Bok Choy in Zone 9A — Northern California

Bok choy is a fantastic addition to any Northern California garden. Its mild, slightly sweet flavor and crisp texture make it incredibly versatile in stir-fries, soups, and salads. Plus, our long growing season and mild winters mean you can enjoy fresh bok choy from your garden for much of the year.

While we face challenges like periodic drought and the occasional heat wave, don't let that discourage you. With careful planning and attention to watering, you can easily grow bok choy in your Zone 9A garden. Our 288-day growing season gives you plenty of time to find the right planting window.

Starting bok choy indoors is an option, particularly for the earliest spring planting. Begin in mid-January through early February, about four weeks before you plan to transplant. This gives the seedlings a head start before our spring temperatures become more consistent.

Use seed trays or small pots filled with a good seed-starting mix. Keep the soil consistently moist using bottom watering to avoid disturbing the tiny seedlings. Provide warmth and plenty of light, either from a sunny window or a grow light.

Transplant your bok choy seedlings outdoors from mid-February through early April. Before transplanting, harden them off by gradually exposing them to outdoor conditions for about a week. This helps them adjust to the sun and wind, reducing transplant shock.

Space the seedlings 6-10 inches apart in well-drained soil. Keep an eye on the weather forecast – those early spring temperature swings can be tough on young plants. If a frost is predicted, cover them with row covers or blankets.

Direct sowing is a simple and effective method for growing bok choy in our climate. Sow seeds directly into the garden from early February through early October. The key is to ensure the soil is consistently moist, especially during germination.

Prepare the soil by loosening it and adding compost. Sow seeds 1/4 inch deep and 6-10 inches apart. Thin seedlings as they grow to provide adequate spacing. Direct sowing works best when the soil temperature is consistently above 50°F.

Consistent watering is crucial for growing healthy bok choy, especially during our dry summers. Aim to keep the soil consistently moist, but not waterlogged, as bok choy is not drought-tolerant. A good rule of thumb is to check the soil moisture about 2 inches deep – if it feels dry to the touch, 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, increase watering to every other day, or even daily, providing up to 2 inches of water per week. Water at the base of the plants to avoid wetting the leaves, which can encourage fungal diseases in our low-humidity environment.

Signs of underwatering include wilting leaves and slow growth. Overwatering, on the other hand, can lead to yellowing leaves and root rot. A layer of mulch, like straw or wood chips, can help retain soil moisture and regulate soil temperature.

You can expect your first harvest of bok choy from late March through mid-December, depending on when you planted. Bok choy matures quickly, usually around 45 days from sowing. You can harvest baby bok choy when it's 6-8 inches tall, or wait for the full-size heads to reach 12-15 inches.

To harvest, simply cut the plant above the base, leaving the roots intact. This allows the plant to potentially regrow for a second harvest, especially in our mild climate. Regularly harvesting outer leaves also encourages continued production.

As our first frost approaches in early December, harvest any remaining bok choy to prevent frost damage. If you have a cold frame or greenhouse, you can extend the harvest season even further.

Here are some common issues you might encounter growing bok choy in Northern California:

Flea Beetles

• What it looks like: Tiny round holes (shot-hole pattern) in leaves, especially on young seedlings. You might see small, dark beetles jumping when disturbed.
• What causes it: These small jumping beetles feed on the leaves of brassicas, causing the most damage to small, young plants. They thrive in dry conditions, which can be common during our spring and fall.
• How to fix/prevent it: Use row covers to protect seedlings during their vulnerable stage. Delay planting until plants are larger and more resilient. You can also try spraying with kaolin clay or neem oil. Interplanting with basil or catnip can help repel them.

Cabbage Worms

• What it looks like: Smooth green caterpillars on your bok choy leaves, with irregular holes chewed in them. You might also notice dark green droppings and white butterflies fluttering around your plants.
• What causes it: These are the larvae of white cabbage butterflies. They can have multiple generations per season, especially if we have a warm fall.
• How to fix/prevent it: Bt spray is very effective. Floating row covers will prevent the butterflies from laying eggs on your plants. Handpicking the caterpillars is also an option. Interplanting with aromatic herbs like thyme or sage can help deter the butterflies, and encourage parasitic wasps that prey on the caterpillars.

Slugs

• What it looks like: Irregular holes in leaves, especially on low-growing plants. You'll see silvery slime trails on the leaves and soil. Damage is usually worse at night and in wet weather.
• What causes it: Slugs are gastropods that feed at night in moist conditions. They hide under mulch and debris during the day.
• How to fix/prevent it: Beer traps (shallow dishes of beer sunk into the soil) are effective. Iron phosphate bait is a pet-safe option. Remove any hiding spots like boards or piles of leaves. Water in the morning so the soil surface dries out by evening. Copper tape around beds and diatomaceous earth around plants can also help.

Bolting in Heat

• What it looks like: Your bok choy sends up a flower stalk when temperatures rise. The leaves become tough and bitter, and the harvest quality declines.
• What causes it: Bok choy is a cool-season crop and will bolt when temperatures consistently exceed its comfort zone.
• How to fix/prevent it: Time your plantings for cool weather in spring and fall. Choose bolt-resistant varieties. Provide shade during warm spells with shade cloth. Mulch to keep the soil cool. Harvest your bok choy before bolting progresses too far.

Northern California Specific Challenges: Our moderate-to-hot summers, combined with low humidity and dry-summer rainfall, mean that bolting and flea beetles are significant concerns. Consistent watering is essential to prevent bolting and keep plants healthy enough to withstand pest pressure. Pay attention to your microclimate, as temperatures can vary significantly even within a small area.

Bok choy benefits from companion planting. Garlic and onions are excellent companions because their strong scent deters pests like aphids and cabbage moths. Beets are also good companions as they don't compete for the same nutrients.

Lettuce and spinach are also good choices, as they are low-growing and won't shade out the bok choy. Avoid planting bok choy near strawberries or tomatoes. Strawberries can attract slugs that will also damage the bok choy, and tomatoes can harbor pests that will attack your bok choy.