Plant gathering allows you to gather wild above-ground plants to add to your food stores. The skill associated with plant gathering is Herbalism. Higher ranks in the skill will increase the harvest by decreasing the time required and increasing the average size of the stack found.
Plant gathering can sometimes make all the difference between surviving and starving. However, the plants you gather may require some processing, and in a few cases (such as hide root), cannot be made into any food item.
To gather plants, at least one of your dwarves must have the "plant gathering" labor designated. Then designate one or more shrubs to be gathered by pressingfor 'designate', and then for 'plants'. Move the cursor to one corner of the area you want to harvest, and hit . Now move diagonally to the far corner of the area, and hit again. This will mark all harvestable shrubs in that area for picking. Alternately, you can left-click individual plants with your mouse.
Your dwarf(s) will then start going out to pick those plants. Every shrub they try may result in a single wild outdoor plant, a stack of several plants, or no plants. The shrub will disappear, regardless of the result. Once the dwarf has any success, they will immediately store the plant in an appropriate food stockpile, even if they do not have the "food hauling" labor enabled.
Choosing an area dense with shrubs (shown by " symbols.) will reduce the travel time of an unsuccessful dwarf from plant to plant. Different plants grow in different biomes, and knowing what biomes are represented on your map, and where, can significantly influence the type of plants you find.
Since you cannot buy above-ground seeds pre-embark, Plant Gathering is also important for obtaining seeds if you plan to do outdoor farming, as the only alternative is to hope that caravans bring some for trade. The plants gathered must first be brewed, milled or eaten to obtain seeds (if any). Seeds are then planted in above ground farm plots by a dwarf with the grower labor enabled, where they will grow reliably and be automatically harvested each season.
Underground shrubs (which only grow once an underground river or pool has been discovered) can be gathered just as those above ground, though they will always be successfully harvested, yielding a random underground crop.