I'm not sure if this is the plant you're describing, but in New England, Monarchs are really attracted to Milkweed. The White Swamp [0] variety may be what you're describing.
[0] https://www.fs.fed.us/wildflowers/plant-of-the-week/asclepia...
[0] https://www.fs.fed.us/wildflowers/plant-of-the-week/asclepia...
In this particular case, though, it was a similar plant that I normally always see in sunny areas. On the edges of my mowed yard, I was seeing a lot of monarchs for a few days.