Sweet Alyssum in Zone 6B — Pacific Northwest

Sweet Alyssum is a delightful addition to any Pacific Northwest garden. Its sweet fragrance, delicate flowers, and low-growing habit make it perfect for edging borders, filling containers, or even acting as a living mulch. Plus, the cool nights we often have here in the PNW seem to intensify its fragrance, making it even more enjoyable.

While our wet springs and mild summers can sometimes present challenges, Sweet Alyssum is well-suited to our 162-day growing season. With a little attention to timing and watering, you can enjoy its blooms from early summer well into the fall.

Starting Sweet Alyssum indoors is an option, though not always necessary in our climate. If you want a head start, especially for earlier blooms, sow seeds indoors from late February through mid March. This gives them about 6 weeks to get established before transplanting.

Use seed trays with a good seed-starting mix and provide warmth (a heat mat is helpful) and bright light (grow lights are ideal, given our overcast spring days). Bottom watering is a great way to keep the soil consistently moist without overwatering the delicate seedlings.

Keep in mind that our cool soil in spring can slow down growth, so indoor starting can give you a jump on the season if you're eager for those first blooms.

Transplant your Sweet Alyssum seedlings outdoors from mid April through late May, after the risk of hard frost has passed. Before transplanting, be sure to harden off your seedlings by gradually exposing them to outdoor conditions for about a week. This helps them adjust to the sun and wind.

Plant them 6-8 inches apart in well-drained soil. Keep an eye on the weather forecast – a late cold snap can still happen, so be prepared to cover your transplants if necessary.

Direct sowing Sweet Alyssum is a simple and effective method, especially if you're not in a rush for early blooms. Sow seeds directly into the garden from mid April through mid August. The soil should be workable and at least somewhat warmed up.

Prepare the soil by loosening it and removing any weeds. Scatter the seeds evenly and lightly rake them in. Keep the soil consistently moist until the seedlings emerge. Thin the seedlings to 6-8 inches apart once they are large enough to handle.

Direct sowing is a great option if you have a large area to cover or want a more natural, informal look.

Sweet Alyssum needs moderate watering to thrive in our Pacific Northwest climate. During our wet springs, you may not need to water at all, but as the dry summer months arrive, regular watering is essential.

A good rule of thumb is to water when the top 2 inches of soil feel dry to the touch – the "finger test." Aim to provide about 1 inch of water per week, either through rainfall or irrigation. Water at the base of the plants to avoid wetting the foliage, which can encourage fungal diseases in our sometimes humid conditions.

Signs of underwatering include wilting and stunted growth. Overwatering can lead to root rot, so ensure the soil is well-draining. A layer of mulch around the plants can help retain moisture and suppress weeds, which is especially useful during our drier periods. Remember that Sweet Alyssum may go dormant in extreme heat, so don't overwater it if it's looking tired during a heat spell; it will bounce back when temperatures cool down.

Sweet Alyssum requires minimal pruning, but a little maintenance can keep it looking its best throughout the growing season. The main thing to watch for is legginess – when the plants get stretched out and sparse in midsummer.

When this happens, shear the plants back by about half. Don't worry, they'll quickly produce fresh growth and flowers. As the first frost approaches around early October, you can leave the plants in place to self-seed or pull them up and compost them.

Sweet Alyssum is primarily grown for its beautiful, fragrant flowers and as a beneficial insect attractant, rather than for a specific harvest. Expect the first blooms to appear around early June, about 45 days after planting. The plants will continue to flower prolifically throughout the summer and into mid October.

There's no need to "harvest" the flowers in the traditional sense. Simply enjoy their beauty and fragrance in the garden. Deadheading (removing spent flowers) can encourage continued blooming, but it's not essential.

As the first frost approaches in early October, you can let the plants go to seed or remove them. The seeds can be saved for planting next year or left in the garden to self-sow.

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 surfaces, with 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, and different species affect different plants. Our wet springs can create ideal conditions for downy mildew.
• How to fix/prevent it: Remove affected leaves immediately to prevent the spread. Improve air circulation around the plants by spacing them properly. Water at the base of the plants to avoid wetting the foliage. Copper-based fungicides can be used as a preventative measure. Look for downy mildew-resistant varieties when purchasing seeds or plants.

Heat Dieback

• What it looks like: The plants may wilt, turn brown, and appear to die back, especially during periods of unusually high temperatures.
• What causes it: While Sweet Alyssum prefers full sun, it can struggle in extreme heat. Although our summers are mild, we occasionally have heat spells that can stress the plants.
• How to fix/prevent it: Provide some afternoon shade during the hottest part of the day. Ensure the plants are well-watered, but avoid overwatering. Mulching can help retain soil moisture and keep the roots cool. The plants will often recover and resume blooming 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. The damage is usually worst on young seedlings.
• What causes it: Flea beetles are small jumping beetles that feed on the leaves of various plants. 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 the plants are larger and more resilient. You can also try coating the plants with kaolin clay or spraying them with neem oil. Interplanting with basil or catnip may help deter flea beetles. Healthy, vigorous plants are better able to outgrow the damage.

Pacific Northwest Specific Challenges: Our mild summers and dry-summer rainfall can be both a blessing and a curse for Sweet Alyssum. While the cool nights enhance its fragrance, the occasional heat spells can cause dieback. Maintaining consistent moisture levels during the dry summer months is crucial. Late blight is not a direct threat to Sweet Alyssum, but the conditions that favor blight (cool, wet weather) can also promote downy mildew. Keep an eye on your plants, especially during periods of high humidity, and take preventative measures as needed.

Sweet Alyssum makes a wonderful companion plant in the Pacific Northwest garden. Its fragrant flowers attract beneficial insects like hoverflies and lacewings, which prey on aphids and other pests.

Good companions include broccoli, potato, and lettuce. Broccoli benefits from the increased pollination and pest control. Potatoes enjoy the ground cover aspect of Sweet Alyssum. Lettuce benefits from the shade provided by the alyssum. Avoid planting it near plants that are highly susceptible to flea beetles, as it can attract these pests.