The genus Delphinium, commonly known as Larkspur, is a highly prized perennial in garden design due to its dramatic, vertically oriented flowers. These plants are instantly recognizable for their towering, candle-like spires, which bring structure and color to mixed borders and cottage gardens. With hundreds of species and cultivars in shades ranging from true blues to purples, pinks, and whites, the Delphinium anchors the summer garden. Understanding the plant’s bloom schedule is important for gardeners planning their seasonal displays.
The Primary Delphinium Bloom Cycle
The main flowering period for Delphinium occurs as the weather transitions from late spring into summer. In most temperate climate zones, this initial flush of blooms begins in June and continues through July. This timing depends on the plant successfully completing its spring growth phase, which culminates in the production of a single, central flower spike.
This initial display features the tallest flower spikes, which can reach six feet or more depending on the cultivar. The florets open sequentially, starting at the base of the spire and progressing upward. Under ideal, cool summer conditions, this primary bloom can last for five to six weeks, providing sustained color.
Once the spike is spent, the plant shifts its energy toward seed production, marking the end of the initial cycle. This is often signaled by the lower foliage beginning to yellow or brown. The bloom’s duration is susceptible to heat, as high summer temperatures cause the flowers to fade more rapidly.
Factors That Determine Exact Bloom Timing
Several environmental and genetic variables determine when a Delphinium begins flowering. The accumulation of spring warmth, known as growing degree days, dictates the start of the bloom. Delphiniums thrive in cool summer conditions; intense heat may shorten or delay the initial bloom as the plant struggles with stress.
The specific variety planted also affects bloom time. Tall varieties, such as the Pacific Hybrid and Elatum groups, follow the mid-June to July schedule. However, types like the Belladonna group are more branching and may initiate flowering earlier, sometimes extending their bloom period into the autumn.
Plant Age and Light Requirements
Established plants that have overwintered bloom more vigorously and reliably than first-year plantings. Plants grown from seed require time to develop a robust root system and may produce smaller, later spikes in their first season. Delphiniums require a minimum of six hours of direct sunlight daily to develop strong flower spikes; insufficient light delays flowering and results in weak stems.
Encouraging a Second Flush of Blooms
Gardeners can encourage a second wave of flowers later in the season. This process begins immediately after the primary bloom fades, typically in mid-to-late July, and involves cutting back the spent flower stalks to prevent the plant from setting seed. Once the first flower spike has finished, the entire spent stem should be cut back close to the ground, leaving only a few inches of stem and the basal foliage intact. This action simulates a reset, forcing the plant to produce new vegetative growth from the crown.
Providing the plant with a deep watering and a dose of potassium-rich fertilizer encourages the development of secondary flower spikes. With proper care, these new spikes will emerge, leading to a second, smaller flush of blooms that occurs in late summer or early autumn, often around late August or September.