The Destiny Of Weed Seeds In The Soil
Author: Allison Kosto, MSU Broadwater County Extension Agent
Allison Kosto
MSU Broadwater County Extension Agent
Where did that weed come from? How did it get there? It’s quite amazing how quickly a site can go from weed-free to infested in just a matter of years. There are a number of factors that can influence this from location to site history to plant biology. However, one factor is seed persistence and viability which can shape the destiny of weed seeds in the soil.
There are multiple ways in which seeds reach the soil. Seeds are naturally equipped with devices that help them spread. Here are a few examples. Houndstongue seeds are covered in tiny hooks that can attach to clothing, fur and feathers. Canada thistle seeds (and dandelions) have feather-like tufts that are light enough to be caught by the wind and float on water. Leafy spurge has seed capsules that ‘explode’ and can project seeds up to 15 feet. However, all seeds have a common goal of finding a section of soil where it can germinate and continue to reproduce.
Once the seeds reach the soil, their next goal is to germinate to produce a plant. Typically, seeds remain dormant until environmental conditions are right. Research has shown that 95 percent of seeds will germinate in the first year if there are favorable conditions. What are the right favorable conditions varies based on the type of seed. Soil temperature, moisture, oxygen and light are all factors that influence germination. For example, some seeds require light to germinate, so if the seeds are buried too deep then they won’t germinate.
A certain number of seeds never make it. Some germinate and die for a variety of reasons. Others decay in the soil, and some become feed for insects, birds and other mammals. In some cases, seeds may remain dormmate for years, even decades, before conditions are right for them to germinate. However, with each passing year, the viability of seeds is reduced.
An example of a plant with long viability is common lambs quarters. It takes twelve years for a 50 percent reduction in seed viability and 78 years for a 99 percent reduction in seed viability. On the other hand, kochia seeds lose their viability quickly with a 50 percent reduction in seed viability in less than 6 months and a 99 percent reduction in 2 years.
Why is this important? Understanding seed bank dynamics can further your ability to control and manage weeds. This is especially true for annual and biennial weeds that reproduce only by seeds. Perennial weeds often use other mechanisms for reproduction so don’t put as much energy into seed production. A good example of an annual weed is cheatgrass. Cheatgrass reproduces only by seed and has relatively short viability in the soil of only three years. Therefore, if you can prevent cheatgrass from going to seed for at least three years, then you should be able to reduce or eliminate cheatgrass. I realize this may not always be practical but can be a great option in some situations.
There are other techniques that can also be used to stop seeds from germinating. Mulch is a great option, where practical, that limits both light and oxygen and reduces or prevents seeds from germinating. Mulching for multiple years may be necessary to further reduce the seed bank. In certain situations, tillage can reduce seed viability. Wild oats in cropland are an example of how tillage can be used to reduce the prevalence of weeds. Wild oats require light to germinate and typically need to be at or near the soil surface. By tilling the soil to burying the seeds at least 4 inches deep, you can significantly reduce the germination rates of wild oats.
Effectively managing weeds is complex and often requires multiple strategies. Understanding the destiny of seeds in the soil is one tool in the toolbox. Contact the MSU Extension Office in Broadwater County at 406-266-9242 or allison.kosto@montana.edu for questions on weed identification and management.
