I've been planning for quite a while to write a guide for the new Farming content that's being added in Mists of Pandaria. After checking out some other guides on the subject, I decided that they were quite thorough. I don't think anyone will have difficulty finding a quality article, both Wowhead and El (of El's Anglin) have excellent coverage. Instead I thought I'd focus on Farms as they relate to tradeskills. There's a lot to learn about Farming and putting it all into one big post is quite overwhelming. I think this way you can read about Farming at your leisure and I will use my blog to cover the details that affect professions.
Hopefully you will all click over and read one of the guides I've linked. I'm going to start out with just a quick overview of Farms here. The quests for Farming are not available until level 90. However, basic functionality for Farming is active at 85. As a result, you can use your 85 alts to farm a small amount of Cooking materials on the first 4 plots that each player gets for Farming. If you want to maximize your Farming output, I highly encourage you to run your less active 85 toons to Halfhill as soon as you can.
When you get there Merchant Greenfield can sell you some of his seeds. You won't be able to purchase the craft-related ones until you're Revered with the Tillers. But you can still buy seeds for vegetable plants. So use your alts to activate the four starter plots and let them plant veggies! Not only will this help you level your Cooking faster, but you can also sell extra veggies to other players. Personally I don't see any reason not to take advantage of this.
It's important to note that crops take a full day to mature, currently resetting at midnight server time. Simply return the next day to harvest your crop. Each plant produces a minimum of 5 veggies and occasionally you'll get bonus veggies. If you plant the crop recommended by Jogu the Drunk, you are guaranteed to get 2-3 extra veggies per plant. There's also a type of plant called "plump" which gives bonus veggies. In addition to bonus veggies, you can also harvest bonus seeds, extra "gifts" for the Tiller NPCs and an item called an Ominous Seed. Planting this seed will create your own veggie pet! Here's a pic I took of Kaliope's Terrible Turnip:
Once you reach level 90 on a toon, you can start working on your Tiller reputation and unlock the other sections of your Farm! This process is covered pretty well in the other guides, but I want to at least discuss the timeline. By doing every quest and every daily quest, I was able to reach Honored with the Tillers on the second day. Each set of dailies will get you 1075 rep (without bonuses) which involves doing 2 daily quests for Farmer Yoon and 3 daily quests for other Tiller NPCs at Halfhill. You also get 50 rep with the Tillers for each crop you harvest (at level 90), so don't forget to pick veggies every day! This plus the occasional progression quests you unlock along the way will help you reach Revered with the Tillers in about 9 days.
Being Revered with the Tillers is where the real fun starts. By that point you should have 12-16 plots activated on your Farm. Now you can purchase all the seeds that let you plant and harvest crafting materials! The seeds you want to plant are: Enigma Seed, Raptorleaf Seed, Windshear Cactus, Magebulb Seed, Snakeroot Seed and Songbell Seed. Each of these plants will produce a different type of crafting component. They mature in one day, just like the other plants. Based on several rounds of testing there doesn't seem to be any benefit to leaving them in the ground longer than one day.
I've been planting combinations of these seeds for over a week now and I have some preliminary numbers on drop rates for you:
Songbell Seed: Always 1 Mote of Harmony (sample size 30+ plantings)
Enigma Seed: 40% silkweed | 20% Fool's Cap | 20% Golden Lotus | 10% Rain Poppy | 10% Green Tea
Raptorleaf: 70% 1x leather | 20% 1x leather + 1x intestine sack | 10% 2x leather
Snakeroot: 80% ghost iron (avg 3.5 ore) | 10% kyparite | 10% black trillium
Windshear Cactus: 1.8 windwool cloth per plant
Magebulb: 90% dust, 10% essences with an avg 3.1 dust per plant
These are all based on sample sizes of about 10 plantings each (except Songbell). The numbers may shift a bit as more players test the content, but I think they're probably enough to give a taste of the output. You can see that Golden Lotus actually comes up somewhat reliably from the Enigma seeds. The Snakeroot plant also has a decent chance of popping up trillium or kyparite. Given the rarity of these items in the wild, I think it could be worth devoting a portion of your farm to them. Obviously time will tell how the market plays out, but this is certainly a low-fuss way to get materials. For the record, each of these specialty seeds cost 1 gold per seed.
I think the most popular item is going to be the Songbell Seed. Given all the hubbub over collecting SoH, I suspect many of us will use our Farm to supplement production of this critical item. If you were to devote your entire Farm to Songbell you could produce almost 2 SoH per day (1.6 technically). I know that even in the Beta I've been using about half of my plots for Songbell :)
I hope this gives you a good idea of how Farming can help you with your tradeskills. If you have any questions please share them in the Comments!