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.
This article originally appeared in the February 7, 1997 issue, p. 8.