Nasturtium in Zone 10B — Southern California

Nasturtiums are a fantastic addition to any Southern California garden. These beauties offer peppery edible flowers and leaves that add a unique zest to salads and other dishes. Plus, they're incredibly easy to grow, providing a long season of vibrant color and culinary delight in our mild climate.

While we contend with drought and occasional heat waves, nasturtiums are surprisingly resilient and rewarding here in Zone 10B. With our extended growing season, stretching nearly year-round, you can enjoy a continuous harvest for many months with proper planning.

Starting nasturtiums indoors isn't essential in our climate, but it can give you a head start. If you're eager to get blooms as early as possible, sow seeds indoors from mid December through early January, about four weeks before the ideal transplanting window.

Use seed trays with good drainage and a well-draining seed starting mix. Keep the trays warm and provide plenty of light—a sunny windowsill or grow light works well. Water from the bottom to encourage strong root growth and prevent damping off. Remember, our very-early spring means you'll be transplanting before you know it!

Transplant your nasturtium seedlings outdoors from mid January through mid February, once they have a few sets of true leaves. 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.

Space the plants about 10-12 inches apart in well-draining soil. Even though our winters are mild, keep an eye on nighttime temperatures and protect young transplants from unexpected cold snaps.

Direct sowing is the preferred method for nasturtiums, as they don't always love being transplanted. Sow seeds directly into your garden beds from mid January through late October. This wide window takes advantage of our long growing season.

Choose a spot with full sun to partial shade. Prepare the soil by loosening it and removing any rocks or debris. Sow seeds about ½ inch deep and 10-12 inches apart. Keep the soil consistently moist until the seedlings emerge, which usually takes about a week or two.

Nasturtiums are surprisingly drought-tolerant and thrive with minimal watering, making them perfect for our water-wise Southern California gardens. During our winter-wet season, you may not need to water them at all, relying on natural rainfall. However, be aware of the Santa Ana winds that can dry out the soil quickly.

During the hotter months, give them a deep watering only when the top two inches of soil feel dry to the touch – use the finger test! Usually, about 1 inch of water per week is sufficient, but adjust based on rainfall and temperature. Water at the base of the plants to avoid wetting the foliage, which can encourage fungal diseases in our low-to-moderate humidity.

Wilting leaves are a sign of underwatering, while yellowing leaves can indicate overwatering. A layer of mulch, like wood chips, can help retain moisture in the soil and reduce the need for frequent watering, while also keeping soil temperatures down during the summer heat inland.

You can expect your first harvest of nasturtium flowers and leaves around mid March through late December, about 55 days after planting. The flowers are ready to harvest when they are fully open and vibrant in color. The leaves can be harvested at any time, but younger leaves tend to be more tender and flavorful.

To harvest, simply snip the flowers or leaves from the plant with scissors or your fingers. Avoid pulling or tugging, which can damage the plant. Regular harvesting encourages continued production of both flowers and leaves.

As our first frost approaches in late December, you can still harvest any remaining flowers and leaves. The seed pods can also be harvested and pickled for a unique caper-like treat.

Here are some common problems you might encounter with nasturtiums in Southern California:

Aphids

• What it looks like: Clusters of small, soft-bodied insects on stems and leaves, often accompanied by sticky honeydew.
• What causes it: Aphids are attracted to stressed plants, and our dry climate can sometimes contribute to stress.
• How to fix/prevent it: Nasturtiums can actually be used as a trap crop to draw aphids away from other plants. You can also spray them off with a strong stream of water or use insecticidal soap.

Cabbage Caterpillars

• What it looks like: Holes in leaves, often with green or brown caterpillars present.
• What causes it: These caterpillars are the larvae of cabbage white butterflies, which are common in our area.
• How to fix/prevent it: Handpick the caterpillars off the plants or use a biological insecticide like Bacillus thuringiensis (Bt). Covering your plants with netting can also prevent butterflies from laying eggs.

Black Fly

• What it looks like: Similar to aphids, these small black flies suck sap from the plant.
• What causes it: Like aphids, they thrive on stressed plants and can multiply quickly in warm weather.
• How to fix/prevent it: A strong spray of water can dislodge them. Insecticidal soap or neem oil are also effective.

Southern California Specific Challenges: Our hot summers inland can stress nasturtiums, making them more susceptible to pests. Ensure adequate watering during heat waves and provide some afternoon shade if possible. The winter-wet season can lead to fungal diseases if plants are overcrowded or lack good air circulation.

Nasturtiums are great companions for many plants in a Southern California garden. They attract beneficial insects and can even act as a trap crop for pests.

Good companions include tomatoes, cucumbers, and squash. Nasturtiums deter pests that bother these crops. They also grow well with broccoli and cabbage, helping to protect them from cabbage white butterflies and other pests.

Avoid planting nasturtiums near fennel, as they can inhibit each other's growth.