The only thing worse than an invasive weed that's hard to get rid of is a hybrid of that weed that's nearly impossible to eradicate.

Creating a robust weed: Hybrid offshoots are more difficult to control

The only thing worse than an invasive weed that’s hard to get rid of is a hybrid of that weed that’s nearly impossible to eradicate. When two plant species contribute to a hybrid, new capabilities for invasion can also be created. Recently formed plant hybrids have been shown to spread rapidly.

A study reported in the current issue of the journal Invasive Plant Science and Management examined the hybridization and spread of Japanese knotweed and giant knotweed in the United States. Previous studies have focused on this occurrence in the plants’ native Asia and in Europe, but not in North America.

Using flow cytometry, researchers investigated DNA content of established plant families in a common garden, seedlings grown from common garden parents, and wild populations. The authors report that not only did the two varieties of knotweed cross-breed in a controlled garden setting, but the hybrids were identified in field populations in Massachusetts as well. Nearly all the created hybrids showed strong growth, seed set, and the production of viable pollen.

Significant introgression, where a hybrid crosses once again with one of its parents, is also suggested by the study results. As introgression progresses, a more diverse swarm of invasive plants is created. Once populations are established, options for eradication become more limited.

The authors emphasize the importance of managing sexual reproduction of invasive species. Careful management of giant knotweed is essential to limiting its role as a parent to a hybrid. Hybrids also should be recognized as invasive species that are diverse, vigorous, and fertile.

Full text of the article, “Viability, Growth, and Fertility of Knotweed Cytotypes in North America,” Invasive Plant Science and Management, Volume 3, Issue 3, July–September 2010, is available at