Nasturtium in Zone 10B — Desert Southwest

Nasturtiums are a fantastic addition to any Desert Southwest garden. Their peppery leaves and bright flowers add a unique flavor to salads and bring a splash of color to your beds. Plus, they're incredibly satisfying to grow, thriving even in our challenging climate.

While our extreme heat and alkaline soil might seem daunting, nasturtiums are surprisingly resilient. With careful timing during our long, 349-day growing season, you can enjoy these beauties from spring through fall.

Starting nasturtiums indoors isn't essential in our climate, but it can give you a head start. If you want to get a jump on the season, sow seeds indoors from mid December through early January, about four weeks before you plan to transplant. Use seed trays with a good seed-starting mix and keep them in a warm spot with plenty of light.

Bottom watering is ideal to prevent damping off; just set the tray in a shallow dish of water until the soil surface is moist. Since our spring comes very-early here, indoor starting is mainly for gardeners eager to harvest as soon as possible.

Transplant your nasturtium seedlings outdoors from late January through mid February. Before planting, harden them off by gradually exposing them to outdoor conditions for about a week. This helps them adjust to the intense Desert Southwest sun.

Space them 10-12 inches apart in well-draining soil. Even in late January, keep an eye out for unexpected temperature swings and be ready to cover them if a cold snap threatens.

Direct sowing is the preferred method for nasturtiums in our region. These plants don't love having their roots disturbed, and direct sowing avoids that issue. You can sow seeds directly into the garden from late January through late October.

Make sure the soil is relatively warm (around 60-70°F) for best germination. Plant seeds about ½ inch deep and 10-12 inches apart. Nasturtiums will thrive even in our alkaline soil, as long as it drains well.

Nasturtiums are remarkably drought-tolerant, making them perfect for desert gardening. The key is to avoid overwatering, which can lead to more foliage than flowers. During the cooler months of spring and fall, water deeply but infrequently – about once a week, or whenever the top two inches of soil feel dry to the touch.

In the extreme heat of summer, you might need to water twice a week, especially if they're in full sun. Use the finger test to be sure; stick your finger into the soil about two inches deep. If it's dry, it's time to water. Aim for about ½ inch of water per week, adjusting based on rainfall.

Water at the base of the plants to avoid wetting the foliage, which can encourage fungal issues, even in our arid climate. Signs of underwatering include wilting leaves and dry, crispy edges. Overwatering can cause yellowing leaves and root rot. A light layer of organic mulch around the base of the plants can help retain moisture and keep the soil cool.

You can expect your first harvest of nasturtium flowers and leaves from mid March through late December, about 55 days after planting. The flowers are ready to harvest when they are 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. Avoid pulling or tugging, which can damage the plant. Regular harvesting encourages more blooms and keeps the plant producing throughout the season.

As the first frost approaches in late December, you can harvest any remaining flowers and leaves to use in salads or other dishes. You can even pickle the seed pods for a caper-like treat.

Even nasturtiums can face challenges in our Desert Southwest gardens. Here are three common problems to watch out for:

Aphids (intentional trap crop)

• What it looks like: Clusters of tiny green or black bugs on stems and leaves, often causing distorted growth.
• What causes it: Aphids are attracted to stressed plants, and can multiply quickly in warm weather. Nasturtiums often act as a trap crop, drawing aphids *away* from your more vulnerable plants.
• How to fix/prevent it: Check plants regularly and squish aphids by hand or blast them off with a strong stream of water from the hose. Since they are a trap crop, you can tolerate some aphids on the nasturtiums themselves.

Cabbage Caterpillars

• What it looks like: Holes in leaves, often with green droppings nearby. You might also spot the caterpillars themselves, which are green and blend in well with the foliage.
• What causes it: These caterpillars are the larvae of cabbage white butterflies, which are common in our region.
• How to fix/prevent it: Handpick caterpillars off the plants regularly. Floating row covers can prevent butterflies from laying eggs on your nasturtiums.

Black Fly

• What it looks like: Similar to aphids, these small black flies cluster on new growth and suck plant juices, weakening the plant.
• What causes it: They thrive in warm, dry conditions.
• How to fix/prevent it: Introduce beneficial insects like ladybugs, which prey on black flies. A strong blast of water can also dislodge them.

Desert Southwest Specific Challenges: The extreme heat of our summers can stress nasturtiums, making them more susceptible to pests and diseases. Providing afternoon shade during the hottest months can help. Our arid humidity means that fungal diseases are less of a problem than in more humid climates, but proper watering techniques are still important to prevent root rot. Monsoon moisture can occasionally lead to fungal issues, so ensure good air circulation around plants.

Nasturtiums make excellent companion plants in the Desert Southwest garden. Plant them near tomatoes, cucumbers, and squash to deter pests like squash bugs and aphids. They also do well with broccoli and cabbage, acting as a trap crop for cabbage white butterflies.

Avoid planting nasturtiums near fennel, as they can inhibit each other's growth. By choosing the right companions, you can create a thriving and pest-resistant garden that takes advantage of our long growing season.