Gymnocoronis spilanthoides, Senegal Tea, is a pond plant which attracts Monarch butterflies without equal. I usually buy it at a aquarium supply store in St. Louis for about $8/bunch. I’ll stick each cutting in some soil and place it in the pond and watch it grow. While the flower is not much to speak of, the fall Monarchs just love it and as the one picture shows, the plant seems to have some sort of chemical which the Monarchs love. I pulled out about half of the plants one day to make more room in the pond and the next day, you can see 27 Monarchs sucking on the roots.
Here’s a website which sells this plant.
Here’s a video I took at the Missouri Botanical Garden.
I found an article which lists Gymnocoronis as an invasive weed in Australia. I would probably suggest not growing this in southern states.