Sweet Alyssum in Zone 6A — Mountain West

Sweet Alyssum is a fantastic addition to any high-altitude garden. It thrives in our intense sunlight and cool nights, providing a continuous display of delicate, fragrant flowers that attract beneficial insects. Plus, it makes a beautiful living mulch for other plants in your garden.

Our short growing season and alkaline soil present challenges, but with proper timing and care, you can easily enjoy Sweet Alyssum's beauty from early June through early October. With our roughly 153-day growing season, you'll find that starting seeds at the right time will get you the most out of this plant.

Starting Sweet Alyssum indoors is an option, especially if you want a head start. Begin in late February through mid March, about 6 weeks before you plan to transplant outdoors. This gives the seedlings a chance to get established before our unpredictable spring weather arrives.

Use seed trays with a good seed-starting mix, provide warmth (a heat mat is helpful), and ensure adequate light (grow lights are ideal). Bottom watering is a great way to keep the soil consistently moist without damping off the seedlings. Keep in mind our spring is often late, so indoor starts allow you to control the environment better during those cool nights.

Transplant your Sweet Alyssum seedlings outdoors from mid April through late May, once the danger of hard 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 sun and cooler temperatures.

Space the plants 6-8 inches apart to allow for their spreading growth habit. Keep an eye on the forecast for any late-season cold snaps, and be prepared to cover your transplants if necessary. Our high-altitude sun can be intense, so consider transplanting on a cloudy day or in the late afternoon to minimize transplant shock.

Direct sowing Sweet Alyssum is a simple and effective method, especially given our relatively short growing season. Sow seeds from mid April through early August, ensuring the soil temperature is consistently above 60°F. This gives the seeds the best chance to germinate quickly.

Prepare the soil by loosening it and removing any weeds. Scatter the seeds thinly and lightly cover them with soil. Keep the soil consistently moist until the seedlings emerge. Space them 6-8 inches apart after they sprout. Direct sowing works well, but be aware that seedlings are more vulnerable to pests and weather fluctuations.

Sweet Alyssum needs moderate watering, especially in our dry Mountain West climate. While it appreciates consistent moisture, it's not drought-tolerant. Water deeply but infrequently, allowing the soil to dry slightly between waterings.

During the spring and fall, when temperatures are cooler, you'll likely only need to water once or twice a week. In the heat of summer, especially with our intense sun, increase watering to every other day or even daily, depending on soil drainage. Use the finger test: if the top 2 inches of soil feel dry, it's time to water. Aim for about 1 inch of water per week, either from rainfall or irrigation.

Water at the base of the plants to avoid wetting the foliage, which can lead to fungal issues in humid conditions. Yellowing leaves can indicate overwatering, while wilting leaves suggest underwatering. Applying a layer of mulch, such as shredded bark or compost, helps retain moisture and regulate soil temperature, which is helpful in our alkaline soils.

Sweet Alyssum requires minimal pruning, but a little maintenance keeps it looking its best. When the plants get leggy or start to decline in midsummer, shear them back by about half. This encourages fresh growth and a new flush of flowers.

Use clean pruning shears to make the cuts. Don't be afraid to prune; Sweet Alyssum responds well to being cut back. As the first frost approaches around early October, you can leave the plants in place to self-seed or remove them to tidy up your garden.

Sweet Alyssum is primarily grown for its ornamental value and its ability to attract beneficial insects, so there is no traditional "harvest." The plants flower continuously from early June through early October, providing a constant source of beauty and support for your garden ecosystem.

Simply enjoy the fragrant blooms and the pollinators they attract. No need to worry about specific signs of ripeness or special harvesting techniques. As the growing season winds down and the first frost approaches in early October, you can choose to let the plants self-seed for the following year or remove them as part of your fall garden cleanup.

Sweet Alyssum can encounter a few issues in our Mountain West gardens:

Downy Mildew

• What it looks like: Yellow patches on leaf tops with fuzzy gray-purple growth on undersides. Leaves brown and die from the bottom up.
• What causes it: This oomycete pathogen thrives in cool, humid conditions, which can occur during our spring and fall temperature swings. Spores spread via wind.
• How to fix/prevent it: Remove affected leaves immediately. Improve air circulation by spacing plants adequately. Water at the base, never overhead. Copper-based fungicides can provide prevention.

Die Back in Extreme Heat

• What it looks like: Plants wilt and may appear to stop growing or even die back during the hottest part of summer.
• What causes it: While our summers are considered moderate-at-altitude, we can still have periods of intense heat, especially with our intense sun. Sweet Alyssum prefers cooler temperatures.
• How to fix/prevent it: Provide some afternoon shade during the hottest months. Ensure consistent watering, and consider mulching to keep the soil cool. The plants should recover when temperatures cool down.

Flea Beetles

• What it looks like: Tiny round holes (shot-hole pattern) in leaves. You might see small dark beetles that jump when disturbed. Worst damage occurs to young seedlings.
• What causes it: These small jumping beetles feed on the leaves, particularly of young plants.
• How to fix/prevent it: Row covers can protect seedlings during the vulnerable stage. Delay planting until plants are larger. Kaolin clay coating or neem oil spray can help. Interplanting with basil or catnip may deter them.

Mountain West Specific Challenges: Our moderate-at-altitude heat and low humidity can be a double-edged sword for Sweet Alyssum. The intense sun can stress plants, making them more susceptible to heat-related issues. Consistent watering and afternoon shade are key to keeping them thriving.

Sweet Alyssum makes a wonderful companion plant in our Mountain West gardens. Plant it near:

• Broccoli: Sweet Alyssum attracts beneficial insects that prey on broccoli pests like aphids and cabbage worms.
• Potato: The fragrant flowers can attract pollinators, improving potato yields. Plus, it acts as a living mulch, helping to retain moisture in our dry climate.
• Lettuce: Sweet Alyssum provides ground cover, helping to keep the soil cool and moist, which is beneficial for lettuce, especially during warmer periods.

Avoid planting Sweet Alyssum near plants that prefer very dry conditions, as its moderate watering needs could create an environment that's too moist for them.