To harvest the plant you can right-click it. Left-clicking will harvest the plant as well as destroy it, leaving the crop sticks. Harvesting Ferru may yield a Small Pile of Iron Dust. Two Small Piles of Iron Dust can be crafted to form one Iron Dust, which can be smelted into Iron Ingots or used in other crafting recipes. Harvesting while destroying can also produce Ferru Seed Bags. The harvest of the plant is determined by the plant's Gain statistic.
A good way to increase the amount of iron dust it drops, how fast it grows and how resistant to trampling it is, is to cross breed Ferru with Ferru as this has the highest chance of getting a Ferru Plant. Collect the seed bags, scan it through a Cropnalyzer and then cross breed the 2 Ferru seed bags that have the highest statistics with each other as this will often yield even higher stats. Then you can continuously breed the plants with higher stats together until you are satisfied. A more in-depth view of crop stats can be found on the Cross Breeding and GGR pages. The stats are referred to as Growth, Gain and Resistance.
Note: Ferru will not grow to its final, harvestable stage without having a block of iron ore beneath it, only to stage 3 of its growth cycle (see photo below). It is also beneficial to treat your plants with Hydration Cells, Weed-Ex and Fertilizer. This process can be automated with a Crop-Matron. It is important to note that excessive use of Weed-Ex has the effect of reducing the stats of crops.
Ferru Growth Stages: