Although it is hard to say report anything good about a troublesome weed like waterhemp, Illinois weed scientists recently reported good news on how rapidly waterhemp dissipates from the seedbank. They allowed a field with a heavy waterhemp population to go to seed in 1996 and tracked waterhemp emergence and the seedbank in no-tilled and chisel-tilled corn and soybeans over the next 4 years. During the subsequent years, they did not allow any waterhemp to survive and produce seed. 

 

In general, the tillage system or the crop grown did not greatly affect the loss from the seedbank. Figure 1 shows the dramatic decline in the total number of waterhemp emerging each year and Figure 2 shows the percent of the seedbank remaining each fall when the plots were sampled. After the first year, just over 60% of the waterhemp seed was lost either to germination, decay, or predation. By year 4 (2000), only 0.004% of the seedbank remained. It is remarkable that after 4 years, the seedbank, which started with 2,460,000 seeds/m² in the top 8 inches, had dropped to a mere 98 seeds/m². This is good news if waterhemp can be prevented from producing seed. If a field as a high waterhemp population that needs to be brought under control, one option might be a few years in alfalfa, which would prevent seed production and allow a substantial level of seed loss. It would likely be more difficult to achieve similar results in corn or soybeans because a few escaping waterhemp can produce significant amounts of seed and replenish the seedbank.   

     

Figure 1. Total number of waterhemp emerging each year during 4 years from a seedbank that started with 2,460,000 seeds/m² in the top 8 inches.

 

 

   

Figure 2. Percent of waterhemp seed remaining in the seedbank after 4 years without any new seed production.