Farming is the profession of growing crops and managing livestock. To get started, you're going to need a fence. Fences can be rented for contribution points from certain NPCs. To find the NPCs from which you can obtain a fence, click the magnifying glass icon near the mini map at the top right of the screen and search for "fence". It will even show you what size of fencing they have to offer. It is important to know that you can obtain any amount of fences you want, but you must place each fence before getting the next (because the npc will just try to take back the fence already in your possession if you do not).
Once you plot out your farming area open up the farming menu on your new farm and place your seeds. (note that plants too closely cluttered can slow growth).Every fence has only a limited number of crop fields it can support at the same time - independent of how many fields it has in total. This is depicted by the "Garden Usage" number as shown on the right (hit 'R' after placing the fence to bring the crop management level up).
Growth conditions define the speed and yield of your farming process:
The temperature level must be appropriate for the crop you want to grow. The little red arrow above the Temperature bar indicates the current temperature while the little yellow arrow below is a slider you can move. This does NOT change the growth condition but allows you to see how the crop growth speed would change if the temperature was different. Based on that you might decide to move your fence or plant this crop somewhere else. The number on the right indicates the growth rate every 5 minutes.
Crops need a certain humidity level to thrive - which level range is optimal depends on the crop. Humidity depends on the level of groundwater which varies with the region.
The (ground) water level of the farm is depicted as "Water" bar below the humidity and should ideally be 100%. That would indicate the ground water reservoir is 100% full. If the ground water reservoir falls below 75% the yield can be affected. Groundwater can be refilled by rain, installing a waterway or by pouring freshwater onto the ground. Waterways can be crafted in the Carpentry Workshop but it takes up one crop field.
Humidity increases over night too. If humidity reaches zero the yield decreases. If the humidity level is too high the plants are at risk of being affected by Blight, a plant diseases. The blue arrow above the humidity bar indicates the current humidity, the yellow arrow below allows you to determine how the crop blight risk would change if the humidity was different. That is purely analytical and does not impact the actual condition! The number on the right indicates the blight risk every 5 minutes.
Plants need a certain amount of nutrition to grow. Fertility of the ground depicts how much nutrition is in the soil, this value can be increased by dropping fertilizer. Fertile soil increases the growth rate.
Healthy plants are growing well and that means pruning them if necessary, keeping insects away (the less cluttered the plants are the less likely insects spread), preventing Blight and chasing off crows. Crows will slow down the growth rate of a plant and can be staved off by installation of a Scarecrow.
If no device is installed to control any of these needs then they must be taken care of manually or by assigning workers to the farm while the plants grow. To assign workers, click on the fence/farm symbol on the main screen top left. Workers only care for crops, they don't harvest, seed or provide experience for you. On the map a symbol indicates the location of placed fences, the general status and hovering over them indicates the detailed condition of each crop.
Pruning can yield - depending on the Farming skill level - items like Weeds and Monk's Branch. Workers can obtain the same products from pruning. They will deposit them in the warehouse - if there are slots available.
Harvest versus Breeding
When the Growth level of a crop reaches 100% it can be harvested for produce/goods, can be kept growing for a while longer (the yield decrease with increasing age of the plant) or bred for further gardening. Harvesting yields a certain amount of the actual crop but can yield extra items. Among them Monk's Branch, Fruit of Nature, Fruit of the Sun.
Breeding can yield one or multiple seeds for this crop of the same or higher quality and several byproducts and rare crafting resources. Byproducts of Gardening can be exchanged for gathering experience, contribution points or high quality seeds depending on the type of the byproduct. The Byproducts are Seed Half Eaten by a Bird, Premature Fruit, Mutant Plant. Byproducts can be bundled/stacked but seeds each take up one storage slot.
Gathered seeds are usually of basic quality but they can yield high quality or better seeds when the mature crops are used for breeding instead of harvesting.
(see page linked above)