Define "northern New Jersey" (what town or county).

Mahwah, NJ

I don't know what goes on in Bergen Cty, but check with your county college if they have a landscaping/horticulure program, or your nearest arboretum (spelling?). Maybe Ringwood Manor (shouldn't be too far from Mahwah?) has some programs, check their website. I was there early in the spring and I think they had a sheet with programs or activities.

Check your library's message board (and nearest townships also) and local newspaper (the one that comes every Thursday or Friday). Also you can call your recreation department and ask them.

I can't find my directory at the moment, but the link to the Garden Club of NJ is below. Send them a note, and they can tell you which local club is closest to you.

Here is a link that might be useful: The Garden Club of NJ

Are you interested in a certain kind of gardening? I am not familiar with a particular group around you but you might look into some of the following.

Bergen County has a Master Gardeners program.
http://njaes.rutgers.edu/mastergardeners/

Native Plant Society of NJ, Northern chapter meets at Pyramid Mountain
http://www.npsnj.org/index.htm

Rose Societies
http://www.ars.org/Districts/penn_jersey.html

There are other specialty societies around. If you post your interest, someone might know.

Frelinghuysen Arboretum is a little South from you but not a bad drive for an occasional workshop. Here in Morris County, several of the parks have programs. You might look into the Parks dept in Bergen County.
http://www.arboretumfriends.org/

Same with Well Sweep - a little far, holds some workshops -
http://www.wellsweep.com/Events1.htm

Sometimes nurseries have workshops. You might look near you.

Here is a map of some gardens in Bergen County. They might have groups around them.
http://ifplantscouldtalk.rutgers.edu/njfgaa/

You might look at gardens on municipal property and see if it is maintained by any group. In our town, there is a Junior Women's club that gets involved with that.

Cook's college will hold special classes for home gardeners. That's a ride for you. Also, adult education programs often have some gardening classes.