As an Auslander, I hesitate to offer ideas to the locals...but since when has talking about plants left me as a wallflower.
critter's mention of Stoneleigh Gardens caught my eye. One of my younger brothers (resident of Hatfield/Lansdale metroplex, PA - near the Pottstown/Perkiomenville megalopolis where I was born and lived till I was 5) volunteers at Stoneleigh. He highly recommends a visit, and pull some weeds/eradicate some invasives while you're there.
I have had the good fortune to have traveled to and visited many of the gardens previously mentioned, and have yet to get to some of them. All good fun. I would advocate for additional places around Philadelphia - there's always more than you have time for, and having alternatives should unexpected obstacles arise makes for a more satisfying trip. But if you are Philly-bound, you'll need a passport:
http://americasgardencapital.o...
Swarthmore College campus in Swarthmore IS the Scott Arboretum - which includes the James R. Frorer Holly Collection and Crum Woods. It is swell.
https://www.scottarboretum.org...
Jenkins Arboretum is on that side of town. A horticultural fellow from the northeast who managed the High Line rehabilitation in NYC had come to Kentucky to manage the 4000 acre Parklands project in Louisville, and then moved back to Philly to manage the newly renovated Jenkins Arboretum. He's a topnotch friendly guy, and you could likely get a personal tour with advance notice.
https://www.jenkinsarboretum.o...
Agree with all the DuPont estates of Longwood, Chanticleer, Winterthur, and I think there are a couple more lesser known. I've only had the pleasure to visit Longwood on several occasions.
Bartram's Garden is doing some interesting inner city work, maintaining an 18th century horticultural enterprise. Medicinal gardens, wonderful handhewn architecture, and of course the 200+ year old Yellowwood (
Cladrastis kentukea) from Kentucky seed - worth some time if you are in the area.
https://www.bartramsgarden.org...
Haverford College campus boasts the Ryan Pinetum, home to 9 PA state champion trees, including interesting uncommon conifers.
https://www.haverford.edu/site...
The Arboretum of the Barnes Foundation is eclectic (my first and last visit was 1988); used to have a big old
Clethra barbinervis.
https://www.barnesfoundation.o...
Tyler Arboretum is in Media PA. It is a diverse place, with grand old specimen trees and many newer features since I last visited long ago. Their former director came to work in Louisville for the parks department in the late 1990s.
https://tylerarboretum.org/
I need to take a break...
