Spring is hopefully just around the corner. A time when dormant buds break, grass begins to grow, and seeds germinate. These seeds have been lying in the soil since last fall, waiting for an appropriate cue such as moisture and a warm soil temperature, to trigger germination. 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 for whether or not the species survives the following year. Of course, not all plants rely totally on seed production in order to multiply. Perennials like ground ivy, quackgrass, and yellow nutsedge also vegetatively reproduce. Yet, all plants, including perennial weeds, persist because of seed production and reestablishment from seed which makes eradication nearly impossible.
Why does seed production make weed eradication nearly impossible? First of all, weeds have the ability in seed production to compensate greatly for loss in numbers, even though their potential seed production capacity varies greatly. Generally, a weed which relies entirely on seed production for survival (annuals and biennials), either produces a large number of small seeds or produces fewer large-sized seeds. For example, common mullein on average produces 220,000 small seeds per plant, while sandbur produces less of a much larger seed (1,100 seeds/plant). The advantages of a large seed versus a lot of seed is a subject ecologists have addressed in numerous textbooks. The take home message was that both seed production strategies have been successful in ensuring the persistence of these weeds.
Perennials do not need to rely entirely on seed production for survival; therefore, not as much energy is put into seed production. For example, two creeping perennials, Canada thistle and leafy spurge, produce only 680 and 140 seeds per plant. Yet, even with only 140 seeds/plant, there would be a potential of 20,000 plants by the end of the second year if all 140 seeds germinated the first year. Just imagine what number could be possible in ten years!
Fortunately, or perhaps unfortunately, not all seeds germinate the following year because of a condition termed seed dormancy. Seed dormancy is when a seed will not germinate even though adequate moisture, oxygen, and temperature conditions exist. This provides the seed a means by which germination is delayed until favorable conditions for growth and establishment in the field are met. The length of this delay depends on the type of seed and the environmental conditions. One report on seeds taken from an archeological excavation in Denmark showed that life spans beyond 1,700 years were possible. Another study found that seeds over 200 years old in adobe bricks could still germinate. However, most studies show that about 95% of the seeds germinate by the first year. In fact, many seeds do not get the chance to germinate because they become a meal for birds, mice, and other small animals.
This seed predation provides little comfort because there always seems to be plenty of weeds to keep us busy hoeing, pulling, and digging. However, one can take comfort in knowing that when you see those weeds it means winter is done and spring has come.
This article originally appeared in the March 24, 1993 issue, pp. , 1993 issue, pp. 29-30.