Staying a Step ahead of the Soil Seedbank

The term ‘soil seedbank’ can be used to describe the storage of seeds from a single species or for all the species in a given area.

Ecosystems experience a variety of stresses such as cold, wildfire, drought, and disturbance, and soil seedbanks are often a crucial survival mechanism for many plants, and they help maintain the long-term stability of ecosystems.

If soil seedbanks are important for plant survival, why are they bad for farmers, gardeners, and land managers?

 

One years’ seeding yields several years of weeding

The soil seedbank serves as the source of future seasons’ weed problems. There is a saying that states, “One year of seed production brings several years of weed infestations”, which implies that not all seeds will germinate in a given year. Some seeds have the ability to remain dormant in the soil seedbank, a trait that nature provides plants to ensure they survive ecosystem imbalances.

The majority of seeds in the soil are deposited by plants that escape control. Therefore, limiting the number of seeds that are deposited into the seedbank is a critical approach to managing weeds.

 

The soil seedbank can be either transient or persistent.

• Transient seedbanks contain seeds that live in the soil for less than a year.
• Persistent seedbanks contain seeds that live in the soil for more than one year. 
  • Regardless of whether a seedbank is transient or persistent, the ultimate goal for farmers, gardeners, and land managers is to limit the number of seeds that are deposited into the soil seedbank annually.

Seed production

Seed production is the ‘deposit’ by nature into the soil, which serves as the ‘bank’. When the time is right, the deposited seed will germinate, grow, reproduce, and yield several more seeds that will be deposited in the soil seedbank. Therefore, from one seed you can yield hundreds or thousands of seed.

One distinction relates to the nature of the deposited seeds and whether or not they can be a short-term (transient) or long-term (persistent) problem. The condition of the seeds produced by plants is predisposed by nature and is governed by the type of life cycle a plant has.

Plants have annual, biennial, or perennial lifecycles which determines their seed producing habits. 

• Plants with an annual lifecycle rely totally on seed production for their survival. Individual annual plants produce numerous seeds, many of which can live for more than one year in the soil. Therefore, annual plants tend to develop a persistent seedbank.
• Plants with a biennial lifecycle rely on production of both seed and vegetative plant parts for their survival. Individual biennial plants produce numerous seeds, a majority of which are short-lived and will not persist in the soil seedbank for too long. Therefore, biennial plants tend to develop seedbanks that are not quite as persistent as seedbanks of annual plants.
• Plants with a perennial lifecycle rely more on vegetative reproduction than on the production of seed for their survival. Therefore, the few seeds that are produced by perennial plants survive in the soil seedbank for a short period, making it a transient seedbank.

Armed with knowledge of how seedbanks are developed it is time to address how one can stay ahead of the soil seedbank.

Three ways to stay ahead of the soil seedbank

1. Limit the number of weedy plants that emerge from the seedbank.
   • Preventing weed seedlings from emerging is the first approach to minimizing the number of weedy plants that eventually grow and produce seed.
   • The most efficient method with this approach is when herbicides are applied prior to emergence of weeds (preemergence weed control).
2. Limit the number of weedy plants that mature, flower, and produce seed.
   • Control plants in their early vegetative stages to prevent them from reaching their reproductive stage. 
   • The most efficient method with this approach is when herbicides are applied after weed emergence and before plants reach their reproductive stage (postemergence weed control).
3. Limit the number of weedy plants that escape preemergence and postemergence control, mature, flower, and go to seed.
   • Weedy plants which escape control are a major contributor to seeds that are incorporated into the soil seedbank. These plants should be prevented from maturing and going to seed.
   • The most efficient method with this approach is when herbicides are applied to control weedy plants late in the growing season (late season postemergence weed control).
     
     

Additional factors that contribute to the depletion of weed seeds in the soil seedbank include:

• Seed decay – pathogens within the soil can infect weed seeds and cause them to decay and die.
• Seed predation – weed seeds in the soil seedbank serve as a source of food for several organisms that live in the soil.  
• Seed death – weed seeds in the soil seedbank tend to lose viability the longer they remain in the seedbank, and they will eventually die.
   

Some Final notes:

Understanding the seedbank is critical for the successful growth of plants.  Whereas nature’s control mechanisms (seed decay, seed predation, and seed death) provide some form of seedbank depletion, they are not enough. Why?

Among several other factors:

• In annual crop production systems, weeds tend to be highly prolific and individual plants can produce over 100,000 seeds.
• Nature’s control mechanisms are not uniformly distributed which means that the rates of seed predation, decay, and death can vary widely among seedbanks within a field.

Herbicide selection when controlling weeds after emergence must consider:

• Size and maturity of the weed.
• Choice of adjuvant that will help provide the best control of weeds. 
  
  

Further Reading:

• Sun Wet Surfactant – Best Postemergence Weed Control with Herbicides 
• How Surfactants Affect Spray Droplet Size 
• Pesticides 101: Introduction to Pesticide Adjuvants and Surfactants
• Everything You Need to Know About Agricultural Wetting Agents
• Cide-Kick II for Excellent Management of Weedy and Invasive Plants 
  
  

References

• Renner, K.A. 2000. Weed Seedbank Dynamics. Michigan State University Extn. Bull. E2717. https://www.canr.msu.edu/uploads/resources/pdfs/weed_seedbank_dynamics_(e2717).pdf. Accessed, February 25, 2022.