Mugwort is a persistent plant.

The seeds of the fall-blooming plant are tiny, and each plant can have up to up to 200,000 that easily travel on the wind, in tire treads or rain runoff. It can sprout on roadsides, vehicle storage lots and gravel roads.

It first came to the United States from Europe in soil used as ship ballast, and has since proven to spread and grow quickly and to be a bad neighbor for native plants.

"It will tend to prevent other plants (from growing) in the same area where it's growing," said Leigh Knuttel, the Connecticut College Arboretum's botanist. "It also chemically alters the soil ... in that area."

Mugwort pollen can cause hay fever. The plant can live in both dry and moist conditions, and very few American caterpillars, rabbits or birds have evolved to eat it. It has spread and threatened human and ecological health along the East Coast, and also can crowd out plants that attract pollinators such as bees and butterflies.

"It is an 'everywhere' problem," Knuttel said.

And mid-September, according to the Connecticut Botanical Society, is the best time to mow or chop it down, before it goes to seed and after the point that the plants will have time to grow back and flower again.

The state botanical society is encouraging town public works departments to mow it now, and for owners of large properties that might have clusters of mugwort to address it.

"It's nice to have vacant lots that are natural," said Sigrun Gadwa, an ecological consultant and a volunteer with the botanical society. "But if there are large patches of mugwort that can be seed sources, it would be nice if they were mowed."

Public works department employees usually mow year-round, Gadwa said, but don't often focus their attention on mugwort or other fall-blooming invasive plants during the time before they go to seed.

Knuttel said the plant has been mostly eradicated in the fenced-in area of the Connecticut College Arboretum but persists on the outer edges of the property close to the roadways that surround it.

"Where we can, we mow," she said. "We definitely try to prevent it from going to seed."

A small study by Jeff Ward, an invasive plant expert at the Connecticut Agricultural Experiment Station, showed that mugwort seed germinates poorly in late September, so if people mow or cut it down now, there is less likelihood that the plant's seeds will spread and germinate, Gadwa said.

"During the first three weeks, the seeds are not viable," she said. "They should try to weed-wack it or use clippers before Sept. 21."

The plants can be managed after then, too, she said, but it requires work. For example, if you just dump it in a pile, you could end up with a mugwort patch in that area.

m.shanahan@theday.com