How to Grow Delphiniums in Australia

Delphinium (Delphinium elatum hybrids and D. grandiflorum, family Ranunculaceae) is a short-lived perennial grown for spires of blue, purple and white flowers. It flowers best where summers stay mild, as in Hobart and Canberra. Many spikes are lost to late staking: the stems are hollow, and one gusty spring westerly can fold them in half.

Three mistakes that matter

1. Staking after the spikes have grown. Tall spikes heavy with flowers snap in wind and rain, and a stake pushed in later damages the roots. Put the stake in at planting time and tie each spike loosely as it lengthens.
2. Planting in wet soil or too deep. Crown rot is almost certain in poorly drained soil or when the crown is buried. Set the crown level with the soil surface, and raise the bed wherever water lies after rain.
3. Expecting tall types to sail through a humid summer. Delphiniums prefer cool summers and struggle through long runs of warm, humid nights. In warm districts, grow them as winter to spring annuals and lean on 'Blue Butterfly', which handles heat better than the tall hybrids.

When to plant

Delphinium seed germinates best at 18 to 24°C and slows or fails once trays run above about 26°C. Sow as the summer heat breaks in autumn, or in late winter or spring where the calendar allows; one sowing a year is enough.

At the warm edge of a window, germinate trays indoors in the coolest bright spot. At the cold edge, a seedling heat mat holds trays near 18°C.

Site, soil and setup

A clump stays in place for several years, so there is no yearly rotation. When a plant rots or fades, set its replacement in fresh soil a little away from the old spot. For soggy ground, see what to do when soil will not drain.

Container growing

The dwarf 'Blue Butterfly' and the branching 'Connecticut Yankees' are the best choices for pots. Give one 'Blue Butterfly' a pot at least 20 cm wide and 20 cm deep, about 6 L. A plant of 'Connecticut Yankees' reaches 80 cm and needs a larger pot. Fill it with premium potting mix, and check for drainage holes.

• Watering. Pots dry out fast in warm weather, so water whenever the top of the mix feels dry, and never leave a pot standing in water. See watering pots in summer.
• Feeding. Water in a fish and seaweed blend at 25 mL per 9 L every fortnight from planting until flowering ends.
• Position. Full sun in cool districts; in hot districts, move the pot into afternoon shade on hot days.

Sowing Seeds

Start delphiniums in trays or punnets and plant them out, because seedlings are easier to keep cool, moist and safe from snails under cover.

Preparation

Wear gloves to handle the seed (see the caution under Harvesting Blooms). Chilling the seed first (cold stratification, a spell of cold, moist conditions that breaks dormancy) improves germination. Mix the seed with a pinch of damp seed raising mix in a lidded tub, and keep it in the fridge for 2 weeks. The packet asks for at least 4 days. Fresh seed germinates best, so keep unused seed sealed in the fridge. More on cold stratification.

Starting under cover

• Fill a clean tray or punnets with fresh seed raising mix, sow the seed 4 to 5 mm deep and firm gently.
• Keep trays in the temperature range under When to plant, in a light, airy place.
• Water only enough to stop the surface drying out, because soggy mix rots seed and seedlings.
• Newspaper laid over the tray holds in moisture. Lift it at the first sign of shoots, since a day or two too long in warm weather gives pale, stretched seedlings.
• Seedlings usually emerge in 10 to 30 days.
• Prick out each seedling into its own small pot once its first true leaves appear.

Sowing direct

In mild spells, seed can also go straight into a prepared bed. Cover it lightly with seed raising mix, lay cardboard over the spot until shoots appear, and watch closely for snails.

Transplanting

Plant out once seedlings are 5 to 10 cm tall, usually 4 to 6 weeks after sowing. Harden them off first (a week or so of gradually longer spells outdoors that toughens soft growth), as in hardening off seedlings. Height decides the spacing and the support.

Water each plant in with seaweed solution at 30 mL per 9 L. Then mulch with pea straw or sugar cane, kept clear of the crown.

Care by growth stage

Steady moisture, regular feeding and early support decide how tall the spikes grow and how long they last. Weeks count from a spring sowing; autumn sowings grow slowly over winter and flower from late winter into spring.

Use soft twine for the ties so it does not bruise the stems. A whole bed can instead grow up through wide netting held flat on posts above the young plants. If growth stalls in poor soil, a light dose of all-purpose controlled release granular (NPK around 14-5-10), scratched into the soil in spring, can help.

Cutting Back

Prompt cutting keeps clumps tidy and, where summer nights stay cool, often brings a second, smaller flush of spikes in autumn.

• Pinch out the growing tip of young plants, if you like, for bushier growth with more, shorter spikes.
• From the second spring, thin each clump to its strongest three or four shoots, so the spikes grow larger and air moves through.
• As the main spike fades, cut it back to just above the side shoots, which flower next.
• Once every spike has finished, cut each stem to 5 cm above the ground.
• In winter, clear away dead growth, and divide crowded clumps in late winter or early spring. See dividing perennials.

Companion planting

Delphiniums sit well in a mixed cottage border, as long as neighbours leave them sun and air.

Grows well with

• Other herbaceous perennials such as salvia, penstemon and shasta daisy, which die back and are cut back on the same timetable.
• Low annuals at the front of the bed, which shade the soil and keep the roots cool without crowding the crown.

Keep apart

• Vigorous spreading plants, because delphiniums do not like competing for space.
• Plants that need frequent sprinkler watering, which keeps leaves wet and invites fungal leaf diseases.
• Vegetable beds and children's play areas, since every part of the plant is poisonous.

Harvesting Blooms

Cut a spike for the vase when the lowest one or two florets have opened; the buds above keep opening after cutting.

• Pick in the cool of early morning or evening.
• Stand each stem in water as soon as it is cut.
• Strip any leaves that would sit below the waterline, because they rot in the vase.
• Leave the bucket somewhere cool for a few hours or overnight before arranging.

Cut spikes usually last about 6 to 8 days in a vase. Ripening fruit gives off ethylene, a gas that makes delphinium florets drop, so keep the vase away from the fruit bowl.

Grow them for the vase, the back of a cottage border, and the bees and butterflies that visit the flowers.

Common problems

Seeds did not come up. Trays kept too warm (see When to plant), a mix kept too wet, or old seed stored warm are the usual causes. Read more

Pale, stretched seedlings. Seedlings left under a cover after they emerge, or raised in poor light, stretch and seldom recover. Read more

Yellow, stunted plants. Slow growth with yellowing leaves usually means the plant is hungry; follow the feeds in Care by growth stage.

Plants fading after two or three years. Most garden delphiniums are short-lived, so raise a few replacements from seed before the old clumps give out.

Plants collapsing at soil level. A soft, dark crown points to crown rot (see Three mistakes that matter).

Pests and diseases

Snails and slugs are the main pests, and powdery mildew is the disease to watch for.

Common garden snail (Cornu aspersum). Snails and slugs chew seedlings and new shoots at night, leaving slime trails; collect them by torchlight or set traps. Read more

Aphids (Aphididae). These small sap-sucking insects cluster on shoot tips, leaves and flower spikes; hose them off or spray insecticidal soap. Read more

Thrips (Thysanoptera). These tiny insects leave white flecks on leaves and can spread tomato spotted wilt virus. Spray insecticidal soap, and keep weeds down, since weeds can harbour both. Read more

Powdery mildew (Erysiphe aquilegiae). White spots spread across the leaves; remove affected leaves and spray 1 part full-cream milk to 10 parts water. Try it on a few leaves 24 hours before spraying the lot, and spray again after heavy rain. Read more

Damping off (Pythium species and Rhizoctonia solani). These soil-borne pathogens rot seedlings at the base, so they collapse in the tray. Read more

Prevention first

• Raise seedlings in fresh seed raising mix in clean trays.
• Space plants as in Sowing Seeds and water at soil level early in the day.
• Keep beds clear of weeds and debris where snails hide by day.
• Give plants full sun where the climate allows, since plants in sun resist powdery mildew better.

Tips

1. Plant tall types a little way out from a solid wall or fence rather than hard against it, where gusts thrash the spikes.
2. Grow 'Galahad' behind 'Casablanca' for a white border that flowers at two heights.
3. Mark each clump with a peg before it dies back in winter, so nobody digs over the resting crown.
4. Label trays and punnets of seedlings as poisonous, so they are never mistaken for edible seedlings.