Nasturtium in Zone 9A — Northern California

Nasturtiums are a fantastic addition to any Northern California garden. Their peppery leaves and bright flowers add a unique zest to salads, plus they're incredibly versatile in the kitchen. The satisfaction of growing these beauties in our Mediterranean climate, with its long growing season, is unmatched, and their ability to attract beneficial insects makes them a true garden ally.

While we face challenges like drought and occasional water restrictions here in Northern California, nasturtiums are surprisingly well-suited to our conditions. With a growing season that stretches for nearly 10 months, you'll have plenty of time to enjoy their vibrant blooms and flavors, especially given their drought tolerance. You'll find that timing your planting right will set you up for success.

Starting nasturtiums indoors is possible, but direct sowing is generally preferred since they don't love being transplanted. However, if you want a head start, especially for an earlier bloom time, you can start seeds indoors from late January through mid February, about four weeks before you plan to transplant.

Use seed trays with good drainage and a well-draining seed starting mix. Keep them warm and provide plenty of light, either with a grow light or a sunny windowsill. Bottom watering is a great way to keep the soil moist without overwatering, which can be especially helpful in our early, sometimes unpredictable spring weather.

If you started your nasturtiums indoors, transplant them outdoors from late February through late March, after any danger of frost has passed. Before transplanting, harden them off by gradually exposing them to outdoor conditions for about a week.

Choose a sunny to partially shaded spot with well-draining soil and space them 10-12 inches apart. Keep an eye on the weather forecast, as a late cold snap can still occur, even in our relatively mild Northern California springs.

Direct sowing is the easiest and most successful way to grow nasturtiums in our region. Sow seeds directly into the garden from late February through late September, giving you a long window to get them established.

Choose a spot with full sun to partial shade, and ensure the soil is well-draining. Nasturtiums aren't picky about soil fertility, and in fact, they often thrive in poorer soils. Sow seeds about half an inch deep and 10-12 inches apart. Keep the soil consistently moist until the seedlings emerge, usually within a week or two.

Nasturtiums are wonderfully drought-tolerant, which makes them perfect for Northern California gardens facing water restrictions. They actually prefer drier conditions and can suffer from overwatering, so less is definitely more.

During our typically dry summers, water deeply but infrequently. Use the "finger test" – stick your finger about two inches into the soil, and only water if it feels dry. During the hottest parts of the summer, you might need to water about once a week, providing about an inch of water. Water at the base of the plants to avoid wetting the foliage, which can help prevent fungal diseases in our low-humidity environment.

In the cooler spring and fall months, you'll likely need to water even less frequently, perhaps only every two to three weeks, depending on rainfall. Watch for signs of underwatering, such as wilting leaves, and overwatering, such as yellowing leaves. A light layer of mulch, like wood chips, can help retain soil moisture and reduce the need for frequent 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.

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 early December, you can still harvest any remaining flowers and leaves, preserving them for later use.

Here are three common problems you might encounter growing nasturtiums in Northern California:

Aphids (intentional trap crop)

• What it looks like: Clusters of small, soft-bodied insects, often green or black, on stems and undersides of leaves. Leaves may be sticky or distorted.
• What causes it: Aphids are attracted to the lush foliage, especially in spring. They thrive in mild, dry conditions, common in our Northern California climate.
• How to fix/prevent it: Nasturtiums can act as a trap crop, drawing aphids away from other plants. You can tolerate them for this reason. If infestations become heavy, spray with a strong jet of water or insecticidal soap.

Cabbage Caterpillars

• What it looks like: Green caterpillars feeding on leaves, leaving holes. You might also see small white butterflies fluttering around your plants.
• What causes it: Cabbage white butterflies lay eggs on nasturtiums, which hatch into hungry caterpillars. They can be prevalent in our long growing season.
• How to fix/prevent it: Handpick caterpillars off plants. Cover plants with netting to prevent butterflies from laying eggs. Bacillus thuringiensis (Bt) is an effective organic control.

Black Fly

• What it looks like: Tiny black flies swarming around the plants, especially new growth. Leaves may be stunted or distorted.
• What causes it: Black flies, also known as fungus gnats, thrive in damp conditions. Overwatering or poorly draining soil can contribute to infestations.
• How to fix/prevent it: Improve drainage and avoid overwatering. Allow the soil to dry out between waterings. Yellow sticky traps can help control adult populations.

Northern California Specific Challenges: Our moderate-to-hot summers and low humidity can stress nasturtiums if they're not properly watered, but their drought tolerance generally makes them resilient. Pay attention to watering needs during heat waves, and provide some afternoon shade if possible.

Nasturtiums are excellent companion plants for many vegetables in Northern California gardens. They attract beneficial insects and can even deter certain pests.

Good companions include tomatoes, cucumbers, and squash, as nasturtiums deter squash bugs and other pests that can damage these crops. They also benefit broccoli and cabbage by attracting cabbage white butterflies away from them, acting as a trap crop. Avoid planting nasturtiums near fennel, as they can inhibit each other's growth.