Sweet Alyssum

Sweet alyssum is one of the most useful plants a vegetable gardener can grow, and most people plant it for the wrong reason. The flowers are pretty — small, fragrant, white or purple clusters that carpet the ground — but the real value is what they attract. Syrphid flies, also called hover flies, come to alyssum for the nectar. Their larvae eat aphids. A border of alyssum around a bed of lettuce or tomatoes is a standing invitation to the insects that keep the pests in check.

The plant is cold-hardy and can be direct-sown four weeks before your last frost. Scatter the tiny seeds on the surface of prepared soil and press them in lightly — they need light to germinate, which tends to happen within a week if the soil stays above fifty degrees. Alyssum grows quickly once it's up, and flowering starts about six weeks after sowing. It reseeds readily, and in many gardens it comes back on its own the following spring.

The catch is heat. Alyssum blooms heavily in cool weather and then, when temperatures climb into the eighties and nineties in midsummer, it slows down or stops altogether. The foliage stays green, but the flowers thin out and the plant looks tired. Most gardeners see this and assume the plant is finished. It is not finished — it is waiting. If you shear it back by half in late July or early August, cutting off the spent blooms and leggy stems, it will flush back out in September and bloom hard through fall, often until frost.

Overwatering is the other common way to lose alyssum, particularly in humid climates. The plant tolerates dry soil well and tends to rot at the crown if kept consistently wet. A weekly deep watering in the absence of rain is usually enough; daily watering in a wet summer can cause the entire planting to collapse into a slimy mat. If you see stems turning brown at the base and the foliage wilting despite moist soil, root rot has likely set in — pull affected plants and let the soil dry out.

Plant it thick. Alyssum is most effective when grown in a solid mat, either as a border along the edge of a bed or as a living mulch between taller plants. Spacing plants six inches apart gives you a dense carpet within a few weeks, and the fragrance — a light honey scent — is strongest in the evening. White varieties tend to be the most fragrant; purple and pink selections are often less so.