Dead Weed Seed

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Learn how to kill weed seeds in soil to prevent weeds and other invasive grasses from continuing to sprout every year. The fate of weed seeds in the soil has been an area of much research in recent years. Most studies have focused on the seeds that successfully produce seedlings since these are the seeds that cause immediate problems for farmers. In most studies, annual emergence typically accounts for 1 to 30% of the weed seed in the soil. Thus, the majority of seeds found in the soil seed bank fail to produce seedlings in any given year. Whether you are planting a winter or upcoming spring crop, re-seeding or enhancing a stand of Conservation Reserve Program (CRP) or State Acres for Wildlife Enhancement (SAFE), or simply interested in seeing if a winter,…

How to Kill Weed Seeds in Soil [5 Easy Methods]

To kill weed seeds in soil you will have to apply one or more of the following methods:

  • Heat soil to temperatures high enough to kill weeds seeds
  • Force seeds to sprout and destroy growing weeds
  • Apply chemical or natural weed killers that prevent weeds from sprouting
  • Use flame weeding to destroy weeds and seeds at once
  • Layer mulch in garden areas to suppress weed sprouting and attract insects that eat seeds

With this arsenal of tricks for killing weed seeds before they sprout, you can stop the spread of weeds in both your lawn and garden.

Table of Contents

5 Tips for Killing Weed Seeds

Rather than spending time and effort battling weeds as they sprout, attack weeds at the source by killing weed seeds. Each of these methods destroys weed seeds, which ensures you won’t have to battle recurring weed invasions. Try these ways to get rid of weed seeds yourself.

Solarization

Soil solarization is a very powerful method for killing weed seeds. Weed seeds begin to die if soil temperatures surpass 108 degrees, with full seed death ensured by soil surface temperatures of 140 degrees or more. Solarization uses clear plastic tarps to trap heat at the soil surface, killing weed seeds within the tarped area. To solarize an area, follow these steps:

  • Clear the area of all vegetation through use of a hoe or other garden implement. Remove any woody stumps
  • Till the soil to further break up any weed root systems left behind.
  • Rake away all vegetation residue
  • Water the tilled and cleared soil with a garden hose until it is damp.
  • Lay a sheet of clear plastic over the area. Weigh it down tightly at the edges
  • Leave the plastic in place for at least two months.

Solarization is the best method to reclaim a weedy garden or other area. It is a “clean slate” for your soil, because seeds will be destroyed by the solar heat trapped beneath the plastic.

It is typically tough to implement solarization in large areas and is not usually suitable for use in lawns, where you may want to preserve grass or other plants. Pre-emergent weed killers and flame weeding are much better for use in lawns.

Till and Kill

Weed seeds can lie dormant in soil for decades and are only “activated” when brought to within an inch of the surface. One method to rid soil of dormant weed seeds is to force these dormant seeds to sprout, then attack them with a powerful natural or chemical weed killer. To do this:

  • In spring, till the affected area. Tilling brings dormant seeds to the surface
  • Water the area for 1–2 weeks with a sprinkler or soaker hose
  • When weeds begin to sprout, apply the weed killer of your choice
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This is another “clean slate” method, where you force weed seeds to show themselves and then kill young weeds before they mature and cast seeds. Because of the invasive tilling step, it is not best used in areas with desirable grasses and plants.

Use Pre-Emergent Weed Killer

Pre-emergent weed killer stops weeds in their tracks. It works by attacking weed seeds just as they begin to germinate, killing them before they even poke above the surface. It’s a weed killer so good, the only sign it’s working is that there will be no new weeds at all.

Fate of weed seeds in the soil

The fate of weed seeds in the soil has been an area of much research in recent years. Most studies have focused on the seeds that successfully produce seedlings since these are the seeds that cause immediate problems for farmers. In most studies, annual emergence typically accounts for 1 to 30% of the weed seed in the soil. Thus, the majority of seeds found in the soil seed bank fail to produce seedlings in any given year. The fate of seeds that fail to germinate and emerge is poorly understood. While some of these seeds are simply dormant and will remain viable until the following year, others are lost due to decay or consumed by insects or small animals. This article will describe results of an experiment that monitored the fate of seeds for the first four years following introduction into the soil.

Methods: Seeds of velvetleaf, waterhemp, woolly cupgrass and giant foxtail were harvested from mature plants during the 1994 growing season. The seeds were cleaned and counted and then buried in the upper two inches of soil on October 21, 1994. Two thousand seeds were buried within a 3 sq ft frame to allow recovery during the course of the experiment. Weed emergence was determined by counting seedlings weekly during the growing season. Emerged seedlings were pulled by hand after counting. In the fall of each year one quarter of the soil within a frame was excavated and the remaining seeds were extracted and counted. Corn or soybeans were planted between the frames during the course of the experiment to simulate agronomic conditions.

Results: The emergence patterns of the four species were described in an earlier article (see emergence patterns). The fate of the seeds (emergence, loss or survival in soil) during the first four years after burial is shown in Figure 1. In the first year following burial waterhemp had the lowest emergence (5%) whereas greatest emergence was seen with woolly cupgrass (40%). Total emergence over the four years ranged from 300 seedlings (15% of seed) for waterhemp to 1020 seedlings (51%) for woolly cupgrass. More than three times as many seedlings emerged in the first year than in subsequent years for velvetleaf, woolly cupgrass and giant foxtail, whereas 140 waterhemp seedlings emerged in 1996 compared to only 100 in 1995.

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Figure 1. Fate of seeds during the four years following burial in the upper two inches of soil. Two thousand seeds of each species were buried in the fall of 1994. The area in white represents the number of intact seeds present in the fall of each year, green represents the total number of seeds that produced seedlings during the four years, and the blue represents the total number of seeds lost. Buhler and Hartzler, 1999, USDA/ARS and ISU, Ames, IA.

Seeds of the two grass species were shorter lived than those of velvetleaf or waterhemp. At the end of the third year (1997) no grass seeds were recovered. Somewhat surprising is that waterhemp seed was more persistent than velvetleaf in this study. Velvetleaf has long been used as the example of a weed with long-lived seeds. In the fourth year of the study four times more waterhemp seedlings than velvetleaf emerged and four times more waterhemp seed than velvetleaf seed (240 vs 60) remained in the seed bank.

For all species except woolly cupgrass the majority of seeds were unaccounted for (the blue portion of the graph) in this experiment. Determining the fate of the ‘lost’ seeds is a difficult task. A seed basically is a storage organ of high energy compounds, thus they are a favorite food source of insects and other organisms. In natural settings more than 50% of seeds are consumed by animals. The importance of seed predation in agricultural fields is poorly understood, but recent studies have shown that predation can be a significant source of seed loss. Another important mechanism of seed loss likely is fatal germination. This occurs when a seed initiates germination but the seedling is killed before it becomes established. Fatal germination probably is more important with small-seeded weeds such as waterhemp and lambsquarters than with large-seeded weeds, but is poorly understood. A better understanding of the factors that influence seed losses might allow these processes to be manipulated in order to increase seed losses.

So what does this mean as far as managing weeds in Iowa. First, consider how the methods used in this experiment might influence the results. The seeds were buried in the upper two inches of soil, the zone most favorable for germination. Most long term studies investigating the persistence of seeds have buried the seeds at greater depths than used here in order to minimize germination. If the seeds were buried deeper one might expect less emergence and greater persistence since the seeds would be at a soil depth with less biological activity. If the seeds had been placed on the soil surface it is likely that there would be more predation, less emergence and shorter persistence.

The results indicate that the seed bank of giant foxtail and woolly cupgrass should be able to be depleted much quicker than that of the two broadleaves. Maintaining a high level of weed control for two years should greatly diminish populations of these weeds in future years and simplify weed management. Unfortunately, a single plant escaping control can produce more seed than was introduced to the soil in these experiments, thus the seed bank can be rapidly replenished any time weed control practices fail to provide complete control. Finally, over 50% of velvetleaf and waterhemp seed was lost in the first two years following burial. However, significant numbers of seed of these species remained four years after burial. This will make populations of these two species more stable over time than those of woolly cupgrass and giant foxtail.

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Doug Buhler is a Research Agronomist at the National Soil Tilth Laboratory, USDA/ARS, Ames, IA.

Is Your Seed Dead or Alive – A Seed Viability Test

Whether you are planting a winter or upcoming spring crop, re-seeding or enhancing a stand of Conservation Reserve Program (CRP) or State Acres for Wildlife Enhancement (SAFE), or simply interested in seeing if a winter, spring or possible summer cover crop planting will help the overall health of your soil, it is important to make sure the seed you plan to use is viable.

A “Seed Tag” usually accompanies seed purchased from seed companies. This tag tells the buyer several things: 1) what variety and species is in the bag, 2) a lot number which tracks where the seed was produced, and 3) information such as the percent purity, percent inert material, percent weed seed, percent noxious weed seed and percent total germination.

Unfortunately, not all seed comes with a tag. Any seed that has been set aside from harvest to be planted for next year’s crop will not have this information. One- or two-year-old seed should be fine to plant, but what if a producer had some seed from 10+ years ago that they wanted to use?

Fortunately, there is a simple way to test seed viability.

You will need:

  • A petri dish (plastic, shallow, flat, clear dish) or a ziplock bag
  • A lid for the petri dish
  • A paper towel
  • Plastic wrap

Then, you’ll follow these five simple steps:

  1. Moisten a piece of paper towel with water and place it in the petri dish;
  2. Evenly place 10 or more seeds on the paper towel. You may want to fold the towel over the seeds so both sides of the seeds are moist;
  3. Place the lid on the dish and wrap tightly with plastic wrap;
  4. Place in a warm environment (i.e., kitchen counter); and
  5. Wait for 7 to 10 days, then check the dish for seeds that have visibly germinated (i.e., sprouted) and count them.

The percentage of germinating seed also known as pure live seed (PLS) will give you a fairly good idea how that same seed should perform when planted provided the seed receives adequate moisture. Adjust seeding rates based on the percentage of germinating the seed.

Example:

Species: Eltan WW

PLS: 80%
Desired seeding rate: 12 seeds/ft row or 50 lbs. /A on 12-inch spacing (St. Andrews Variety Trial Seeding Rate)

The seeding rate of bulk seed: 50/0.80 = 55.5 pounds of bulk seed

Therefore, you will need an additional 5.5 lbs/acre of seed to reach either the desired 12 seeds/ft row or 50 lbs/acre based on a PLS of 80%.

For help with converting lbs/acre to seeds/ft row, lbs/acre to seeds/acre or vice versa for both, please visit our Crop Tools & Calculators page and check out the Seeding Rate Converter calculator.

For questions or comments, contact Dale Whaley by email at [email protected] or by phone at 509-745-8531.

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