Nasturtium in Zone 8B — Desert Southwest

Nasturtiums are a fantastic addition to your Desert Southwest garden. Their peppery flowers and leaves add a unique zest to salads, and the seed pods can be pickled for a caper-like treat. Plus, they’re beautiful and attract pollinators, making them a valuable and delicious addition to your garden.

While our extreme heat and arid climate present challenges, nasturtiums are surprisingly resilient when planted at the right time. With a long growing season of around 279 days, you have ample opportunity to enjoy these beauties in your Zone 8B garden.

Starting nasturtiums indoors is an option, but direct sowing is generally preferred since they don't love being transplanted. If you want a head start, sow seeds indoors from late January through mid February, about four weeks before you plan to transplant. Use seed trays with a good seed-starting mix.

Keep the trays warm and provide plenty of light, either with a grow light or a sunny window. Bottom watering is ideal to prevent damping off; simply place the tray in a shallow dish of water and let the soil absorb moisture from the bottom. Remember, our very-early spring can be deceptive, so keep an eye on the weather before transplanting.

Transplant your nasturtium seedlings outdoors from early March through early April, after the last expected frost. 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.

Choose a spot with full sun to partial shade and space the plants 10-12 inches apart. Be mindful of unexpected temperature swings that can still occur in early spring. Protect young transplants with shade cloth if necessary.

Direct sowing is the recommended method for nasturtiums, as they don't always handle transplanting well. You can direct sow seeds outdoors from early March through late September, giving you two distinct growing seasons. Wait until after the last frost to sow.

Prepare the soil by loosening it and removing any rocks or debris. Nasturtiums aren’t picky and actually thrive in poor soil. 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 drought-tolerant and thrive in drier conditions, making them well-suited for our arid Desert Southwest climate. Overwatering is a common mistake that leads to more foliage and fewer flowers, so less is definitely more. During the cooler months of spring and fall, water only when the top 2 inches of soil feel dry to the touch – use the finger test.

In the extreme heat of summer, you'll need to water more frequently, typically about once a week, providing about 1 inch of water. Always water at the base of the plant to avoid wetting the leaves, as our arid climate doesn't encourage fungal diseases. Signs of underwatering include wilting leaves, while overwatering can cause yellowing leaves and root rot.

A light layer of organic mulch, like compost or shredded bark, can help retain moisture in the soil during the hottest months. Remember to adjust your watering schedule based on monsoon moisture, which can reduce the need for supplemental watering.

You can expect your first harvest of nasturtium flowers and leaves from late April through early 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.

Gently snip the flowers or leaves from the plant, being careful not to damage the surrounding foliage. Regular harvesting encourages continued production throughout the growing season. As the first frost approaches in early December, harvest any remaining flowers and seed pods. The seed pods can be pickled and enjoyed as a caper substitute, extending your harvest even further.

Here are three 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 or yellowing foliage. *What causes it:* Aphids are attracted to stressed plants. In our Desert Southwest climate, this is often due to heat stress or inconsistent watering. *How to fix/prevent it:* Nasturtiums are often used as a trap crop for aphids, drawing them away from other plants. You can spray aphids off with a strong stream of water or introduce beneficial insects like ladybugs.

Cabbage Caterpillars *What it looks like:* Holes in leaves, often with green droppings nearby. You may also see small green caterpillars on the plant. *What causes it:* Cabbage white butterflies lay their eggs on nasturtiums, and the caterpillars hatch and feed on the leaves. *How to fix/prevent it:* Handpick caterpillars off the plants or use a biological insecticide like Bacillus thuringiensis (Bt). Covering plants with lightweight row covers can also prevent butterflies from laying eggs.

Black Fly *What it looks like:* Similar to aphids, these tiny black flies cluster on new growth, sucking sap and weakening the plant. *What causes it:* Like aphids, black fly are attracted to stressed plants. Dry conditions and alkaline soil can exacerbate the problem. *How to fix/prevent it:* Spray plants with insecticidal soap or neem oil. Ensure plants are adequately watered and consider amending the soil with compost to improve its nutrient content and drainage.

Desert Southwest Specific Challenges: The extreme heat and arid humidity of the Desert Southwest can stress nasturtiums, making them more susceptible to pests. Providing afternoon shade during the hottest months and ensuring consistent watering can help keep your plants healthy.

Nasturtiums are excellent companion plants for several vegetables in the Desert Southwest garden. Plant them near tomatoes to deter whiteflies, or alongside cucumbers and squash to repel squash bugs and vine borers. They also benefit broccoli and cabbage by attracting cabbage white butterflies away from these crops. Avoid planting nasturtiums near fennel, as they can inhibit each other's growth.