With little competition for sunlight and nutrients, waterhemp can produce up to 1 million seeds, though it’s more commonly about 250,000 per plant. Waterhemp seeds stay viable in the soil for about four years with 10% survival after three years and less than 1% survival after four years, according to Purdue Extension.
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Like waterhemp, Palmer amaranth can produce up to 1 million seeds without competition, but more commonly produces about 460,000 seeds per plant. Seeds buried deeper than 2” in the soil likely won’t germinate, and after three years in the soil only 20% of the seeds are still viable, according to University of Georgia Extension.
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Giant Ragweed can germinate even after being buried up to 6” deep in the soil, but the weed seed bank can be depleted by 99% in just two years. Each plant produces between 1,900 and 5,500 seeds when in competition with corn and soybeans, according to Michigan State University Extension.
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Winter and summer annual marestail [horseweed] produces about 200,000 seeds per plant that can travel up to 400’ by wind. Marestail germinates more in the fall than any other time and goes dormant when buried deeper than 1” in the soil. Seed longevity hasn’t been extensively studied, though there is a report of finding viable seeds after 20 years, according to Michigan State University Extension weed science.
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