Sweet Alyssum in Zone 7B — Pacific Northwest

Sweet Alyssum is a must-have in any Pacific Northwest garden. Its delicate, honey-scented flowers attract beneficial insects and add a touch of charm to borders, containers, and even rock gardens. Plus, the cool nights and mild summers of our region help intensify its fragrance, making it a sensory delight.

While we might face challenges like cool spring soil and the occasional slug, Sweet Alyssum thrives with a little attention. Our long 193-day growing season gives you plenty of time to enjoy its continuous blooms from mid-May well into November.

Starting Sweet Alyssum indoors isn't essential, but it can give you a head start, especially if you're eager to see those first blooms as early as possible. If you choose to start indoors, aim for mid-February through early March, about six weeks before you plan to transplant.

Use seed trays filled with a good seed-starting mix and keep them consistently moist, bottom watering can help prevent damping off. Provide warmth and plenty of light, especially under our overcast spring days, a grow light will be your best friend.

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

Space them 6-8 inches apart in well-drained soil. Keep an eye out for slugs, which can be particularly active during our wet springs.

Direct sowing Sweet Alyssum is a simple and effective method, especially since our mild summers offer a long window for success. Sow seeds directly into the garden from early April through late August.

Prepare the soil by loosening it and removing any weeds. Sweet Alyssum prefers slightly cooler soil, so aim for a time when the soil temperature is consistently above 60°F. Space the seeds 6-8 inches apart.

Sweet Alyssum prefers moderate moisture, but it's crucial not to overwater, especially with our tendency for wet springs. During the dry summer months, water deeply about once a week, providing around 1 inch of water.

Use the finger test: stick your finger into the soil about 2 inches deep. If it feels dry, it's time to water. Water at the base of the plants rather than overhead to minimize the risk of fungal diseases during our dry-summer humidity.

Signs of underwatering include wilting and dry, crispy leaves. Overwatering can lead to yellowing leaves and root rot. A light layer of mulch, like composted leaves, can help retain moisture and suppress weeds.

Sweet Alyssum requires minimal pruning, which is great for busy PNW gardeners! When the plants get leggy or stop blooming profusely in midsummer, shear them back by about half.

This encourages fresh growth and a new flush of flowers. At the end of the season, after the first frost around late October, you can leave the plants in place as a living mulch or remove them and add them to your compost pile.

Sweet Alyssum is primarily grown for its ornamental value and as a beneficial insect attractant, so there's no need to "harvest" flowers in the traditional sense. Expect the first blooms around mid-May, about 45 days after sowing.

The plants will continue to flower throughout the summer and into the fall, often lasting until the first frost in late October or early November. Deadheading spent flowers can encourage continued blooming, but it's not strictly necessary.

Here are some common problems you might encounter with Sweet Alyssum in the Pacific Northwest:

Downy Mildew

• What it looks like: Yellow patches on the upper leaf surface, with a fuzzy gray-purple growth on the undersides. Leaves will eventually brown and die, starting from the bottom of the plant.
• What causes it: This is caused by an oomycete pathogen that thrives in cool, humid conditions. The spores are spread by wind, making it common during our wet springs.
• How to fix/prevent it: Remove affected leaves immediately to prevent further spread. Improve air circulation around your plants by spacing them adequately. Water at the base of the plants, avoiding overhead watering. Copper-based fungicides can be used as a preventative measure.

Heat Dieback

• What it looks like: Plants may wilt, turn brown, and appear to die back during periods of extreme heat.
• What causes it: Although we have mild summers, Sweet Alyssum can struggle when temperatures occasionally rise above 90°F.
• How to fix/prevent it: Provide afternoon shade during heat waves. Water deeply during hot spells. The plants will often recover when temperatures cool down.

Flea Beetles

• What it looks like: Tiny, round holes (shot-hole pattern) in the leaves. You might also see small, dark beetles that jump when disturbed. Damage is usually worst on young seedlings.
• What causes it: Flea beetles are small jumping beetles that feed on the leaves.
• How to fix/prevent it: Use row covers to protect seedlings during their vulnerable stage. Delay planting until plants are larger and more resilient. Apply kaolin clay or neem oil spray. Interplant with basil or catnip, which can help repel these pests.

Pacific Northwest Specific Challenges: Late blight isn't a direct threat to Sweet Alyssum, but our dry-summer humidity can create favorable conditions for downy mildew, so good air circulation and watering practices are key. While slugs are always a concern in PNW gardens, they don't usually target Sweet Alyssum as much as other plants.

Sweet Alyssum makes a great companion plant in the Pacific Northwest vegetable garden. It attracts beneficial insects like hoverflies and lacewings, which prey on aphids and other pests.

Good companions include:

• Broccoli: Sweet Alyssum attracts beneficial insects that help control broccoli pests.
• Potato: The fragrant flowers can deter potato beetles.
• Lettuce: Sweet Alyssum acts as a living mulch, suppressing weeds and helping to retain moisture.

Avoid planting it near plants that are highly susceptible to flea beetles, as it may attract them to the area.