Most persistent weed seeds

Seed Persistence

One of the most frequently asked questions about unwanted plants (weeds) is “How did they get there?” Actually, the plant you have growing originated from seed that has been lying in the soil since last fall or longer, waiting for the appropriate environmental cues such as moisture, warm soil temperatures, and light to trigger its germination. The weed seed could be in your own garden soil, the soil that was transplanted along with a new plant from a neighbor, relative, or friend, or possibly even in the soil of a purchased container-grown plant. Isn’t it amazing that a seed, which can be as large as a 20 pound coconut or as small as false pimpernel which requires 150 million seeds to weigh 1 pound, holds the key to the survival of the species.

Why is it so hard to eliminate weeds? First, weeds have the ability to compensate for the destruction of seeds or plants by producing large amounts of seed. Seed production potential varies from species to species. Annuals and biennials that rely entirely on seed production for survival, either produce a large number of small seeds (common mullein produces 220,000 seeds per plant) or produce fewer large-sized seeds (sandbur produces 1,100 seeds per plant). Weeding the garden for an afternoon demonstrates that both methods work very well in ensuring persistence. Perennials do not rely entirely on seed production so they don’t expend as much energy in seed production. However, they still produce enough seeds to ensure survival as well as persistent roots on stems. Leafy spurge produces only 140 seeds per plant, yet there would be the potential of 20,000 plants by the end of the second year if all 140 seeds germinated the first year and survived to maturity or seed production. Just imagine what could be possible in ten years!!

Fortunately, or unfortunately depending upon how you look at it, not all seeds germinate the year after they were produced because of seed dormancy. Seed dormancy prevents seed germination even though adequate moisture, oxygen, temperature, and light conditions occur. The length of this delay depends on seed type and environmental conditions. Seeds also survive because environmental conditions aren’t ideal. For example some seeds require light in order to germinate. If seeds are buried a few inches beneath the soil surface, the seeds cannot germinate. However, once the soil is tilled and the seeds are turned up to the surface, all systems are go.

How long can seeds survive? If the proper external conditions are not met, the seeds will remain in a quiescent state or a state of persistent viability. This varies by species from a few weeks to several hundred years. Seeds discovered during an archeological excavation were still viable 1,700 years after the site was abandoned. Other studies found that seeds over 200 years old in adobe bricks could still germinate. Most research studies show that about 95% of the seeds will germinate the first year if a favorable environment exists. The good news is that many seeds do not get the chance to germinate because they become a source of food for birds, mice, and other animals. Unfortunately even with seed predation, there are always enough seeds that germinate to keep gardeners busy hoeing, pulling, and digging.

What can be done to reduce seed persistence? First, do not allow weed seeds to germinate. Mulching is an excellent way to prevent many weeds from germinating. Second, do not allow unwanted plants to go to seed. This is also true for some desirable plants that have a tendency to become “weedy”. Remove the flowers on these plants as soon as they begin to fade.

Most persistent weed seeds

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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.

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.

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