Arugula in Zone 4A — Mountain West

Arugula is a fantastic addition to any Mountain West garden. Its peppery bite adds zest to salads and sandwiches, and it's incredibly versatile in everything from pesto to pizza toppings. Plus, the satisfaction of harvesting fresh greens just weeks after planting is especially rewarding in our short growing season.

Our high altitude and intense sun can be tough on some plants, but with a little planning, you can enjoy a steady supply of arugula. The key is understanding our 118-day growing season and taking advantage of the cool weather windows for planting.

Starting arugula indoors is definitely an option, but honestly, it's often not necessary given how quickly it germinates. If you want a very early start, aim for early to late April, about three weeks before you plan to transplant.

Use seed trays or small pots filled with a good seed-starting mix. Keep the soil consistently moist, watering from the bottom to avoid damping-off. A warm location and grow lights will give them a strong start, but remember that our late springs can still bring unexpected temperature swings, so be prepared to protect seedlings.

Transplant your arugula seedlings outdoors anytime from early May through mid June, once the danger of a hard frost has passed. Before transplanting, harden them off by gradually exposing them to outdoor conditions for about a week.

Space the plants about 6 inches apart in well-drained soil. Keep an eye on the weather forecast; even in May and June, we can get unexpected cold snaps, so be prepared to cover your seedlings if necessary.

Direct sowing arugula is the easiest way to grow it in the Mountain West. You can sow seeds directly into the garden from late April through late July. The great thing about direct sowing is that you can get multiple harvests by sowing every few weeks.

Prepare the soil by loosening it and removing any large clumps. Sow seeds about ¼ inch deep and 6 inches apart. Keep the soil consistently moist until the seedlings emerge. Be aware that arugula is fast-growing and tends to bolt (go to seed) in hot weather, so focus on planting during cooler periods.

Consistent moisture is key to happy arugula in our dry Mountain West climate. While arugula isn't drought tolerant, its peppery flavor can intensify if it's slightly stressed. The trick is to find the right balance.

In the spring and fall, when temperatures are cooler, you may only need to water once or twice a week, providing about 1 inch of water each time. During the warmer summer months, especially with our intense sun, you'll likely need to water every other day, ensuring the soil stays consistently moist. Use the finger test: stick your finger about 2 inches deep into the soil. If it feels dry, it's time to water.

Water at the base of the plants to avoid wetting the leaves, which can increase the risk of fungal diseases, even with our low humidity. Signs of underwatering include wilting leaves and stunted growth. Overwatering can lead to yellowing leaves and root rot. A light layer of organic mulch, like straw or shredded leaves, can help retain moisture and keep the soil cool.

You can expect your first arugula harvest about 40 days after planting, which typically means early June if you started early, and continuing through late September. The leaves are ready to harvest when they are about 2-3 inches long. Smaller leaves will have a milder flavor, while larger leaves will be more peppery.

To harvest, simply snip off the outer leaves with scissors or a knife, leaving the inner leaves to continue growing. This cut-and-come-again method will allow you to harvest arugula continuously throughout the growing season. Be sure to harvest before the plants start to flower, as the leaves will become bitter once they bolt.

As our first frost approaches in mid- September, harvest any remaining leaves to enjoy in salads or other dishes. You can also try covering the plants with a frost blanket to extend the harvest a bit longer.

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

Flea Beetles

• What it looks like: Tiny round holes (shot-hole pattern) in leaves. You might see small, dark beetles that jump when disturbed. The damage is worst on young seedlings.
• What causes it: These small jumping beetles feed on the leaves, especially of 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 less susceptible to damage. Coating the leaves with kaolin clay can deter the beetles. Neem oil spray can also help. Interplanting with basil or catnip may help repel them. Healthy, vigorous plants often outgrow the damage.

Bolting in Heat

• What it looks like: The arugula sends up a flower stalk, and the leaves become tough and bitter. Harvest quality declines significantly.
• What causes it: Arugula is a cool-season crop, so extended warm temperatures above 80°F will cause it to bolt.
• How to fix/prevent it: Time your plantings for cool weather in spring and fall. Choose bolt-resistant varieties if possible. Provide shade during warm spells using 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 the undersides of leaves. You might also notice 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 from the hose. Encourage natural predators like ladybugs and lacewings. For heavy infestations, use insecticidal soap or neem oil. Avoid excess nitrogen fertilizer, which attracts them.

Downy Mildew

• What it looks like: Yellow patches on the tops of leaves with fuzzy gray-purple growth on the undersides. Leaves will brown and die from the bottom up.
• What causes it: A fungal pathogen that spreads in cool, humid conditions via wind-carried spores.
• How to fix/prevent it: Remove affected leaves immediately to prevent the spread. Improve air circulation around the plants. Water at the base, avoiding overhead watering. Copper-based fungicides can be used for prevention. Look for and plant resistant varieties when available.

Mountain West Specific Challenges: Our moderate-at-altitude heat, low humidity, and dry rainfall can be both a blessing and a curse for arugula. The intense sun can stress the plants, making them more susceptible to bolting. However, the low humidity also reduces the risk of fungal diseases. Consistent watering is crucial to prevent the plants from drying out, but avoid overwatering, which can lead to other problems.

Arugula benefits from companion planting. Growing it near lettuce and spinach helps create a beneficial microclimate with slightly more shade and moisture. Carrots and beets make good companions because they don't compete for the same nutrients and their root systems don't interfere with arugula's shallow roots. Onions help repel some common arugula pests.

Avoid planting arugula near strawberries, as they are susceptible to some of the same fungal diseases, which can increase the risk of problems in your garden.