Nasturtium in Zone 9B — Southern California

Nasturtiums are a fantastic addition to any Southern California garden. Their peppery leaves and vibrant flowers add a unique flavor to salads, and the cheerful blooms attract beneficial insects. Plus, they thrive in our long growing season, offering a year-round harvest of edible delights.

Our mild winters and extended warm periods mean you can enjoy nasturtiums almost continuously here in Zone 9B. While we face challenges like drought and occasional heat waves, careful planning and water-wise gardening practices will ensure your nasturtiums flourish during our 334-day growing season.

Starting nasturtiums indoors is an option, but honestly, they do so well direct-sown that it's usually not necessary here. If you really want a head start, you can begin in late December through mid January, about four weeks before you plan to transplant. Use seed trays with good drainage and keep them in a warm, bright location – a sunny windowsill works well.

Ensure the soil stays consistently moist, and bottom watering is a great way to prevent damping off. Remember, our very-early spring means you'll need to be ready to move them outside fairly soon.

Transplant your indoor-started nasturtiums outdoors in early February through early March. Before planting, harden them off by gradually exposing them to outdoor conditions for about a week. This helps them adjust to the sun and wind.

Plant them 10-12 inches apart in well-draining soil. Keep an eye on the weather forecast – even though we rarely get frost, a sudden cold snap can still damage young plants.

Direct sowing is the preferred method for nasturtiums, as they don't always love having their roots disturbed. Sow seeds directly into the garden from early February through mid October, giving you plenty of flexibility. Choose a spot with full sun to part shade.

Prepare the soil by loosening it and removing any large rocks or debris. Plant seeds about ½ inch deep and 10-12 inches apart. Keep the soil consistently moist until germination, which usually takes about a week or two, depending on soil temperature.

Nasturtiums are wonderfully drought-tolerant, making them perfect for our water-conscious Southern California gardens. They actually thrive in slightly poor, dry soil, so resist the urge to overwater. Too much water or fertilizer will encourage lush foliage at the expense of flowers.

During our winter-wet season, from late fall through early spring, rainfall might be enough to keep them happy. However, from late spring through early fall when we get those hot days, check the soil moisture regularly using the "finger test" – stick your finger about 2 inches deep into the soil. If it feels dry, it's time to water.

Give them a good soaking, aiming for about 1 inch of water per week during the hottest periods. Water at the base of the plants to avoid wetting the leaves, especially in our low-to-moderate humidity, which can help prevent fungal diseases. Yellowing leaves can indicate overwatering, while wilting leaves signal underwatering. A light layer of mulch around the base of the plants can help retain moisture in the soil and suppress weeds.

You can expect your first harvest of nasturtium flowers and leaves around late March through late December, about 55 days after planting. The flowers are ready to harvest when they're fully open and brightly colored. The leaves can be harvested at any time.

To harvest, simply snip off the flowers or leaves with scissors or your fingers. Be gentle to avoid damaging the plant. Regular harvesting encourages continued production of both flowers and leaves.

As we approach late December and the possibility of our first frost, you can still harvest any remaining flowers and leaves. The seed pods can also be harvested and pickled for a caper-like treat.

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

Aphids

• What it looks like: Clusters of tiny green, black, or brown bugs on stems and undersides of leaves; sticky honeydew; distorted growth.
• What causes it: Aphids are attracted to stressed plants, and warm weather encourages their rapid reproduction. Our hot, dry summers can stress plants, making them more susceptible.
• How to fix/prevent it: Nasturtiums are often used as intentional trap crops for aphids, drawing them away from other plants. Blast aphids off with a strong stream of water from the hose. You can also introduce beneficial insects like ladybugs or use insecticidal soap.

Cabbage Caterpillars

• What it looks like: Holes in leaves, chewed edges, and green droppings. You might spot the caterpillars themselves – small, green worms.
• What causes it: Cabbage white butterflies lay their eggs on nasturtiums, and the caterpillars hatch and start munching.
• How to fix/prevent it: Handpick caterpillars off the plants. Cover plants with netting to prevent butterflies from laying eggs. Bacillus thuringiensis (Bt) is an effective organic treatment.

Black Fly

• What it looks like: Similar to aphids, you'll see clusters of small, black flies on new growth.
• What causes it: Black fly thrive in warm conditions and can quickly multiply, especially on stressed plants.
• How to fix/prevent it: Use the same strategies as for aphids: strong water spray, insecticidal soap, and encouraging beneficial insects.

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!), providing some afternoon shade, and using mulch can help your plants stay healthy and resist problems.

Nasturtiums make excellent companions for many garden plants. Plant them near tomatoes, cucumbers, and squash to help deter pests like squash bugs and aphids. They also work well with broccoli and cabbage, as they can attract cabbage white butterflies away from these crops. Avoid planting nasturtiums near fennel, as it can inhibit their growth. They are generally considered good companions for most plants, creating a beneficial ecosystem in your Southern California garden.