Larkspur is a hardy annual that sends up slim spikes of spurred flowers in blue, violet, pink, lilac and white. It gives you the look of delphinium in a single season, with far less fuss. It is one of the best cut flowers for fresh bunches and for drying, and it self-sows gently in a cottage border. It is easy to grow if you sow it at the right time: into cool soil, not warm.
Sowing and planting
Larkspur germinates in cool soil and struggles in warm soil. It also grows a taproot (a single, deep main root) and dislikes being moved. Direct sowing is the easiest method.
Fall sowing. In zones 7 to 10, sow outdoors 6 to 8 weeks before your first frost, or once the soil has cooled in autumn. Seedlings form low rosettes, overwinter, and flower earlier and taller than spring-sown plants. In zones 5 and 6, sow in late autumn just before the ground freezes. The seed then sits dormant and germinates in early spring.
Spring sowing. In colder zones, sow as soon as you can work the soil in early spring, 2 to 4 weeks before your last frost. Light frost does not harm seedlings. Sowing late, into warm soil, gives poor germination and short spikes.
How to sow. Rake the soil to a fine surface. Sow thinly and cover the seed with 1/8 to 1/4 inch (3 to 6 mm) of soil, since darkness helps germination. Keep the soil evenly moist until seedlings appear. Thin them to 8 to 12 inches (20 to 30 cm) apart once they have two true leaves (the leaves after the first seed leaves).
Chilling the seed. Larkspur seed germinates better after a cold spell. For spring sowing, seal the seed in a bag in the refrigerator for 1 to 2 weeks first. Fall-sown seed is chilled naturally in the soil.
Starting indoors. It is possible but tricky. Sow into deep cells or biodegradable pots 6 to 8 weeks before your last frost, and keep them in a cool room around 55°F (13°C). Harden off the seedlings over a week by getting them used to outdoor conditions, then plant out without disturbing the roots.
Succession sowing. A second sowing 2 to 3 weeks after the first in spring stretches the flowering season. It rarely pays to sow once the weather turns warm.
Growing on
Watering. Water seedlings during dry spells so the soil stays evenly moist. Established plants cope with short dry periods. Water at the base of the plant to keep leaves dry and reduce mildew.
Feeding. Larkspur does not need rich soil. Dig in a little compost before sowing. Too much nitrogen gives soft, floppy stems and fewer flowers.
Mulching. Once seedlings are a few inches tall, spread a thin mulch of compost or shredded leaves. It keeps roots cool, which larkspur likes. Keep it away from the stems.
Staking. Tall varieties bend in wind and rain. Stretch horticultural netting horizontally about 12 to 18 inches (30 to 45 cm) above the soil while plants are young, and let them grow through it. In a border, twiggy sticks or a ring of stakes and twine also work.
Pinching. You do not need to pinch larkspur. The main spike opens first, then side spikes follow.
Deadheading. Cut off faded spikes above a side shoot to encourage more flowers. Leave some spikes to set seed if you want plants to self-sow.
Companions. Larkspur suits cottage plantings with poppies, cornflowers, nigella and sweet peas. It likes the same cool start.
Pollinator value. Bumblebees and other long-tongued bees work the spurred flowers, and hummingbirds visit too.
Harvesting
Fresh cutting. Cut spikes when one-third to half of the florets are open. Cut early in the morning into a bucket of cool water. Strip leaves that would sit below the waterline. Fresh stems usually last a week or more in a vase.
Drying. Larkspur is one of the easiest flowers to dry. Cut spikes when most florets are open but before the bottom ones fade. Tie small bunches and hang them upside down somewhere dark, dry and airy for 2 to 3 weeks. The colour holds well.
Saving seed. Let a few spikes turn brown and dry on the plant. The slim pods split open when ripe. Cut the spikes before they shatter and shake the seed into a paper bag. Store it in a labelled envelope somewhere cool and dry, out of reach of children and pets. Mixed plantings cross, so colours in the next generation may vary.
Wear gloves when you handle the plants, pods and seed, and wash your hands afterwards.
All parts of larkspur are poisonous if eaten, especially the seeds and young plants. Keep children, pets and livestock away, never grow it near food crops you might confuse it with, and wear gloves when cutting or saving seed.
Problems and pests
White, powdery coating on leaves
This is powdery mildew, a fungal disease. It shows up late in the season or in crowded, humid plantings. Thin plants for better airflow and water at the base. Remove badly affected leaves. Late in the season, simply pull spent plants.
Seedlings fail to come up
Usually warm soil, or seed sown too shallow in the light. Sow into cool soil in fall or early spring, cover the seed fully, and chill spring-sown seed first. Old seed also loses vigour, so use fresh seed where you can.
Plants flower early on short, thin spikes
Warm weather or a late sowing. Larkspur rushes to flower once heat arrives. Sow earlier, favour fall sowing where your winters allow, and give plants light shade in hot areas.
Sticky shoots and curled tips
Aphids feed on young growth and flower buds. Wash them off with a jet of water, and let ladybugs and other predators do their work. For heavy infestations, use insecticidal soap, following the label and avoiding hot sun.
Stems collapse at soil level
Crown rot from waterlogged soil. Grow larkspur in well-drained ground, avoid overwatering, and keep mulch clear of the stems. Remove affected plants.
Varieties worth growing
- 'Giant Imperial' series: tall, well-branched plants in mixed and single colours; a long-standing cut-flower choice.
- 'QIS' series: tall, uniform spikes in separate colours, bred for cut-flower growers.
- 'Frosted Skies': pale blue-white semi-double flowers with a darker blue edge.
- 'Earl Grey': unusual smoky silver-mauve flowers; popular with florists.
- 'Misty Lavender': soft lavender-pink flowers on tall stems.