AWBURY ARBORETUM If you haven't been recently, you should visit again. It's here in Germantown! Lovely old hardy citrus along the wall near the 'secret garden' which smell great in bloom. The original landscaper introduced the navel orange in California and I wonder if he actually planted these mature specimens? Great place to volunteer when they have work days as a they have only a meager endowment left.
BARNES Original Museum has wonderful plants on the grounds, a small shop, and a horticultural certificate program. Some of their coolest trees are a monkey tree, and a small grove of different mature stewartia with irresitable bark. PLANT SALE usually first weekend in May. Best to get there at beginning. (Members allowed in early.) Shop for unique plants, many propagated from the Barnes collection, including Stewartia, magnolias, greenhouse specimens, annuals, and perennials. All funds raised support the Barnes’s horticultural education programs. Prices are often very reasonable.
MORRIS ARBORETUM There's always something changing here. Even the old Mill is now working. I almost always volunteer for the plant sale (and spend more money than I should). By the way, if you buy a lot it's very well worth volunteering for the discount. The PLANT SALE is the second weekend in May. To get the best selection, join the Arboretum and you can go a day earlier (even on Thursday if you are a major donor). (Or wait til late on the last day and pick from the few plants left at a discount.) The staff who work at the plant sale are very knowledgeable and helpful. I save up my questions over the year and usually get them answered. (Like why can't I get hollyhocks to overwinter?)
SCOTT ARBORETUM, Swarthmore College The arboretum spreads over the campus. Huge variety of plants, a PLANT SALE which is now in spring, and some of the very best container plantings I've seen. If you look around there are info boxes nearby which list the plants.
Most of the other area greater philadelphia gardens are listed here.CHANTICLEER is over the top spectacular, and BARTRAM'S' is sweet and cool by the river. Their plant sale is early May, day earlier if you are a member.
INTERNATIONAL USEFUL WEB SITES
There are two major world wide gardening sites. I generally go to Dave's for information, but the forums can make the most difference for finding really helpful suggestions. For instance, Dave's doesn't have a camellia forum but the garden web does. (When researching Camellias I ran into an invitation to a garden in N.J. full of Camellias and many plants I'd only seen in books. (Steve has offered palms to people planting and caring for them in a public place on the forum here. You do have to pay to read some of the forums, but that is often where to find the most info, help, and encouragement. My membership often lapses, but in the winter months it's fun to mess around on them). A plant I was trying to identify for over a year was ID'd within hours when I posted a picture on Dave's ID forum. I was able to find someone growing a Choisiya in Philly when I had to figure out what exposure it might like best.
The best feature of Dave's is that they rank mail order companies. You can see how many people had good or bad experiences with a given company. And you can often find which nurseries sell a specific rare plant.