The delphinium, often known as larkspur, is a striking herbaceous perennial prized for its soaring, dramatically colorful flower spikes. These plants add vertical drama and a classic cottage garden feel to any landscape. Successful establishment and magnificent blooms depend entirely on planting them at the correct time, whether using nursery stock or starting from seed.
Planting Established Starts and Bare Roots
Planting established starts or dormant bare roots is the most straightforward way to introduce delphiniums, and the ideal time is during the cool seasons.
Early spring is the preferred window, specifically after the last threat of a hard freeze has passed, but while the soil temperatures remain cool. Planting at this time allows the root system to establish fully before the stress of summer heat arrives, leading to the best chance for first-year flowers.
A secondary, equally effective planting window opens in early autumn, typically six to eight weeks before the ground is expected to freeze solid. This fall planting provides the roots with a long period of cool, moist soil to grow without the plant expending energy on flowering. In regions with mild winters and intensely hot summers (zones 8 and 9), planting in late fall or very late winter is often favored to bypass heat shock.
Timing for Starting Delphinium Seeds
Propagating delphiniums from seed follows a longer timeline because the seeds require cold stratification to break dormancy and trigger germination. Stratification mimics a winter environment and involves placing the seeds in a moist medium in a refrigerator for one to three weeks before sowing.
For spring planting, seeds should be started indoors approximately eight to ten weeks before the last expected frost date. This lead time allows the seedlings to develop into robust young plants ready for hardening off and transplanting once conditions warm up.
Alternatively, seeds can be sown directly outdoors in late winter or very early spring when frosts are still occurring, which naturally provides the necessary cold treatment. Direct sowing done in late spring or early summer will generally not bloom until the following year.
Essential Site Requirements for Success
Proper site selection is important for delphinium longevity, regardless of the planting method chosen. These plants thrive in a location that receives full sun, defined as six to eight hours of direct light daily. In hotter climates, providing protection from the intense afternoon sun is recommended to prevent scorching.
Delphiniums require a rich, deeply worked soil that has been amended with organic matter like compost or aged manure. The planting site must offer excellent drainage, as waterlogged soil leads to crown and root rot, which is the primary cause of plant failure. Delphiniums prefer a soil pH that is neutral to slightly alkaline, ideally ranging between 6.5 and 7.5.
Immediate Post-Planting Establishment Care
Once the delphinium start or bare root is situated, focused care ensures a smooth transition and establishment. First, provide a deep, thorough watering immediately after planting to settle the soil and eliminate air pockets around the roots. This initial drenching is more beneficial than frequent, shallow watering.
Applying a two-inch layer of organic mulch around the base of the newly planted crown helps keep the shallow root zone cool and consistently moist during the initial growth phase.
Finally, because the stems are hollow and the flower spikes become top-heavy, setting up support is necessary early on. Staking should be done when the plants are young, often when they reach about 12 inches in height, to prevent strong winds or heavy rain from snapping the stalks.