Arugula is a fast-growing, cool-season salad green often ready to harvest just 4 weeks after seeding. It prefers rich, slightly acidic soil with a pH of 6 to 6.8 , but tolerates a wide variety of conditions. Arugula is related to plants in the cabbage family like kale, collards, and broccoli, so as you rotate crops in your garden, try to avoid planting arugula after other cabbage family crops.
Arugula seeds germinate quickly even in cold soil, so plant them as soon as soil can be worked in spring. Plant seeds ¼ inch deep and 1 inch apart in rows, or broadcast alone or mixed with other greens. As the plants grow, gradually thin them to 6-inch spacings (and don’t let your thinnings go to waste - use them in salads!) Mature plants will form a rosette of deeply lobed leaves. Plant in successions every 2 to 3 weeks until about a month before your average first frost date for a continuous supply. Since arugula is low-growing and somewhat shade-tolerant, it is a good choice for intercropping. Try growing it along the edges of beds and around the base of taller plants.
To harvest arugula, pick off the outside tender leaves at the base of the plant, or cut the whole plant about an inch above the ground. Leave the center growing point so the plant can re-grow. Larger leaves tend to get tough and very bitter tasting so harvest them small. Hot weather can cause more bitter flavor to develop in arugula, and also cause the plants to bolt (go to seed). You can slow bolting by reducing heat with shade and avoiding moisture stress with regular watering.
Arugula is insect-pollinated, so different varieties should be separated from each other by ½ mile to ensure pure seed. It is also self-sterile (individual plants cannot pollinate themselves), so you’ll need at least five arugula plants to ensure viable seeds. A population of about 20-50 plants is better if you plan to save seed for multiple generations. Common arugula or rocket (Eruca sativa) is an annual, unlike many other members of the Brassicaceae family, and will bolt in hot weather even if it has not been exposed to cold temperatures. Wild arugula (Diplotaxis tenuifolia) is an exception - it is perennial, and may not flower every year.
Arugula will grow flower stalks similar to those produced by members of the mustard family. The seed pods will turn brown and become brittle when the seeds are ready. Once the pods begin to turn brown, clip the stalks from the plants (or just pull the plants up) and lay them out on a tarp somewhere protected from rain to finish drying. Some of the seed pods will shatter as they dry, so make sure to have something underneath the drying stalks to catch stray seeds. Allow seeds to dry out until they are too hard to be dented with your fingernail. Thresh the seeds by crushing the pods with your hands (wear work gloves) or by shaking the dry stalks upside down in a clean trash can or other large container, hitting the stalks against the sides. If desired, you can further clean the seeds by winnowing with the help of a fan running on low speed.