Sweet Alyssum in Zone 4B — Mountain West

Sweet Alyssum is a delightful addition to any Mountain West garden. Its sweet fragrance and delicate flowers attract beneficial insects, making it a great companion plant for vegetables. Plus, it thrives in our cool nights and intense sunlight, adding beauty and function to your garden.

Our short growing season and alkaline soil can be challenging, but Sweet Alyssum is fast-growing. With a 128-day growing season, you'll have plenty of time to enjoy its blooms, whether you start seeds indoors or sow them directly.

Starting Sweet Alyssum indoors isn't essential, but it can give you a head start, especially if you're eager for those early blooms. If you choose this method, sow seeds indoors from mid-March through early April, about six weeks before you plan to transplant them outdoors.

Use seed trays or small pots filled with a good seed-starting mix. Keep the soil consistently moist by bottom watering, and provide warmth and plenty of light. Remember, spring in the mountains often has temperature swings, so a controlled indoor environment helps ensure consistent germination.

Transplanting Sweet Alyssum outdoors is best done after the danger of frost has passed, typically from early May through mid-June. Before transplanting, harden off your seedlings by gradually exposing them to outdoor conditions for about a week.

When transplanting, space the plants 6-8 inches apart. Keep an eye on the weather forecast – a late frost or sudden hailstorm can damage young transplants. Be ready to cover them if needed.

Direct sowing Sweet Alyssum is a simple and effective method, especially since our summers have intense sunlight. Sow seeds directly into well-prepared soil from early May through late July.

Ensure the soil is loose and the soil temperature is consistently above 60°F. Lightly cover the seeds with soil and keep them moist until germination. Direct sowing is less work, but be aware that young seedlings are more vulnerable to pests and our unpredictable spring weather.

Sweet Alyssum needs consistent moisture, but it doesn't like soggy soil. In our dry Mountain West climate, regular watering is crucial, especially during the hottest part of the summer.

During the spring and early summer, water deeply once or twice a week, providing about 1 inch of water each time. Use the "finger test" – if the top 2 inches of soil feel dry, it's time to water. As temperatures rise, increase watering to two or three times a week. Aim to water at the base of the plants to avoid wetting the foliage, which can encourage fungal diseases, even though our humidity is low.

Signs of underwatering include wilting leaves and stunted growth. Overwatering can lead to root rot, indicated by yellowing leaves and a mushy stem base. Adding a layer of mulch, like pine needles or shredded bark, helps retain moisture and regulate soil temperature.

Sweet Alyssum requires minimal pruning, but a little maintenance keeps it looking its best. When plants become leggy or stop blooming profusely in midsummer, shear them back by about half.

This encourages fresh growth and a new flush of flowers. Use sharp scissors or pruning shears to make clean cuts. There's no need to prune seed heads unless you want to prevent self-seeding. As the first frost approaches in mid-September, you can leave the plants as is, or lightly trim them to tidy up the garden.

Sweet Alyssum is primarily grown for its ornamental value and its ability to attract beneficial insects. You don't "harvest" it in the traditional sense. Expect the first blooms to appear from mid-June through late September, depending on when you planted.

The flowers will bloom continuously throughout the growing season. Simply enjoy the blooms and the benefits they bring to your garden. As the first frost approaches in mid-September, the plants will naturally decline. You can leave them in place to self-seed or remove them to prepare your garden for winter.

Here are some common problems you might encounter with Sweet Alyssum in the Mountain West:

Downy Mildew

• What it looks like: Yellow patches on the tops of leaves, with a fuzzy, gray-purple growth on the undersides. Leaves brown and die from the bottom up.
• What causes it: This is an oomycete pathogen that spreads in cool, humid conditions via wind-carried spores. Though our region is dry, cool nights and infrequent rain can create favorable conditions.
• How to fix/prevent it: Remove affected leaves immediately. Improve air circulation around the plants. Water at the base, avoiding overhead watering. Consider using a copper-based fungicide as a preventative measure.

May Die Back in Extreme Heat

• What it looks like: The plant wilts and appears stressed, even with adequate watering. Leaves may turn brown and crispy.
• What causes it: While our summers are moderate-at-altitude, Sweet Alyssum can struggle during the hottest stretches. The intense sun and dry air exacerbate the problem.
• How to fix/prevent it: Provide some afternoon shade during the hottest part of the day. Ensure consistent watering. The plants usually recover when temperatures cool down in the evening.

Flea Beetles

• What it looks like: Tiny, round holes (shot-hole pattern) in the leaves. You might see small, dark beetles that jump when disturbed.
• What causes it: These are small jumping beetles that feed on the leaves. They are most damaging to small, young plants.
• How to fix/prevent it: Use row covers to protect seedlings during their vulnerable stage. Delay planting until plants are larger and more resilient. Kaolin clay coating can deter the beetles. Neem oil spray can also be effective.

Mountain West Specific Challenges: The intense sun at our high altitude can stress Sweet Alyssum, even though it enjoys full sun. Our alkaline soil might require amendments to ensure optimal growth. However, the low humidity generally keeps fungal diseases at bay, and the cool nights enhance the flower's fragrance.

Sweet Alyssum makes a great companion plant in your Mountain West garden.

• Broccoli: Sweet Alyssum attracts beneficial insects like hoverflies and lacewings, 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: As a ground cover, Sweet Alyssum helps retain soil moisture and suppress weeds, benefiting lettuce in our dry climate.

Avoid planting Sweet Alyssum near plants that are highly susceptible to flea beetles, as it can sometimes attract these pests.