Problem: These weeds produce a LOT of seed
- Palmer amaranth and waterhemp are dioecious species, with separate male and female plants. Female plants will bear the seed at end of the season, so populations will have a mix of plants with and without seed.
Seedheads of Palmer amaranth (above) and waterhemp (below).
- Both species can produce over 1 million seeds per plant.
- Stop seed to prevent rapid increases in population and slow the development of herbicide resistance.
The impact of this much seed—doing the math
- Single female plant produces 1 million seeds
- Assume 20 percent of these are viable
- Assume only 25 percent germination following year equals 50,000 plants
- Even assuming 99 percent control that equals 500 plants (assume half are female) and……
- 250 female waterhemp plants produce 250 million seed
- Following year—equals 12.5 million plants
- Even with 99 percent control, that can mean 125,000 plants
- Use an appropriate herbicide program.
- Scout and destroy plants or remove from field before mature seed develop, when seed are absent or still green and soft.
- Seedheads with mature seed—turning dark and hard—should be bagged on-site and removed.
|Don't let this (above)...||become this (above).|
How long does the seed last in soil?
- Approximately 80 percent of the seeds lose viability within the first 12 months, and about 5 percent remain viable after 36 months.
- Seed initially lose viability more rapidly when left on the surface, compared with burial 6 inches deep.
- Viable seed levels will decrease rapidly with several years of effective control, but a small amount of seed can survive more than three years.
|Effect of time and burial on viability of waterhemp and palmer amaranth seed. (Korres et al, Weed Science, 2018)|
What is the effect of tillage on seed distribution and next year’s population?
- A moldboard plow buries much of the seed deep enough that it cannot emerge, greatly reducing population the following year. Other types of tillage result in shallower burial, which causes a slight reduction in the following year’s population, compared to no-till where seed remains on the surface.
- Deep tillage offers the most immediate help with remediation of a new infestation of Palmer amaranth or waterhemp. This can only be used once though.
- Shallow tillage can help with control when integrated with an appropriate herbicide program.
|Effect of tillage type on vertical distribution of seed in soil (left) and emergence the following year (right). (Farmer et al, Weed Technology, 2017)|