To get rockarrot, you first need to harvest the seeds out of bushes (the same way you'd harvest berries). It's a bit uncommon but super easy with a herbivore tame (Bronto is the best tame). Then you need to grow it in a crop plot using 1 seed, a water source, and fertilizer (or just regular poop [size matters]). If the plot runs out of any of these resources then the plant will die immediately! It takes a while to grow to full size but once it does it gives the vegetables fairly fast.