Garden Web has various online discussion groups. Some are for just
one type of plant, like Camellias, Hydrangeas, Azaleas and
Rhododendrons, but there are also two general ones that would
interest you - one on trees, and the other on shrubs.
Here are a few of the shrubs being discussed now: Calycanthus,
Corylus, Daphne, Gardenia, Hamamelis, Juniperus, Viburnum. As you
can see, they cover a wide range.
Here are a few of the tree discussions: Avocado, Magnolia, Paulownia,
Persimmons from seed.