Nasturtium in Zone 7B — Desert Southwest

Nasturtiums are a fantastic addition to any Desert Southwest garden. Their peppery leaves and bright flowers add a unique zest to salads, while their vigorous growth provides ground cover and attracts beneficial insects. Plus, the satisfaction of harvesting edible flowers and seeds from your own garden is hard to beat, especially when those plants thrive with minimal fuss.

Desert gardening presents unique challenges, like our intense sun and alkaline soil. But with strategic timing and smart watering, nasturtiums can flourish during our long, 245-day growing season. You can enjoy their beauty and flavor from spring all the way into fall.

While nasturtiums thrive when direct sown, starting seeds indoors can give you a head start, especially if you're eager for an early spring bloom. Start seeds indoors from mid February through early March, about four weeks before you plan to transplant them outdoors. A simple seed tray with a humidity dome, placed in a warm spot with plenty of light, will do the trick.

Consider bottom watering your seedlings to encourage strong root growth and prevent damping off. Remember, our very-early spring means you'll want sturdy seedlings ready to face the elements as soon as the weather allows. This approach is especially useful if you're aiming for an early harvest before the extreme heat arrives.

Transplant your nasturtium seedlings outdoors from late March through mid April, once the danger of frost has passed. Before transplanting, harden off your seedlings by gradually exposing them to outdoor conditions for about a week. This helps them adjust to the intense Desert Southwest sun and temperature fluctuations.

Space your plants 10-12 inches apart to allow for their trailing growth habit. Keep an eye on the weather forecast – even in late spring, we can experience unexpected temperature swings that might stress young transplants.

Direct sowing is the preferred method for nasturtiums, as they don't always love being transplanted. Sow seeds directly into the ground from late March through early September. This extended window allows you to take advantage of our two growing seasons.

Prepare the soil by loosening it and ensuring good drainage. Nasturtiums actually prefer slightly poor soil, so no need to over-amend. Sow seeds 1/2 inch deep and space them 10-12 inches apart. Keep the soil consistently moist until germination, which usually takes about a week. Soil temperatures between 65-70°F are ideal for germination.

Nasturtiums are surprisingly drought-tolerant, making them a great choice for water-wise Desert Southwest gardens. The key is to avoid overwatering, which can lead to more foliage than flowers. Less is definitely more when it comes to nasturtiums.

During the spring and fall, when temperatures are milder, water deeply but infrequently, about once a week. Use the finger test: if the top two inches of soil are dry, it's time to water. Aim for about 1 inch of water per week. During the extreme heat of summer, you might need to water twice a week, but always check the soil moisture first.

Water at the base of the plant to avoid wetting the foliage, which can increase the risk of fungal diseases in our arid climate. Signs of underwatering include wilting leaves and stunted growth. Overwatering can cause yellowing leaves and root rot. A light layer of organic mulch, like shredded bark, can help retain moisture and regulate soil temperature.

You can expect your first harvest of nasturtium flowers and leaves from mid May through mid November, 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, being careful not to damage the plant. Regular harvesting encourages continued production of both flowers and leaves. As the first frost approaches in mid- November, harvest any remaining flowers and seed pods. You can pickle the seed pods for a caper-like treat or save them for planting next year.

Here are some common problems you might encounter with nasturtiums in the Desert Southwest:

Aphids

• What it looks like: Clusters of tiny green, black, or brown insects on stems and leaves, often causing distorted growth.
• What causes it: Aphids are attracted to stressed plants and can multiply quickly in warm weather. In the Desert Southwest, they thrive during the spring and fall when temperatures are mild.
• How to fix/prevent it: Nasturtiums are often used as a trap crop for aphids, drawing them away from other plants. You can also blast them off with a strong stream of water from your hose. Introduce beneficial insects like ladybugs to control aphid populations naturally.

Cabbage Caterpillars

• What it looks like: Green caterpillars chewing holes in leaves, leaving behind droppings.
• What causes it: Cabbage white butterflies lay their eggs on nasturtiums, and the caterpillars hatch and feed on the foliage. They are more prevalent during the warmer months.
• How to fix/prevent it: Handpick caterpillars off the plants. Cover plants with floating row covers to prevent butterflies from laying eggs. Bacillus thuringiensis (Bt) is an effective organic control method.

Black Fly

• What it looks like: Small black flies swarming around the plants, especially in moist areas.
• What causes it: Black flies are attracted to wet soil and decaying organic matter. Overwatering or poor drainage can create ideal conditions for them.
• How to fix/prevent it: Improve drainage by amending the soil with compost or perlite. Avoid overwatering. Use yellow sticky traps to catch adult flies.

Desert Southwest Specific Challenges: The extreme heat and intense sun can stress nasturtiums, making them more susceptible to pests and diseases. Provide afternoon shade during the hottest months to prevent sunscald. Our arid climate means that spider mites can also become a problem, so keep an eye out for them and treat them with insecticidal soap if necessary.

Nasturtiums are excellent companion plants in the Desert Southwest garden. They attract beneficial insects and can help deter pests from other crops.

Good companions include tomatoes, cucumbers, and squash. Nasturtiums deter squash bugs and cucumber beetles, while their presence near tomatoes can help repel aphids. They also work well with broccoli and cabbage, as they can attract cabbage white butterflies away from these brassicas, acting as a trap crop.

Avoid planting nasturtiums near fennel, as they can inhibit each other's growth. Also, while nasturtiums tolerate poor soil, avoid planting them near plants that require rich, heavily fertilized soil, as the nasturtiums will likely suffer.