Sweet Alyssum in Zone 7A — Mountain West

Sweet Alyssum is a winner in our Mountain gardens. Its delicate, honey-scented flowers add beauty to borders, attract beneficial insects, and can even act as a living mulch. Plus, those cool Mountain nights enhance its fragrance.

Our high altitude, intense sun, and short growing season can be challenging, but Sweet Alyssum's relatively quick 45-day maturity makes it a perfect fit. With a 173-day growing season, you'll have plenty of time to enjoy these blooms, especially with careful planning.

Starting Sweet Alyssum indoors gives you a head start, but it's not always necessary given our relatively short maturity time. If you want earlier blooms, aim to sow seeds indoors from mid February through early March, about six weeks before you plan to transplant.

Use seed trays with a good seed-starting mix, and provide warmth (a heat mat helps) and plenty of light (grow lights are ideal). Bottom watering is key – set the tray in a shallow dish of water and let the soil soak it up to prevent damping off. Remember, spring in the Mountains can be late, so be patient!

Transplant your Sweet Alyssum seedlings outdoors from early April through mid May, after the danger of hard frost has passed. Before transplanting, harden them off by gradually exposing them to outdoor conditions for about a week.

Space them 6-8 inches apart to allow for their spreading growth habit. Keep an eye on the weather forecast – a late spring cold snap can still happen! Be ready to cover the young plants if needed.

Direct sowing is a perfectly viable option for Sweet Alyssum here in the Mountain West, and can save you some effort. Sow seeds directly into the garden from early April through mid August.

Make sure the soil is finely raked and relatively warm. Gently press the seeds into the soil, spacing them 6-8 inches apart (thin later if needed). Keep the soil consistently moist until germination. Direct sowing is a great way to extend your bloom season, with successive sowings every few weeks.

Watering Sweet Alyssum correctly is crucial, especially given our dry Mountain climate. While they need moderate moisture, they're not drought-tolerant and our low humidity means they'll dry out faster than you think.

During the spring and fall, when temperatures are cooler, water deeply about once a week, providing about 1 inch of water. Use the "finger test" – if the top 2 inches of soil are dry, it's time to water. In the heat of summer, you may need to water every other day, especially if they're in full sun. Water at the base of the plants to avoid wetting the foliage, which can encourage fungal diseases in our cool evenings.

Signs of underwatering include wilting leaves and dry, crispy edges. Overwatering can lead to yellowing leaves and root rot. A layer of mulch, like shredded bark or compost, will help retain moisture in the soil and keep the roots cool.

Sweet Alyssum requires minimal pruning, but a little attention keeps them blooming beautifully. The main thing is to shear them back when they start to look leggy, usually in midsummer.

Simply cut back the plants by about half, removing spent flower heads. Don't worry about being too precise – they'll bounce back quickly with fresh growth and more flowers. As the first frost approaches in early October, you can let them be. They might even self-seed for next year!

Technically, you don't "harvest" Sweet Alyssum in the traditional sense. They flower continuously, from late May through mid October, and are valued more for their ornamental qualities and beneficial insect attraction than for a harvestable product.

Enjoy the continuous blooms! The flowers attract pollinators like bees and butterflies, which are essential for a healthy garden ecosystem. As the growing season winds down and temperatures drop, the plants will naturally slow their production.

Here are some common problems you might encounter with Sweet Alyssum in our Mountain gardens:

Downy Mildew

• What it looks like: Yellow patches on the upper leaf surface, with a fuzzy, gray-purple growth on the undersides. Leaves start to brown and die 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, those cool nights can create enough moisture for downy mildew to take hold.
• How to fix/prevent it: Remove affected leaves immediately to prevent spread. Improve air circulation around the plants. Water at the base, avoiding overhead watering. Copper-based fungicides can be used as a preventative measure.

Heat Dieback

• What it looks like: The plants may stop blooming or even die back in the hottest part of summer.
• What causes it: While our summers are moderate-at-altitude, Sweet Alyssum can still struggle in prolonged heat, especially with intense sun.
• How to fix/prevent it: Provide afternoon shade during the hottest days. Water deeply and consistently. Mulch around the plants to keep the soil cool. Don't worry, they'll usually bounce back once temperatures cool down again.

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 small jumping beetles feed on the leaves. They can be particularly damaging to young seedlings.
• How to fix/prevent it: Row covers can protect seedlings during their vulnerable stage. Delay planting until plants are larger. You can also try spraying with neem oil or interplanting with basil or catnip to deter them.

Mountain West Specific Challenges: The intense sun can scorch foliage if plants aren't adequately watered. Our alkaline soil can also affect nutrient uptake, so amend with compost to improve soil health. Hail can be a concern, so consider providing temporary protection if severe weather is forecast.

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

It pairs well with broccoli because its flowers attract beneficial insects that prey on broccoli pests. It also complements potatoes, attracting hoverflies that help control aphids. Lettuce also benefits from Sweet Alyssum's presence, as it provides ground cover and attracts pollinators.

I'm not aware of any truly "bad" companions for Sweet Alyssum, but avoid planting it near anything that requires very dry soil, as its watering needs are moderate.