How Can Spring Tillage Impact Weed Pressure This Year?

Weed management can seem like a never-ending project. On one hand, you deal with annual weed emergence—and on the other hand—you know that your soil holds a seed supply (or seed bank), ensuring you haven’t seen the last of your weeds. 

The Never-Ending Battle: Understanding Weed Seed Banks

The number of seeds in the soil varies widely, but a Minnesota study found that an “average” square foot of soil, 6" deep, contained between 100 and 3000 viable weed seeds. Those seeds are patiently waiting for favorable conditions that allow them to germinate. An Iowa State University study showed that annual emergence accounts for somewhere between one and thirty percent of all the weed seeds present in the soil. 

How Many Weeds Seeds Are Lurking in Your Soil?

So what determines the actual weed emergence each year? In any one year, weeds seeds in the seed bank may germinate and grow, germinate and die before you see them, decay in the soil, or remain dormant. How will the conditions in your field determine which may occur?

📸: @PBouman via Canva/Featuring agricultural tillage in spring.

The general viability of weed seeds will decline with every year in the soil, but some can survive for many years:

• Most annual grasses have a very short viability in the soil and can be depleted in 2-3 years, yet wild oats have been shown to be viable for 3–6 years, annual ryegrass up to 9 years, and barnyard grass up to 13 years.

• Several broadleaf weed seeds are depleted quickly in the soil. Most kochia and Russian thistle seed will germinate or decay the year following production, and most remaining seeds do not last past 3 years.

• Broadleaf weeds with seed viability in the medium range of 4 to 6 years are waterhemp, palmer amaranth, and common cocklebur.

• Other species of broadleaf weed seeds can remain viable for decades, as jimsonweed has been shown to have a 90% germination rate after 40 years in the soil, while field bindweed seeds can be viable for more than 50 years.

• Other research has shown that Canada thistle seeds can remain viable for 20 years, velvetleaf and common lambsquarters can be viable for 40 years, morningglory 50 years, and velvetleaf for 60 years.

• In the unbelievable category, mustard seeds were once excavated from a monastery in Denmark that dated back 600 years, and several germinated.

📸: @ligora via Canva/Featuring an example of deep tillage impacting weeds seed bank dynamics.

The Fate of Weed Seeds: Germination vs. Dormancy

Weed germination is similar to crop seed germination and is affected by many environmental factors, including soil temperature and moisture, but weeds are also subject to dormancy. Dormancy of weed seeds acts as a survival mechanism, spreading out germination over many years to ensure some weeds will germinate under better-growing conditions and produce seeds to prolong the species and keep weed scientists employed.

The Secret Weapon of Weeds: Seed Dormancy Explained

• Primary dormancy refers to the amount of recently produced seed that does not germinate the following year, even though conditions are good. Seed coat thickness accounts for much of the dormancy among weeds. Common cocklebur is interesting as each bur contains two seeds, one large and one small. The larger seed usually germinates the next season, while the smaller seed does not usually germinate until a year or two later due to germination inhibitors in the seed.
• Secondary dormancy occurs when seeds cannot germinate due to the absence of suitable environmental conditions; e.g., weeds seeds that are buried deeper in the soil with conditions less conducive to germination will remain dormant and have longer viability than seeds near the soil surface. So while it may be tempting to use deep tillage to bury weed seeds, other older seeds may be brought close to the surface with deep tillage, triggering germination. Seeds left near the surface with reduced tillage will be more susceptible to predator insects and birds.

Breaking the Seed's Defenses: Stratification and Scarification

To ensure seeds do not germinate too fast, many seeds require stratification or scarification. Stratification is a required cold treatment, while scarification is a mechanical breakdown of the seed coat, allowing water and oxygen to enter the seed and start the germination process.  

Managing the Seed Bank: A Balancing Act

The weed seed bank is a dynamic balance sheet where a certain percentage of seeds are withdrawn each year. A zero-tolerance policy on weed seed production can quickly deplete the seed supply, while any weed escapes that drops seed or seed entering your field with manure, wind, water, or waterfowl can quickly add to it.

📸: @Matauw via Canva/Featuring tillage in spring (Left) and no-till (Right).

This content is being brought to you in partnership with Grow Smart^® Live and contributing guest authors. BASF provides the information in this article as a service to its customers; however, the views expressed by guest writers are their own and do not necessarily reflect the views of BASF.