I've wondered about this for a long time. Why are there such good lists for
cars and so few for other subjects?

The excellent car lists are based on two factors. First, all the cars in
question are almost exactly the same, with tiny variations. That makes it so
a list member in Alaska can read a post on an idle stabiliser problem, go
out to his or her van, and find exactly the same device located at exactly
the same location  on his engime compartment wall. We automatically  have
something in common. Second, there are large numbers of these cars produced,
which helps garner a large number of listees. A list dealing with antique
Wedgewood stove, for example, would satisfy the first requirement, but
therre are too few in existence to attract a wide following.

Compare this to "gardening," where the variables are almost infinite, and a
list member in Alaska will have nothing in common with one in Arizona. It's
hard to relate to a cactus with pumpkin rot when you're in the Yukon. The
element of commonality is too general in a "gardening" list.

A list equivalent in specificity to Vanagon.com would be some something
like, "The ferms in low elevation southern Oregon" group.

I did find a pretty good site on rhododendrons a few years back, but my soil
didn't correspond to theirs...