this: effective long-term management requires preventing weeds from going to seed and the exhaustion of seeds in the soil. This is an uphill task, but maybe, we will win someday. If you are interested in estimating how your weed control practices may impact your seedbank over time, Dr. Andrew Kniss from the University of Wyoming developed a model that can provide some insight.
Figure 2. Twitter post from 2021 showing an emerged weed seed unearthed after 141 years. Visit the project website to learn more.