Sounds like a wonderful project. Are you looking for information on native plants such as wildflowers or cultivated plants such as roses, peonies, etc. ?

I would like to include native wildflowers.

Just off the top of my head: mayflowers, trilliums, lady slippers, Quenn Anne's lace, brown-eyed susans, buttercups, asters, and daisies. Lots more---check with your library about borrowing a book by Judith Johnson:"The Heritage of Our Maine Wildflowers. . ."

Thank you very much for the information on wildflowers and the suggested reading. I appreciate your time.

Queen anne's lace and oxeye daisy are not native, but would have been brought over by the settlers. I might add wild strawberry to that list though, as well as violets and blueberries.

The state arboreum in Augusta has a heritage garden, and if I remember correctly there is a sign with names of the plants. I work very close and will try to walk over on my lunch hour someday next week and write them down.

cattails and daylillies are not indigenous either.

Settlers at that time were no doubt interested in survival rather than ornamental gardening. That being said, the source you should consult is John Josselyn's "New England's Rarities". He visited New England in about your time frame and his small book contains probably the earliest plant list made in New England. The book has often been reprinted and is often quoted. Your library can get it for you--or it might even be online, if you Google his name. Let us know how it turns out.

A few of the old species bearded iris survived the trip over. Lot of dual purpose plants would have been grown. Chamomile, calendula come to mind. Pretty flowers with some medicinal value. Calendula was also used as dye and to color up food a bit. Comfrey has interesting leaves.
Dill is a cool looking plant.
Chives in bloom are lovely.
Things that will look nice and serve a purpose at the same time would have been favored.

Squashes
corn
sheaves of grain
apple trees

I did get to the Arboreum this week, but was disappointed that there was no sign telling what the plants were, I guess my memory was wrong about that.
The garden was in rather poor shape, but what I could recognize was:lavender, catnip, alchemilla, vervain, dianthus (bluegreen leaved variety), circle flower winter savory (still had a readable tag), tansy, and a Damson plum tree (prunus domestica).
There were also 2 types of artimesia, one A. pontica aka Roman wormwood, which I grow in my garden, and a more green leaved one which I think is the common wormwood. I had that in a different garden, but it was even more invasive than the other artimesias.
Sorry I couldn't be more help.