Nasturtium in Zone 10A — Southern California

Nasturtiums are a fantastic addition to any Southern California garden. The peppery flavor of the leaves and flowers adds a unique zest to salads, and they're incredibly versatile in the kitchen. Plus, their vibrant colors bring a cheerful splash to your garden beds nearly year-round.

Our SoCal climate presents some unique challenges, like occasional drought and summer heat inland, but with a little planning, you can enjoy a bountiful harvest. With our long 349-day growing season, you'll find Nasturtiums are easy to grow.

Starting Nasturtium seeds indoors is possible, but honestly, it's usually not necessary here in Southern California. Since our springs start so early, direct sowing is generally more successful. However, if you want a head start, especially for an extra-early bloom, you can start seeds indoors from mid-December through early January, about four weeks before our typical last frost.

Use seed trays with a good seed-starting mix, keep them in a warm spot, and provide plenty of light – a simple grow light works wonders. To avoid damping off, water from the bottom by placing the tray in a shallow dish of water until the soil surface is moist.

If you started your Nasturtium seeds indoors, you can transplant them outdoors from late January through mid-February. Before transplanting, it's crucial to harden them off. This means gradually exposing them to outdoor conditions over a week.

Space the seedlings 10-12 inches apart in your garden bed. Keep an eye on the weather forecast; even though our winters are mild, a surprise cold snap can still happen, so be prepared to cover your young plants if needed.

Direct sowing is the preferred method for Nasturtiums, as they don't always love being transplanted. You can direct sow from late January through late October in Southern California, taking advantage of our long growing season.

Choose a spot with full sun to part shade. Prepare the soil by loosening it and removing any large rocks or debris. Nasturtiums aren't picky about soil, so don't worry about adding too much fertilizer. Sow seeds 1/2 inch deep and 10-12 inches apart. Keep the soil consistently moist until the seedlings emerge, which usually takes about a week.

Nasturtiums are remarkably drought-tolerant, which is a huge plus for us here in Southern California with our ongoing water restrictions. They actually thrive in slightly poor, dry soil, and overwatering can lead to more leaves than flowers. Remember, less is more with these guys.

During our winter-wet season, you might not need to water them at all unless we're experiencing a particularly dry spell. From spring through fall, when the weather heats up and rainfall is scarce, check the soil moisture regularly. Stick your finger about 2 inches deep into the soil; if it feels dry, it's time to water. Aim for about 1 inch of water per week during the hottest months.

Water at the base of the plants to avoid wetting the leaves, especially in our low-to-moderate humidity. This helps prevent fungal diseases. Signs of underwatering include wilting leaves and stunted growth, while overwatering can cause yellowing leaves and root rot. Mulching around the base of the plants with organic matter like straw or wood chips can help retain soil moisture and suppress weeds.

You can expect your first Nasturtium harvest around mid-March if you planted in late January, and you can continue harvesting through late December. Both the flowers and leaves are edible, so keep an eye out for fully open flowers for the best flavor. The seed pods can also be harvested and pickled for a caper-like treat.

When harvesting, gently snip off the flowers or leaves with scissors or your fingers, being careful not to damage the rest of the plant. Regular harvesting encourages more flower production, so don't be shy! As our mild winters approach in late December, you can still harvest occasionally. The plants might slow down their growth, but they can often provide a few more harvests before fully resting.

Here in Southern California, the top three problems you might encounter with Nasturtiums are aphids, cabbage caterpillars, and black fly.

Aphids

• What it looks like: Small, pear-shaped insects clustered on stems and leaves, often accompanied by sticky honeydew and sooty mold.
• What causes it: Aphids are attracted to stressed plants and can proliferate quickly in warm, dry conditions, which we often experience during Santa Ana winds.
• How to fix/prevent it: Nasturtiums are often used as a trap crop to lure aphids away from other plants. You can blast them off with a strong stream of water from the hose, or introduce beneficial insects like ladybugs.

Cabbage Caterpillars

• What it looks like: Green caterpillars munching on leaves, leaving behind ragged holes. You might also see their droppings (frass) on the leaves.
• What causes it: Cabbage white butterflies lay their eggs on Nasturtiums, and the larvae (caterpillars) hatch and start feeding. They thrive in our warm weather.
• How to fix/prevent it: Handpick the caterpillars off the plants and drop them into a bucket of soapy water. You can also cover your plants with row covers to prevent the butterflies from laying eggs.

Black Fly

• What it looks like: Tiny black flies clustered on new growth, causing distorted leaves and stunted growth.
• What causes it: Black fly, like aphids, are attracted to stressed plants and can quickly multiply in favorable conditions.
• How to fix/prevent it: Improve air circulation around the plants by spacing them properly. You can also use insecticidal soap or neem oil to control infestations.

Southern California Specific Challenges: Our hot, dry summers and occasional Santa Ana winds can stress Nasturtiums, making them more susceptible to pests. Consistent watering (but not overwatering!) and providing some afternoon shade during the hottest months can help keep your plants healthy and productive.

Nasturtiums are great companions in the Southern California garden. Plant them near tomatoes to help repel whiteflies and aphids, protecting your tomato crop from these common pests. They also work well with cucumbers and squash, attracting pollinators to improve fruit set and acting as a trap crop for squash bugs. Broccoli and cabbage also benefit from Nasturtium companions, as they can deter cabbage moths and other brassica pests. Avoid planting them near fennel, as fennel can inhibit the growth of many plants, including Nasturtiums.