Mustard Greens in Zone 7B — Northern California

Mustard greens offer a peppery kick to your salads and stir-fries, and they're incredibly rewarding to grow in Northern California. Our long growing season means you can enjoy multiple harvests, and their slight frost tolerance lets you push the season a bit on both ends. Plus, they thrive in the same conditions as many other Mediterranean crops that do so well here.

While we face challenges like drought and increasingly hot summers, especially during fire season, careful timing and water-wise gardening practices will keep your mustard greens happy. With our 245-day growing season, you have plenty of opportunities to get a successful crop.

Starting mustard greens indoors isn't essential, but it can give you a head start, especially for an early spring harvest. Sow seeds indoors from early to late February, about three weeks before you plan to transplant. This is especially useful if you want to get a jump on the early warmth we often experience in spring.

Use seed trays or small pots filled with a good seed-starting mix. Keep them consistently moist – bottom watering works great to prevent damping off – and provide warmth and light (a sunny windowsill or grow light works). Remember, spring can come early here, so be ready!

Transplant your mustard greens seedlings outdoors from early March through mid April. This timing allows them to establish before the summer heat kicks in. 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 the plants 6-12 inches apart. Keep an eye on the weather; we can still get some unexpected cool snaps in early spring, so be ready to cover them if needed.

Direct sowing mustard greens is a simple and effective method in our climate, thanks to our long growing season. You can sow seeds directly into the garden from mid February through mid September. This wide window allows for multiple plantings, ensuring a continuous harvest.

Prepare the soil by loosening it and adding compost. Mustard greens prefer a soil temperature of at least 45°F for germination. Sow seeds about ¼ inch deep and space them 6-12 inches apart. Thin seedlings as needed.

Mustard greens need consistent moisture, especially during our dry summers. They are not very drought-tolerant, and underwatering can lead to bolting and a bitter taste. Aim to keep the soil consistently moist, but not waterlogged.

During the cooler spring and fall months, you might 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 other day, or even daily, providing 1-2 inches of water per week. Use the finger test: stick your finger about 2 inches 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 encourage fungal diseases in our low-humidity environment.

Wilting leaves are a sign of underwatering, while yellowing leaves and soggy soil indicate overwatering. A good layer of mulch, like straw or wood chips, will help retain moisture and keep the soil cool, which is especially important as we head into fire season.

You can expect your first harvest of mustard greens from early April through late November, about 45 days after planting. The best time to harvest is when the leaves are young and tender. Smaller leaves will have a milder flavor, while larger leaves tend to be spicier.

Harvest by snipping off the outer leaves, leaving the inner leaves to continue growing. This allows for a continuous harvest throughout the season. Be sure to harvest before the plants start to flower (bolt), as the leaves will become bitter.

As our first frost approaches around mid- November, harvest any remaining leaves. You can also try covering the plants to extend the harvest a bit longer, as mustard greens can tolerate a light frost.

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

Flea Beetles

• What it looks like: Tiny round holes (shot-hole pattern) in leaves, especially on young seedlings. You might also see small, dark beetles jumping when disturbed.
• What causes it: These small jumping beetles feed on the leaves. They're most damaging to small, young plants.
• How to fix/prevent it: Use row covers to protect seedlings. Delay planting until plants are larger. You can also try spraying with kaolin clay or neem oil. Interplanting with basil or catnip can help deter them. Healthy, vigorous plants are more resistant to damage.

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 or curled and distorted new growth.
• What causes it: These sap-sucking insects reproduce rapidly, especially in warm weather. Ant colonies may farm them for honeydew.
• How to fix/prevent it: Knock them off with a strong water spray. Encourage ladybugs and lacewings (natural predators). Use insecticidal soap for heavy infestations. Neem oil also works well. Avoid excess nitrogen fertilizer, as it attracts them.

Cabbage Worms

• What it looks like: Smooth green caterpillars on the leaves. You'll see irregular holes chewed in the leaves and dark green droppings. You might also see white butterflies hovering around the plants.
• What causes it: These are the larvae of white cabbage butterflies. They can have multiple generations per season in our warm climate.
• How to fix/prevent it: Bt spray is very effective. Use floating row covers to prevent egg laying. Handpick caterpillars. Interplant with aromatic herbs like thyme and sage. Encourage parasitic wasps.

Bolting in Heat

• What it looks like: The plant sends up a flower stalk. The leaves become tough and bitter.
• What causes it: Mustard greens are a cool-season crop and will bolt (go to seed) when temperatures rise. This is especially common during our hot summers.
• How to fix/prevent it: Time your plantings for cool weather (spring and fall). Choose bolt-resistant varieties. Provide shade during warm spells. Mulch to keep the soil cool. Harvest before bolting progresses too far.

Northern California Specific Challenges: Our moderate-to-hot summers, combined with low humidity and dry conditions, can stress mustard greens. Consistent watering is key to prevent bolting and keep the leaves tender. Shade cloth can be a lifesaver during heat waves, and mulching is essential to retain moisture and keep the soil cool. Also, be mindful of fire season and adjust your watering accordingly to keep plants hydrated.

Mustard greens benefit from several companion plants. Onions and garlic can help deter pests like aphids. Beets are good neighbors because they don't compete for the same nutrients. Dill attracts beneficial insects like ladybugs, which prey on aphids. Lettuce provides ground cover, helping to keep the soil cool and moist.

Avoid planting mustard greens near strawberries or tomatoes. Strawberries are susceptible to similar fungal diseases, and planting them together can increase the risk of infection. Tomatoes can compete for nutrients and sunlight, hindering the growth of mustard greens.