The Lowdown on Flowering Weeds & Viable Seeds

As the days shorten and the heat soars, those weed escapes in your fields may be getting ready to make their annual seedbank deposit sooner than you realize. 

For many weed species, plants can produce viable weed seeds shortly after flowering, warns University of Delaware Extension weed specialist Mark VanGessel. In one multi-state study from 2016, researchers found that some weed species had viable seed within two weeks of the first flower appearing. 

The good news? This means that late-season weed management tactics – such as hand-pulling, mowing or chemical termination – can make a decent dent in the seedbank by reducing the number of seeds going into the soil.

It can be hard to determine when flowering lambsquarters (above) moves into the immature seed stage. But like many other species, the weed is capable of producing viable weed seed in those early reproductive stages. (Photo credit: Mark VanGessel, University of Delaware)

The Fast Lane

Some of the hastiest species in the trial were giant foxtail and common ragweed. Other species aren’t far behind them, with jimsonweed, velvetleaf and common lambsquarters all showing some viable seeds at the “immature seed” phase of development, which follows close behind flowering. 

Don’t be fooled by the term “immature,” VanGessel adds. Seeds at the immature stage were often green and could easily be crushed, but these seeds continued to ripen even if the plant was uprooted or chopped up, he explains.

The signs of a flowering Palmer amaranth plant are subtle, but this is a critical period to keep the weeds from depositing thousands of seeds into the soil seedbank. (Photo credit: Mark VanGessel, University of Delaware)

What does this mean for late season weed management? Get those flowering weeds out of your fields, whenever possible, VanGessel says.

It can make a big difference – the same study found that terminating weeds at the immature seed stage did reduce the total number of weed seeds headed to the seedbank by 64% to 100%, compared to waiting to control them when fully mature. 

“Within two weeks of flowering, the seeds on some plants will be viable, meaning they are contributing to the weed seedbank and capable of producing seedlings next year or beyond,” VanGessel explains. “So if you are pulling weeds to stop them from producing seeds, you should physically remove the weeds from the field because they may already have mature seeds.” 

And while herbicides might kill weed escapes, don’t count on them to kill the developed weed seeds, VanGessel adds. The study found that spraying glyphosate on weeds late in the season had no impact on reducing seed viability.

So start scouting and move fast! 

“Many weeds have started to flower in the past week or two,” VanGessel says of his mid-Atlantic region. “And weeds, depending on the species, will continue to flower from now until frost.” 

Some weeds have showy flowers that make it easy to determine when flowering begins; while others have small nondescript flowers that can make it difficult to determine when they start to flower.

The large, bright flowers of morningglory plants are much easier to spot than the tiny yellow-green blooms of Palmer amaranth. (Photo credit: Mark VanGessel, University of Delaware)

See more resources from GROW on weed identification here and here

Text by Emily Unglesbee, GROW and Mark VanGessel, University of Delaware