[FishDr]

As others have said, it depends on the size bass you're after and the size flies you're planning on throwing.

My first bass rod was a 9-weight that I used extensively fishing for bass up to 7 pounds in the California delta. The fish were generally larger than the bass you come across in CO (average at my favorite spot was about 3 pounds) and were close to, or in, heavy cover (water hyacinths, tules, and some massive logjams). I threw a lot of big (1/0 - 3/0) poppers as well as some heavily weighted bunny flies and crawdad imitations and there was always a real chance of stumbling across a striped bass, so having the extra backbone came in handy. Out here in CO I use the rod a bit for largemouth, but with a couple of exceptions, few lakes have large populations of big fish so it might be overkill. The 9-weight now sees most of its action on wipers and northerns, so if you're planning on tackling those species with the same rod, going with a bit of overkill might not be a bad idea.

My second bass rod is a 7-weight. I bought it to throw smaller (8 - 1) poppers and various streamers and nymphs at CO bass, especially smallmouth and spots. It has also become my favorite rod when I'm stalking bigger carp (10 - 20 pound class) because it has the backbone (and the real has the backing capacity) to give me a fighting chance when a 15-pounder heads for the other side of the lake at flank speed, or decides that becoming intimately familiar with the interior of a weedbed is suddenly a good idea.

In either case, definitely get a line designed for throwing bass flies...they can be quite bulky and the specially designed taper makes it a little easier to get the flies into those sweet spots where you just know a big fish is waiting patiently for something to wander by.

 

[FishDr]

You can catch bass, sometimes big bass, on a 5-weight or even a 3-weight, but it's not necessarily optimal.  I used to do a lot of fishing for bluegill, green sunfish, and some aggressive bluegill X redear sunfish hybrids on this one creek in northern California using poppers on either a 3 or a 5 weight.  It was a rare trip where a largemouth bass didn't find the little size 8 - 12 popper attractive and came up to eat. I normally didn't lose the fish, but landing them required more finesse than would have been necessary with a 9-weight.  I have to admit it was fun when a 2 - 4 pound largemouth came up to say hello, but sometimes I felt pretty undergunned (especially on the 3-weight). Of course, there was the one bass that ate the bluegill that ate the popper (the bass then went around the drowned tree, through the willows and onwards, but that's another story)...

Here's one of those "incidental" bass that took a liking to a little bluegill popper.

Also, I'd hate to try to throw a big fly, like a 1/0 or 2/0 Dahlberg Diver or worse still a 3/0 bunny fly that's waterlogged, into the wind on a lighter rod...a better caster than me could do it, but I don't like to struggle any more than I have to. Distance is normally not a problem because I, like Castell (welcome!), do most of my bass fishing out of a float tube or pontoon boat. Those man-powered craft are a great aid to poor casters like myself...if you can't cast far enough, just close in!