Arugula (Eruca sativa) is a leafy-green annual commonly grown as a salad green. Although it's easily grown throughout the United States as a cool-season crop in outdoor gardens, it also can be grown indoors year round for harvest as a leafy green or as microgreens.
If you have a south-facing window that gets full-sun exposure for at least four hours per day, then you might be able to grow arugula indoors there using only natural light, but you'll likely have better results if you grow the plant under a fluorescent grow light fixture. This is especially true if you intend to grow arugula through winter when the days are shorter. Use a timer to keep the lights on for 10 to 14 hours each day
Fill a tray, small pot or window box with a moisture-retentive potting mixture. A container at least 4 inches deep allows plenty of room for root development. Always use a container with holes in the bottom for drainage.
Water the soil thoroughly a few hours before planting so that it's evenly moist but not soggy.
Scatter arugula seeds about 1/2 inch apart on the surface of the soil, and then scatter 1/4 inch of soil over the seeds.
Water the soil gently after sowing, taking care not to disturb the seeds.
Arugula seeds germinate quickly, and seedlings often emerge in less than one week. When the seedlings are a few inches tall, thin them -- removing some of them -- so that the remaining individual plants are 4 to 6 inches apart.
Fertilizer and Water
Keep an eye on the soil's moisture level, and water the plants on a regular basis so that the soil remains moist but not soggy. Water carefully from above so that you don't damage the plants, or set the pot in a tray filled with 1 inch of water so that the soil soaks up water from below. Do not, however, allow the pot to sit in standing water if the soil is saturated.
Begin fertilizing when the plants are 4 to 6 inches tall, using a balanced, such as 10-10-10, liquid fertilizer diluted with water to one-half the rate recommended on the fertilizer's label. Use the fertilizer-water mixture to water the plants' soil every two weeks.
Begin harvesting when the plants are about 6 inches tall, which they usually are four to six weeks after the seeds were sown. Harvest a plant's outer leaves, cutting the leaves close to the base of the plant and leaving the inner leaves to continue growing.
To harvest the plants as microgreens, cut them with scissors just above the level of the soil. Harvest just after the emergence of the first set of true leaves, and replant seeds immediately to get started with a new crop. Each plant's first set of leaves aren't true leaves; its following leaves are true leaves.