First of all, you have to do a quest called "Cleansing Felwood". The quest mob is located at the blue diamond in the map below:
Once you have done that quest, return to her and she will offer several methods of obtaining stuff for her to make plant salve with. Either from mob kills, herb harvests or from skinning. Choose the one you want, and start harvesting stuff. You'll want at arseload of the shards, goop or whatever, so you can get 40-50 salves or more. Once you have those, then you need to scout out the Night Dragon and Whipper Root Tuber plants. The Green Diamonds on the map represent the Tubers, the Red Diamonds are the Night Dragons.
The plants are on a 24 minute timer. Currently, when you right-click a plant, it will open a dialog box like a quest. If it needs to be cleansed, you can do that. If it is cleansed already, you'll be offered the chance to pick the fruit or whatever. Now, if you're fast, you can quickly click the dialog "Pick the fruit if the Night Dragon" then swiftly right-click the plant again and you can pick it again. Or, get this mod:
http://www.curse-gaming.com/mod.php?addid=1996
After putting it in your IF folder under Add-ons, restart the game and type "/turnin on". Then, all you have to do on a plant that is up is just spam right-click and it will repeatedly harvest the fruit for you 4-5 times. You can really net some decent amounts of these that way.
After you harvest a plant, make a note of the time and you can come back to it in 24 mins.
Most of the time, there are a group of people gathering them, and they'll have the timers up on all of them. The way it is set up, if you have an epic mount and go from plant to plant, and by the time you get them all, the first one will be back up when you get to it. This means that a group orbiting the whole place will get them all. What you should try to do if you go up there is ask in general if anyone is gathering them and join them.
When gathering them in a group (without the turnin mod) usually someone says "Open" and everyone opens the dialog box without clicking pick. Then there is a countdown and on one, everyone picks and trys to get a second go.
Supposedly in 1.9, they are removing the dialog box/quest box method of gathering them, and you'll loot them like everything else. This means only one person will get the fruit every 24 mins, so individuals will probably be camping spawns. Dunno how that will work yet, but I imagine a lot of people will stop using them, and go with pots. But in the mean time, people are gathering fruits by the hundreds. One of our GMs has 800ND and about 900 tubers.