I wouldn't sweat the particulars of most plants, they mostly want the
same things, adequate moisture, sunlight and food. If you try and
match the "needs" that you find people say each plant needs you won't
be able to grow anything!!! There are a few exceptions,as in
everything! In my expeience as long as the soil isn't bone dry, soggy,
in the deep shade or on the south side of a sunny wall almost every
thing thrives despite the "needs" I see in the literature on plants! I
have black eyed susan and purple coneflower in more shade than they
should like and on and on.
I suggest (based on my experience in the mid atlantic region
zone7-6) see what is "required" for the plants you want and try and
come close to it but don't worry about trying to push the limits! Give
the plants room, I fertilize lightly with 10-10-10 in early spring and
for plants that like richer soil I side dress them with an extra dose.
There are limits of course and I guess knowing them comes with
experience in your particular site, I grow larkspur that tops out at
6-7 FEET, others in my area think 2 feet is doing well, but I can't
keep primrose alive (despite matching it's needs!) for anything!
All in all take the experts opinions with a grain of salt! In my
experience most things will grow and even thrive in most sites, just
beware of the extreme sites, as noted before, in them stick to the
plants that like the conditions, at first! A small tree wil eventually
lightly shade an overly sunny site and you can water a dry site....
but beware the soggy site!!!! :) Good Luck and sorry if this was no
help!