Arugula in Zone 4B — Mountain West

Arugula is a fantastic addition to any Mountain West garden. Its peppery bite adds zing to salads, pizzas, and pesto, and it thrives in our cool spring and fall weather. Plus, the satisfaction of harvesting fresh greens just weeks after planting is especially rewarding given our short growing season.

Our high altitude and intense sun can be tough on some crops, but arugula handles it well with a little attention. With a 128-day growing season, you can easily squeeze in multiple plantings of this quick-growing green. Timing is key, but you'll be enjoying fresh arugula in no time.

Starting arugula indoors is definitely an option, especially if you want a very early start. Begin your seeds indoors from early to late April, about three weeks before you plan to transplant them outside. Use seed trays and provide warmth (a heat mat is helpful) and plenty of light, especially since our spring light can still be weak.

Bottom watering is beneficial to prevent damping off. While direct sowing is generally preferred due to arugula's quick growth, starting indoors can give you a head start and protect young seedlings from early spring pests.

Transplant your arugula seedlings outdoors from early May through mid June, once the danger of hard frost has passed. Make sure you harden them off for about a week before transplanting. Gradually expose them to increasing amounts of outdoor sunlight and wind to toughen them up.

Space the seedlings about 6 inches apart. Keep an eye on the weather forecast; even in May and June, we can get unexpected cold snaps or even late-season snow in the Mountains. Be prepared to cover your transplants if needed.

Direct sowing is the easiest way to grow arugula in our climate. Sow seeds from mid April through late July for a continuous harvest. Prepare the soil by loosening it and removing any rocks or debris. Arugula prefers soil temperatures around 50-65°F for optimal germination.

Sow seeds about ¼ inch deep and 6 inches apart. Keep the soil consistently moist. Arugula is fast-growing and can bolt (go to seed) in the heat, so planting in cooler weather is crucial for the best flavor and longest harvest.

Arugula needs consistent moisture to thrive, especially in our dry Mountain West climate. While it isn't drought-tolerant, you can influence the flavor with your watering practices. Aim to keep the soil consistently moist, but not waterlogged.

In the spring and fall, when temperatures are cooler, you may only need to water once or twice a week. During the warmer summer months, especially with our intense sun, you might need to water every other day. Use the "finger test" – stick your finger about 2 inches into the soil. If it feels dry, it's time to water. Aim for about an inch of water per week, either from rainfall or irrigation.

Water at the base of the plants to avoid wetting the leaves. This is especially important during cooler evenings to prevent fungal diseases. Signs of underwatering include wilting leaves and a bitter taste. Overwatering can lead to yellowing leaves and root rot. Mulching around your arugula plants with straw or wood chips helps retain moisture and regulate soil temperature.

You can expect your first arugula harvest from late May through late September, depending on when you planted. Arugula is ready to harvest about 40 days after planting. Young leaves, about 2-3 inches long, have a milder flavor. Larger leaves are more peppery.

Harvest by snipping off the outer leaves with scissors, leaving the inner leaves to continue growing. This encourages continuous production throughout the season. To extend your harvest as the first frost approaches in mid- September, consider covering your plants with a row cover or bringing them indoors to a container.

Here are some common problems you might encounter growing arugula in the Mountain West:

Flea Beetles

• What it looks like: Tiny round holes (shot-hole pattern) in the leaves, especially on young seedlings. You might see small dark beetles jumping when disturbed.
• What causes it: Flea beetles are small jumping beetles that feed on 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 plants are larger and more resilient. Kaolin clay coating can deter beetles. Neem oil spray can also help. Interplant with basil or catnip. Healthy, vigorous plants can often outgrow the damage.

Bolting in Heat

• What it looks like: The arugula plant sends up a flower stalk. The leaves become tough and bitter, and the harvest quality declines.
• What causes it: Bolting is triggered by extended warm temperatures. Arugula, being a cool-season crop, bolts when days consistently exceed 80°F.
• How to fix/prevent it: Time your plantings for the cooler spring and fall weather. Choose bolt-resistant varieties if available. Provide shade during warm spells with shade cloth. Mulch around the plants to keep the soil cool. Harvest frequently to encourage continued 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 a sticky honeydew residue and curled or distorted new growth.
• What causes it: Aphids are sap-sucking insects that reproduce rapidly, especially in warm weather. Ant colonies may farm them for honeydew.
• How to fix/prevent it: A strong spray of water from the hose can knock aphids off the plants. Encourage ladybugs and lacewings, which are natural predators. Insecticidal soap can be used for heavy infestations. Neem oil also works systemically. Avoid excess nitrogen fertilizer, which attracts aphids.

Downy Mildew

• What it looks like: Yellow patches on leaf tops with fuzzy gray-purple growth on the undersides. Leaves brown and die from the bottom up.
• What causes it: Downy mildew is caused by 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 by spacing them properly. Water at the base of the plants, never overhead. Copper-based fungicides can be used for prevention. Plant resistant varieties if available.

Mountain West Specific Challenges: Our moderate-at-altitude summer heat can cause arugula to bolt quickly. Our low humidity helps reduce the risk of fungal diseases, but the dry conditions mean you need to be diligent about watering. The intense UV can also stress the plants, so providing some afternoon shade during the hottest part of the summer days is beneficial.

Arugula benefits from companion planting in our Mountain gardens. Plant it alongside lettuce and spinach, as they share similar growing requirements and won't compete for resources. Carrots and beets are good companions because they help loosen the soil, making it easier for arugula roots to grow. Onions are also beneficial because their strong scent can deter pests.

Avoid planting arugula near strawberries. Strawberries and arugula can compete for the same nutrients in the soil, potentially hindering the growth of both plants.