Sweet Alyssum in Zone 6B — Mountain West

Sweet Alyssum is a fantastic addition to any high-altitude garden here in the Mountain West. Its delicate, honey-scented blooms attract beneficial insects, making it a valuable companion plant for your vegetables. Plus, those cool nights even in summer intensify the fragrance, creating a lovely sensory experience.

Our short season, alkaline soil, and intense UV light can be a challenge, but Sweet Alyssum's quick maturity and adaptability make it a winner. You can easily fit it into our 163-day growing season with the right timing. Let's dive into how to make the most of this charming plant.

Starting Sweet Alyssum indoors isn't strictly necessary, but it can give you a head start, especially if you're itching to get gardening during those late February through mid March days. This is about 6 weeks before you would transplant. It's a nice way to get a jump on our late spring.

Use seed trays filled with a good seed-starting mix. Keep them consistently moist by bottom watering to prevent damping off. Provide warmth and plenty of light, either from a sunny window or grow lights. Remember, our spring can be quite variable, so indoor starts offer a bit more control.

Once the danger of hard frost has passed, typically mid April through late May, it's time to transplant your Sweet Alyssum seedlings outdoors. Before planting, harden them off by gradually exposing them to outdoor conditions for about a week. This will help them adjust to the intense sun and cooler nights.

Space the plants 6-8 inches apart to allow for their spreading growth habit. Keep an eye on the weather forecast, as we can still get some late-season cold snaps. Be prepared to cover your transplants if temperatures dip unexpectedly.

Direct sowing Sweet Alyssum is a convenient option, especially since it germinates quickly. You can sow seeds mid April through early August, giving you plenty of time to enjoy their blooms. The key is to wait until the soil has warmed up a bit.

Prepare the soil by loosening it and removing any rocks or debris. Sprinkle the seeds lightly on the surface and gently press them into the soil. Keep the soil consistently moist until the seedlings emerge. Space the plants 6-8 inches apart as they grow.

Sweet Alyssum needs moderate watering, especially in our dry Mountain climate. While it's not particularly drought-tolerant, it also doesn't like to sit in soggy soil. Finding the right balance is key, especially with our low humidity and limited rainfall.

During the active growing season, aim to water deeply about once a week, providing around 1 inch of water. Use the "finger test" – if the top 2 inches of soil feel dry, it's time to water. In the heat of summer, Sweet Alyssum may go dormant, so reduce watering during this period. It bounces back quickly when temperatures cool down.

Water at the base of the plants to avoid wetting the foliage, which can encourage fungal diseases. Overhead watering is less of a concern here due to our low humidity, but it's still best to be cautious. Signs of underwatering include wilting leaves and dry soil, while overwatering can lead to yellowing leaves and root rot. A light layer of mulch can help retain moisture in our dry climate.

Sweet Alyssum requires minimal pruning, making it a low-maintenance choice for our busy gardening season. However, a little attention can keep it looking its best.

When the plants get leggy or stop blooming profusely, usually in midsummer, shear them back by about half. Don't worry, this encourages fresh growth and a new flush of flowers. As the first frost approaches in early October, you can leave the plants in place to self-seed or remove them to tidy up the garden.

Sweet Alyssum is more of a living mulch and beneficial insect attractant than a crop to harvest, so there's no real "harvest" in the traditional sense. You can expect the first blooms to appear about 45 days after planting, generally from late May through mid October.

Simply enjoy the continuous display of fragrant flowers throughout the growing season. The flowers attract pollinators like bees and butterflies, which are essential for a healthy garden. As the season winds down and the first frost approaches, you can let the plants go to seed, or compost them.

Even in our relatively disease-free climate, Sweet Alyssum can encounter a few problems. Here's how to identify and address them.

Downy Mildew

• What it looks like: Yellow patches on the upper leaf surface, with fuzzy gray-purple growth underneath. Leaves start browning and dying from the bottom of the plant upwards.
• What causes it: An oomycete pathogen that thrives in cool, humid conditions. While our humidity is generally low, prolonged periods of rain can create favorable conditions.
• How to fix/prevent it: Remove affected leaves immediately to prevent spread. Improve air circulation around plants by spacing them properly. Water at the base of the plants, avoiding overhead watering. Consider using a copper-based fungicide as a preventative measure.

May Die Back in Extreme Heat

• What it looks like: Plants appear stressed, wilted, or may even stop blooming during the hottest part of the summer.
• What causes it: While our summers are moderate-at-altitude, intense UV light and periods of drought can stress Sweet Alyssum.
• How to fix/prevent it: Provide some afternoon shade during the hottest months. Ensure consistent watering, especially during dry spells. Mulch around the plants to help retain moisture.

Flea Beetles

• What it looks like: Tiny, round holes (shot-hole pattern) in the leaves. You might also see small, dark beetles jumping when disturbed. The damage is usually worst on young seedlings.
• What causes it: These small, jumping beetles feed on the leaves, causing damage. They are more prevalent during warm, dry periods.
• How to fix/prevent it: Protect seedlings with row covers during their vulnerable stage. Delay planting until plants are larger and more resilient. You can also try coating the leaves with kaolin clay or spraying with neem oil. Interplanting with basil or catnip may also help deter flea beetles.

Mountain West Specific Challenges: Our moderate-at-altitude heat, low humidity, and dry rainfall patterns generally benefit Sweet Alyssum, reducing the risk of many common diseases. However, the intense UV light can still stress the plants, so providing some afternoon shade is helpful. Consistent watering is essential to combat our dry climate.

Sweet Alyssum makes an excellent companion plant in the Mountain West garden.

• Broccoli: Sweet Alyssum attracts beneficial insects like hoverflies and ladybugs, which prey on aphids and other pests that can damage broccoli.
• Potato: The fragrant flowers can help deter potato beetles, a common pest in our region.
• Lettuce: Sweet Alyssum acts as a living mulch, helping to suppress weeds and retain moisture in the soil around lettuce plants.

Avoid planting Sweet Alyssum near plants that are highly susceptible to fungal diseases, as it can potentially harbor these pathogens.