Arugula in Zone 9A — Northern California

Arugula is a fantastic addition to any Northern California garden. Its peppery bite adds a delicious zing to salads, pizzas, and pesto, and it's so easy to grow here. Plus, with our long growing season, you can enjoy fresh arugula for most of the year.

We definitely have our challenges, like the occasional drought and those unpredictable temperature swings, but arugula is surprisingly resilient. And with a 288-day growing season, you have plenty of time to get multiple harvests.

Starting arugula indoors isn't strictly necessary in our mild Northern California climate, but it can give you a jump start, especially for an early spring harvest. Start your seeds indoors from early to late January, about three weeks before you plan to transplant them outside.

Use seed trays or small pots filled with a good seed-starting mix. Keep the soil consistently moist (bottom watering works great!) and provide warmth and plenty of light, either from a sunny window or a grow light. Because our springs often start early, this gives you a head start before the "official" transplant window.

Transplanting arugula seedlings outdoors can be done from early February through mid March, depending on your microclimate and the weather forecast. Before transplanting, harden off your seedlings by gradually exposing them to outdoor conditions for about a week.

Choose a spot with full sun to partial shade and space your seedlings about 6 inches apart. Keep an eye on the forecast for any late frosts and be prepared to cover your seedlings if necessary.

Direct sowing is the easiest way to grow arugula in Northern California, and it works well for most of the year. You can sow seeds directly into the garden from late January through mid October, taking advantage of our long growing season.

Prepare your soil by loosening it and adding some compost. Arugula prefers well-drained soil with a pH between 6.0 and 7.0. Sow seeds about ¼ inch deep and space them about 6 inches apart. Keep the soil consistently moist until the seeds germinate, which usually takes about a week. Remember that arugula bolts in the heat, so focus on cooler weather plantings.

Arugula needs consistent moisture, especially during our dry Northern California summers. Aim to keep the soil consistently moist, but not waterlogged. The "finger test" is a good way to check: stick your finger about 2 inches into the soil. If it feels dry, it's time to water.

During the cooler spring and fall months, you'll likely only need to water once or twice a week, providing about 1 inch of water each time. In the heat of summer, you may need to water every day or every other day, providing up to 2 inches of water per week. Water at the base of the plants to avoid wetting the leaves, which can increase the risk of fungal diseases in our humid microclimates closer to the coast.

Signs of underwatering include wilting leaves and slow growth. Overwatering can lead to yellowing leaves and root rot. A layer of mulch, like straw or wood chips, can help retain moisture in the soil and suppress weeds, which is especially helpful during our hot, dry summers.

You can expect your first arugula harvest about 40 days after planting, which means if you plant in late January, you could be harvesting as early as early March. Arugula is ready to harvest when the leaves are 2-3 inches long. Young leaves have a milder flavor, while larger leaves have a more peppery taste.

To harvest, simply snip off the leaves with scissors or your fingers, being careful not to damage the plant. Harvest the outer leaves first, allowing the inner leaves to continue growing. This will encourage continued production. As the first frost approaches in early December, harvest any remaining leaves. You can also cover the plants with a frost blanket to extend the harvest a bit longer.

Here are some common problems you might encounter growing arugula in Northern California:

Flea Beetles

• What it looks like: Tiny round holes (shot-hole pattern) in the leaves, especially on young seedlings. You might also see small, dark beetles jumping when disturbed.
• What causes it: These are caused by small jumping beetles that feed on the leaves. They are most damaging to small, young plants.
• How to fix/prevent it: Use row covers to protect seedlings during their vulnerable stage. You can also delay planting until the plants are larger and better able to withstand the damage. Kaolin clay coating and neem oil spray can also help. Interplanting with basil or catnip may deter them.

Bolting in Heat

• What it looks like: The plant sends up a flower stalk, and the leaves become tough and bitter.
• What causes it: Arugula is a cool-season crop and bolts when temperatures rise.
• How to fix/prevent it: Time your plantings for cooler weather in spring and fall. Choose bolt-resistant varieties if possible. Provide shade during warm spells, and mulch to keep the soil cool. Harvest frequently to delay bolting, and harvest any remaining leaves once bolting begins.

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: Aphids are sap-sucking insects that reproduce rapidly, especially in warm weather.
• How to fix/prevent it: Knock them off with a strong spray of water. Encourage ladybugs and lacewings, which are natural predators. Use insecticidal soap for heavy infestations, or neem oil, which works systemically. Avoid excess nitrogen fertilizer, as this can attract 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: This is caused by a pathogen that spreads in cool, humid conditions via wind-carried spores.
• How to fix/prevent it: Remove affected leaves immediately. Improve air circulation by spacing plants properly. Water at the base, never overhead. Copper-based fungicides can be used for prevention. Plant resistant varieties where available.

Northern California Specific Challenges: Our moderate-to-hot summers, low humidity, and dry-summer rainfall patterns mean that keeping arugula consistently watered is crucial to prevent bolting and maintain good flavor. Be vigilant about watering during heat waves, and consider providing shade during the hottest part of the day.

Arugula benefits from companion planting in a Northern California garden. Plant it alongside lettuce and spinach, as they share similar growing requirements and won't compete for resources. Carrots and beets make good companions because they help loosen the soil, which arugula appreciates. Onions can help deter pests, protecting your arugula from unwanted visitors. Avoid planting arugula near strawberries, as they can attract pests that also affect arugula.