i've used a lot of disparate internet sources from googling and duckduckgo-ing like wikipedia and whatnot, but i've also got a few sources i used consistently for reference! here they are. hooray for the internet and the library!

Go Botany
HOSTS - a Database of the World's Lepidopteran Hostplants
UMass Amherst's Weed Herbarium
iNaturalist
Wildlowers in the Field and Forest (Clements & Gracie; 2006)
A Guide to Invasive Plants in Massachusetts (Somers, Kramer, Lombard, Brumback; 2008)
Weeds of the Northeast (Uva, Neal, & DiTomaso; 1997)
Wildflowers of New England (Elliman & New England Wild Flower Society, 2016)
The Audubon Society Field Guide to North American Wildflowers, Eastern Region (Knopf, 1985)
Wild Urban Plants of the Northeast (Del Tredici, 2010)