Where can they be grown?

They like a damp position, they hate being dry or drying out.
Sun or part shade is best.
Gunnera manicata is the enormous leaved one that you can stand under with 10ft+ leaves, or Gunnera tinctoria that gets to around 6 foot high with smaller leaves.
They do form slowly spreading clumps and dont run for meters.
However, Im not sure of the temperate of Pennsylvania!
If you dont get frosts then they will seed and probably grow, and become a weed- they produce thousands of seeds!

If you do get frosts, then the seeds wont germinate and thereofre wont be a weed problem!
They aren't hardy in frosty conditions and so will need cutting back after a frost and then covering with a thick straw mulch and the old leaves put over the top to keep out winter wet, as they wont like winter wet and the cold.
If you dont get frosts or colds spells, then they will be everygreen all year.

Mine in england has to be wrapped in winter, but on the east coasts of ireland and cornwall, they can grow all year and become a weed esp ireland
.
Some countries have banned the plants as they are so invasive e.g Chile as the weather is so warm all year and irradication is a constant thing!

Negative effects are that the seedlings can (if your not a cold country) out compete native and local species as there huge sized leaves block out light and rain from the soil under the plants and doesnt allow anything else to grow underneath its enormous prickly leaves.

Hope this helps, any more questions please ask!

i have grown a gunnera manicata in a very large pot for about three years now.the leaves can get to about two foot across,some smaller.the leaves feel really rough and strange.it does well in it pot as long as you water it well.i will post some photos of it now.

They also like a relatively rich soil thats constantly moist, the better the soil and root run, the bigger and happier the plant