The new Cataclysm version of the charts can be found here: http://reviveandrejuvenate.blogspot.com/p/leveling-herbalism-cataclysm-edition.html
Just about everyone will level Herbalism at some time in their playing career. Back before leveling was speeded up, it was fairly easy to keep up your herb-picking skill with the level of herbs in the area you were questing in at the time, as long as you ventured off the beaten track a bit.
Nowadays with heirloom items, refer-a-friend and just speedier leveling in general, it is such a pain to keep your herbalism up to speed as you level that it is pretty much easier to just leave it until you're high level and go back to pick flowers later. If you're taking herbalism on your Death Knight, it's the only option you have.
There are a lot of herbalism guides out there - googling for "herbalism guide" turns up a load of them. The ones at thenoobschool.com and almostgaming.com even have maps with routes for you to take for each zone. Most of the others just contain text-based lists informing where to go next.
When leveling Leesah's herbalism I was following one of these guides, doing laps of Swamp of Sorrows, when I realised it would be useful to have a chart of which herbs were available in which zones, with the levels those herbs were available. Such a chart would be a handy reference to work out the optimal location for my current level of herbalism.
So, I made one. Two charts, in fact. This is the chart for 1-300 herbalism. The 300-450 part is further down in the post. Click the image to make it bigger.
How to use the chart
The number in each box is the number of nodes the herb spawns in in each zone, based on data from wowhead.com and wowwiki.com's herb pages.
I have put an asterisk in the starter area boxes because there are a LOT of nodes in each of these zones. It should be pretty easy to get your herbalism to 50 so that you can start picking Mageroyal in the next zone you go to.
Look at the herbs that you are able to pick at your current level of herbalism. Where do those herbs spawn the most?
For example, Aefa's herbalism (yes I am leveling two toons with herbalism) is currently at 170. At this level, herbs like Stranglekelp, Bruiseweed and Wild Steelbloom are still green. I am picking and still expecting to level from Kingsblood, Liferoot and Grave Moss (if I find any) and lately have started being able to pick Fadeleaf and Goldthorn.
Based on the chart, the ideal areas for Aefa to level up herbing are Stranglethorn Vale or Arathi Highlands. This comes as no surprise - STV is like a great big herb garden. With the most herb nodes of any zone this side of Outlands it is an ideal zone to get up to about 230 herbalism, until you reach the Sungrass phase and herbs like Kingsblood start going gray.
After 230, the next areas that have good levels of nodes include Ferelas, Hinterlands, Swamp of Sorrows and Azshara. Hinterlands is far easier to get around than the mountainous Ferelas and Azshara, so it gets my vote for sprinklings of Sungrass and huge ruins complexes full of trolls and Purple Lotus, and Swamp of Sorrows has enough Blindweed to make anyone blind.
Another interesting thing came out after analysing this data: You don't need to have 300 herbalism before you go to Outlands. Hellfire Peninsula has plenty of Golden Sansam and Dreamfoil to get you up there, so you can safely go to the GOGG at about 260-280 herbalism.
Speaking of Outlands, here's the Outlands and Northrend chart:
Some of the herbs have question marks in the level field as the information was not available in the sources I was working with. If anyone has this information please let me know and I'll update the chart.
Once you get to Outlands, there are herbs all over the place and Felweed stays green until 400, so getting up to at least 350 herbalism to start your Northrend gathering is easily done.
Leveling Alchemy or Inscription? If you are an alcho or a scribe, you can use the charts to locate the herbs you need for the current potions or inks, if the prices of such items or of the herbs themselves are too high on your server.
Any Goblins in the house? Find out which herbs sell for the most per stack on your server, consult the chart to locate them and go on a herbing spree. Time is money, friend! Of course, if you were a real Goblin you'd just work out which herbs were underpriced, buy them and sell at a profit. But where's the fun in that?!
There is one last thing that I can't help you with, and that's that you must remember to train the next level of Herbalism before levels 75, 150, 225 and 300!
Happy Herbing :)