Simple Idea:
Each Herb that grows in the land also bears a seed.
Each seed can be planted anywhere, after a period of time a new herb grows where the seed was planted.
Depending on the type of land (forest, desert, water, mountain) it may or may not grow. Some seeds are corrupted, and won't grow no matter where they are planted.
Example:
A small red herb grows here.
A small red seed is on the ground.
Get herb
You get a small red herb!
Get seed
You get a small red seed!
travel
Plant red seed
You sow the red seeds into the fertile land with a wicked spade.
return later*
A small red herb grows here.
A small red seed is on the ground.
Another spin on this could be making farmer a perk at startup (much like heroism, addict, etc)
Druids and gnomes can farm automatically. Undeads and Demons cannot farm.
This would also create a "Seed economy" in the lands.
its a neat idea, however..I see no real need for it. There is far from a shortage of herbs.
It if doesn't grow - it dies. Maintaining the status quo will just allow the mud to continue slowly fading. I don't feel much about the idea either way, but i think its always a bad idea to just be negative about something without much of a reason.
I (we) are not negative, I just see no reason for this. Do you?
Gathering seeds and cultivating plants is not exactly FLish, is it?
Ironically, the game I was playing before I came to FL was called bushtarion, and was exactly about gathering seeds and growing plants 
stop playing farmville mali
The only issues I have with the idea is that eventually you run out of space. The whole mud is @ 20,000 rooms. many of them are not in forest/grassland areas so I dare say that nearly half of the rooms are not, so that leaves 10,000 rooms. Then subtract the couple of rooms that have herbs/bushes already in them, and we are still left with over 9,000, but the problem is that give it six months to a year, and those rooms would all have herbs in them. I really like the idea, however described as you did, would not work so well. With so many herbs around, I would very easily be able to go anywhere, and find exactly what I need all within a couple of rooms movement, without cost.
Possible solution, druids and maybe the perk as suggested allow for them to farm the land and cause the herb to grow there for a few months. Although there would certainly have to be a timer on this to grow to a point it would be able to be picked and used, maybe give it 3 days. After you pluck it, it takes additional farming and watering. For those without the ability to cast or use an item to control weather, you could just pour out your water from a jug/barrel to accomplish this.
I like the idea. I do not agree with Foxx or Kyzarius. I think it gives Forsaken Lands something different. Just because there is no need for this type of idea that doesn't mean it would not be a good addition. Everyone wants to scream about role playing but when people give ideas that could essentially assist in role playing you shoot it down because it doesn't belong in Forsaken Lands.
Now saying that I do realize that there are more important things in Forsaken Lands that needs to be worked on. A farming system is most likely low on the priority list. I still think it's a decent idea.
I can see Cephirus point. A system would have to be worked out of course so that not every room could be farmed.