Arugula in Zone 9B — Desert Southwest

Arugula is a fantastic addition to any Desert Southwest garden. Its peppery bite adds a unique flavor to salads and sandwiches, and it's incredibly versatile in everything from pasta dishes to pesto. Plus, harvesting your own greens, even just a little bit at a time, is incredibly satisfying.

Our extreme heat and intense sun can be tough on many plants, but with careful timing, you can enjoy a steady supply of arugula for much of our 313-day growing season. By taking advantage of our two growing seasons, you can easily manage the challenges and savor this delicious green nearly year-round.

Starting arugula indoors is definitely an option, especially if you want a head start on the season. Sow seeds late December through mid January, about three weeks before you plan to transplant. Use seed trays with a good seed-starting mix, and keep them warm (around 70°F) and under grow lights.

Bottom watering is key to avoid damping off. This is where you pour water into the tray beneath the seed cells, allowing the soil to absorb the moisture from the bottom up. Since our spring character is very-early, starting indoors gives you a jump on the season and a way to get your first harvest going before the heat ramps up.

Transplant your arugula seedlings outdoors late January through early March. Before planting, harden them off by gradually exposing them to outdoor conditions for about a week. This helps them adjust to the intense Desert Southwest sun and wind.

Space the seedlings about 6 inches apart in well-draining soil. Keep an eye on the weather forecast; even though our winters are mild, a late frost can still happen, so be ready to cover your seedlings if needed.

Direct sowing arugula is easy and often the preferred method, especially for succession planting. Sow seeds directly into the garden early January through late October. Arugula germinates best in cool soil, so aim for a soil temperature between 45°F and 75°F.

Prepare the soil by loosening it and adding compost for fertility. Sow seeds about ¼ inch deep and 6 inches apart. Keep the soil consistently moist. Arugula is fast-growing, but it bolts quickly in the heat, so direct sow during our cooler months for the best results.

Arugula needs consistent moisture to thrive, but it doesn't like soggy soil. In the Desert Southwest, this means paying close attention to your watering schedule, especially during our extreme summer heat.

During the cooler months (late fall, winter, and early spring), water deeply about once or twice a week, providing about 1 inch of water each time. Use the "finger test" – stick your finger about 2 inches into the soil; if it feels dry, it's time to water. As temperatures rise, increase watering frequency to every other day or even daily during the hottest stretches, providing about 1-2 inches of water per week.

Water at the base of the plants to avoid wetting the leaves, as this can encourage fungal diseases, even in our arid climate. Signs of underwatering include wilting leaves and slow growth. Overwatering can lead to yellowing leaves and root rot. Mulching around your arugula plants with a layer of organic material like straw or wood chips helps retain moisture and keep the soil cool.

You can expect your first arugula harvest mid February through late December, about 40 days after planting. Harvest young leaves when they are 2-3 inches long for a milder flavor; larger leaves will have a more peppery taste.

To harvest, simply snip off the outer leaves with scissors or your fingers, leaving the inner leaves to continue growing. This encourages continued production. Harvest frequently to prevent bolting. As our first frost approaches in mid- December, harvest any remaining leaves and consider succession planting indoors for a winter crop.

Here are some common problems you might encounter with arugula in the Desert Southwest:

Flea Beetles

• What it looks like: Tiny round holes (shot-hole pattern) in leaves, especially on young seedlings. You might see small, dark beetles that jump when disturbed.
• What causes it: Small jumping beetles that feed on the leaves. They're most damaging to small, young plants.
• How to fix/prevent it: Use row covers to protect seedlings during their vulnerable stage. Delay planting until plants are larger and more resilient. A coating of kaolin clay can deter them. Neem oil spray can also help. Interplant with basil or catnip to repel them. Healthy, vigorous plants often outgrow the damage.

Bolting in Heat

• What it looks like: The plant sends up a flower stalk, and the leaves become tough and bitter. Harvest quality declines rapidly.
• What causes it: Extended warm temperatures. Arugula is a cool-season crop and bolts when days consistently exceed 80°F.
• How to fix/prevent it: Time your plantings for the cooler parts of the year (spring and fall). Choose bolt-resistant varieties. Provide shade during warm spells with shade cloth. Mulch heavily to keep the soil cool. Harvest frequently to encourage leaf production before bolting progresses.

Aphids

• What it looks like: Clusters of tiny soft-bodied insects (green, black, or white) on stems and leaf undersides. You might also see sticky honeydew residue and curled or distorted new growth.
• What causes it: Rapidly reproducing sap-sucking insects. Populations explode in warm weather. Ant colonies may farm them for honeydew.
• How to fix/prevent it: A strong spray of water can knock them off. Encourage beneficial insects like ladybugs and lacewings. Insecticidal soap can be used for heavy infestations. Neem oil works systemically. Avoid excess nitrogen fertilizer, as it attracts them.

Downy Mildew

• What it looks like: Yellow patches on leaf tops with fuzzy gray-purple growth on undersides. Leaves brown and die from the bottom up.
• What causes it: An oomycete pathogen that spreads in cool, humid conditions via wind-carried spores.
• How to fix/prevent it: Remove affected leaves immediately. Improve air circulation around the plants. Water at the base, never overhead. Copper-based fungicides can be used for prevention. Plant resistant varieties where available.

Desert Southwest Specific Challenges: Our extreme heat makes bolting the biggest concern. Our arid climate can stress plants, making them more susceptible to pests. While monsoon moisture can be welcome, it can also create conditions favorable for downy mildew if air circulation is poor.

Arugula benefits from companion planting. Lettuce and spinach make good neighbors because they share similar growing requirements and won't compete for resources. Carrots and beets help loosen the soil, which benefits arugula's root growth. Onions deter pests like aphids. Avoid planting arugula near strawberries, as they can attract slugs and snails, which can also damage arugula leaves.