Planting summer bulbs at the correct time is fundamental to a successful season of blooms, as improper timing can lead to rot. Summer bulbs, which include corms, tubers, and rhizomes, are planted in the spring and typically flower from mid-summer until the first fall frost. Common examples include the towering flower spikes of gladiolus and the complex, layered blooms of dahlias. Proper spring planting provides the precise conditions needed for root development before the heat of summer arrives.
The Crucial Role of Soil Temperature and Frost
The primary indicator for planting summer bulbs is the state of the soil, specifically its temperature and the absence of freezing conditions. These bulbs originate from warmer climates and lack the hardiness to survive a hard frost once they sprout. Planting before the last expected frost date means emerging shoots risk being killed by a late cold snap.
Soil temperature must be consistently warm enough to encourage root growth without causing the bulb to decay. The ideal range for most summer bulbs is when the soil temperature reaches a consistent 55°F to 60°F, measured a few inches deep. Planting into cold, waterlogged earth is detrimental because the dormant bulb cannot draw up moisture effectively and becomes vulnerable to fungal pathogens. Wet conditions encourage rot, turning the bulb into a soft, unusable mass. Waiting for the soil to warm up ensures the bulb is immediately stimulated to grow roots in a safe, hospitable environment.
Timing Variations for Tender and Hardy Summer Bulbs
While the general rule is to wait for the soil to warm, the exact planting time varies based on the specific bulb’s hardiness level. Summer bulbs are typically categorized into two main groups: tender and hardy. Tender summer bulbs cannot withstand cold temperatures in the soil and require the warmest conditions before planting outdoors.
This group includes popular varieties such as dahlias, begonias, and caladiums. For these, planting must be delayed until well after the last frost date has passed and the soil is firmly within the 60°F to 70°F range. Gardeners often start these sensitive tubers indoors in pots four to six weeks earlier to give them a head start, transplanting the established plants outside only when the weather is reliably warm.
In contrast, hardy summer bulbs possess greater tolerance for cooler spring soil and can be planted earlier than their tender counterparts. This category includes bulbs like certain lilies, alliums, and crocosmia. These varieties can typically be placed in the ground as soon as the soil is workable in early to mid-spring, often a month or more ahead of the tender varieties. Their ability to handle slightly lower temperatures allows for a much earlier start to the season.
Staggered Planting for Continuous Summer Blooms
Once the optimal planting window has opened for a specific bulb type, a technique known as staggered planting can be used to extend the overall flowering period. This method is particularly effective for bulbs that bloom rapidly after planting but only flower once, such as the gladiolus. Instead of planting the entire supply of corms at the same time, the gardener divides them into smaller batches.
A common practice is to plant a new batch of gladiolus corms every ten to fourteen days, starting with the first group immediately after the last frost has passed. Since gladiolus generally take about 90 days from planting to bloom, this successive planting ensures that as the flowers from the first batch begin to fade, the next group is just reaching maturity. Continuing this process until mid-summer provides a steady, uninterrupted supply of fresh blooms until the first frost of autumn.