Bulbs are plants that survive unfavorable periods by retreating into underground storage organs. For Seattle gardeners, understanding the precise timing for planting these dormant structures is necessary to ensure a display of color in subsequent seasons. The timing separates hardy varieties that require a cold dormancy period from tender types that cannot tolerate a freeze. Success depends on aligning planting with the specific needs of the plant and the regional weather patterns.
Optimal Timing for Spring-Flowering Bulbs
Spring-flowering bulbs, such as tulips, daffodils, crocuses, and hyacinths, must be planted in the fall to successfully flower. This autumn window, generally spanning from late September through November, allows the bulbs sufficient time to establish a root system before colder weather begins. Planting should ideally happen when the soil temperature drops below 50 degrees Fahrenheit, which encourages root development rather than premature top growth. The goal is to get the bulbs in the ground before winter conditions make working the soil difficult.
The cold period is a biological requirement for these hardy bulbs, signaling the plant to redirect its energy toward forming the future stem and flower. Spring bloomers typically require at least 11 to 13 weeks of temperatures below 45 degrees Fahrenheit to meet this chilling requirement. Planting too early, when the soil is still warm from summer, can cause the bulbs to sprout prematurely, leaving the fragile shoots vulnerable to subsequent cold snaps. For the best spring display, aim to complete the planting of varieties like alliums and grape hyacinths by early to mid-November.
Timing for Summer-Flowering Bulbs
Summer-flowering bulbs, which often include tender varieties like dahlias, gladiolus, and tuberous begonias, follow a different planting schedule. These plants are sensitive to cold and do not require a chilling period to initiate blooming. They must be planted in the spring, well after the last threat of frost has passed in the Seattle region. The average last frost date in Seattle is typically in early April, making late April or May the appropriate planting window.
Planting these tender varieties is safest when the soil has warmed to about 60 degrees Fahrenheit, which usually occurs as spring progresses into early summer. Since these bulbs are not cold-hardy, they are often treated like annuals or are lifted from the ground in the fall before the winter rains begin. These types are planted to bloom from mid-summer into the fall, extending the season of color long after the spring bloomers have faded into dormancy.
Seattle’s Climate Impact on Bulb Planting
The Seattle area presents unique conditions that influence bulb planting practices. While the mild winters are beneficial for many plants, the lack of prolonged, deep cold can affect certain spring-flowering bulbs. Some tulip varieties, for example, need a specific duration of cold dormancy below 45 degrees Fahrenheit to flower reliably. Gardeners may need to select cultivars known to perform well in warmer zones or consider pre-chilling bulbs in a refrigerator for 6 to 8 weeks before planting to simulate the necessary cold period.
The high volume of rainfall from November through April necessitates careful attention to soil composition and drainage. Bulbs, especially in dormancy, are susceptible to rotting in consistently saturated soil. To prevent this, planting sites must have well-draining soil, sometimes requiring the addition of organic matter or sand to improve porosity. Planting depth is also important; planting spring bulbs deeper, at two to three times the bulb’s height, helps protect them from temperature fluctuations.