Plant Fiber is a byproduct item in RUST and is used in farming, spawnable via the F1 console. Players may only acquire plant fiber by picking crops that are explicitly player-grown. Wild-grown plants do not yield plant fiber. It is then only useful as Fertilizer, created by placing it into the Composter.
Fertilizer serves two potential purposes. It may be used to improve the ground quality of both the Large and Small Planter Boxes. Alternatively, it can be harvested and sold directly to Bandit Camp at a rate of 3 Scrap per 2 fertilizers.
For more information regarding plant fiber, fertilizer, and its assorted uses in plants, check out our extensive Farming guide.