How to Grow Lobelia in Australia

Lobelia (Lobelia erinus) belongs to the bellflower family, Campanulaceae, and is grown for masses of tiny flowers along bed edges and spilling from baskets. It comes from southern Africa and flowers best in mild weather, so in warm parts of Australia it earns its place through winter and spring. The seed is as fine as dust and needs light to sprout, so the usual mistake is burying it.

Three mistakes that matter

1. Covering the seed. Lobelia seed germinates better in light, and even a thin blanket of mix can leave a tray bare after three weeks. Press it onto the surface of damp mix and leave it uncovered, as set out in Sowing Seeds.
2. Letting pots and baskets dry out. Lobelia has little tolerance of drought, and a basket that dries through on a hot, windy day can wilt, stop flowering or die back. Check pots daily in warm weather and water before the mix dries out.
3. Planning a summer show in a hot garden. Lobelia stops flowering in heat, and plants sitting in wet soil through summer often rot. In warm zones, time sowings for winter and spring flowers, following When to plant.

When to plant

Seed comes up in 14 to 21 days in a warm spot, ideally 21 to 26°C. Autumn is the main sowing time in every zone, so plants flower through the mild months ahead. Southern gardens can sow again under cover in late winter for spring and summer flowers.

Early in an autumn window, keep trays in bright shade until the heat breaks. Young plants cannot take frost, so plant spring trays out only after the last frost.

Site, soil and setup

Give lobelia at least 3 hours of direct sun a day. Full sun suits it in cool and southern gardens, while hot inland and northern gardens should give it morning sun and afternoon shade. It needs soil rich in organic matter that holds moisture yet drains freely, and it grows in acid, neutral or alkaline soil.

Damping off fungi survive on the old roots and leaves of earlier crops. Start every tray in new mix, and plant out away from any spot where seedlings collapsed the year before.

Container growing

Both varieties suit pots, window boxes and hanging baskets. Give a group at least 10 L of potting mix; a 30 cm pot holds about 13.5 L and needs drainage holes. Legionella bacteria can multiply in bagged mix, so dampen it before opening the bag, wear a mask and gloves, and wash your hands afterwards.

• Watering: whenever the top of the mix feels dry, soak it until water runs from the base.
• Feeding: potted plants run short of food sooner. From the first flowers, water on fish and seaweed blend at 25 mL per 9 L every fortnight.
• Position: morning sun and afternoon shade in hot zones, full sun in cool ones, and shelter from drying wind.

Sowing Seeds

Start lobelia in trays, because the seed is too fine to sow straight into a bed and seedlings are easier to keep damp under cover.

Starting in trays

• Fill a punnet or tray with fresh seed raising mix, firm it lightly and water it well.
• Mix the seed with a pinch of dry sand, sprinkle it thinly over the surface and press it in without covering it. Read more
• Cover the tray with a clear lid or plastic, keep it warm and lift the cover once shoots appear.
• Keep the mix lightly moist, never soggy: stand the tray in shallow water until the surface darkens, then lift it out to drain.
• Expect shoots in 14 to 21 days.

Transplanting

• Once seedlings are large enough to handle, prick out little clumps 5 cm apart, or one clump to a cell. Each clump grows on as one plant.
• Harden off the clumps (give them a little more time outdoors each day over about a week, until they cope with sun and wind).
• Plant out 4 to 8 weeks after sowing, once clumps are 5 to 10 cm tall and the last frost has passed.
• Space 'Emperor William' 10 cm apart and 'Cascade Mixed' 20 cm apart.
• Settle each clump in with half-strength seaweed solution, 15 mL per 9 L.

Both varieties are grown the same way; their habit decides where each goes.

Care by growth stage

Steady moisture decides how long lobelia flowers. It has little tolerance of drought, yet wet soil in summer heat rots the roots. Water when the surface starts to dry, and feed lightly.

If growth stalls in poor soil, scratch a light dose of all-purpose controlled release granules (NPK around 14-5-10) into the soil around the plants.

Deadheading

Annual lobelia needs no fiddly picking of single faded flowers. A trim after each flush, or once heat stalls the plants, brings on fresh growth and new flowers.

• After each flush: cut stems back to just above a set of lower leaves.
• In a hot spell: trim plants that have stopped flowering and keep them watered; they often flower again once the weather cools.
• After trimming: water on seaweed solution at 30 mL per 9 L to help the plants recover.
• End of the season: pull out tired plants, compost them and sow fresh trays for the next window.

Companion planting

Lobelia belongs at the front of a bed, so its best partners grow taller behind it or share its basket.

Grows well with

• Petunias: in a mixed hanging basket, petunias fill the centre while lobelia spills over the edge.
• Violas: another cool-season flower, sown in the same autumn window and happy in part shade.
• Sweet alyssum: a second low edging plant for the front of a bed or the rim of a basket.
• Vegie patch edges: the flowers draw butterflies and other pollinators into the garden.

Keep apart

• Lavender: it wants dry, lean soil, while lobelia needs steady moisture.
• Zucchini: its leaves spread a metre or more and smother low edging plants.

Harvesting Blooms

Flowers open about 13 to 14 weeks after sowing, and short sprigs suit tiny posies once most flowers on a stem are open.

• Cut in the cool of the morning, when stems are full of water.
• Take short sprigs from the fullest part of the plant, just above a leaf.
• Stand the stems in water straight away.
• Wash your hands once you finish.

Sprigs wilt quickly out of water, so keep them in a cool room and top up the glass each day. Lobelia is ornamental only: every part is poisonous if eaten. Keep posies and pots out of reach of children and pets.

Common problems

Bare tray after three weeks. Seed covered with mix or a surface left to dry is the usual cause, as the first of the Three mistakes that matter explains. Read more

Seedlings stretched and floppy. Too little light after they come up makes them race upwards, so move the tray to the brightest spot as soon as shoots show. Read more

Flowering stops in summer. Heat is the cause, not a fault in the plant; trim and keep watering as set out in Deadheading.

A basket wilts an hour after watering. Mix that has dried right out sheds water down the sides, so stand the basket in a bucket of water until the bubbling stops. Read more

Pests and diseases

Serious trouble is uncommon, and most of it strikes seedlings or plants stressed by heat.

Garden snail (Cornu aspersum) and grey field slug (Deroceras reticulatum). They graze seedlings and soft shoots at night, most of all after rain, and leave silvery trails. Hunt them by torchlight about two hours after sunset, and set beer traps nearby. Read more

Two-spotted mite (Tetranychus urticae). Leaves turn pale and finely stippled, and numbers build fast in hot, dry weather. Hose the undersides of the leaves and keep plants well watered. Read more

Damping off (Pythium species and Rhizoctonia solani). Seedlings pinch at soil level and topple, worst in mix that stays wet. Remove the fallen seedlings and let the surface dry slightly between waterings. Read more

Prevention first

• Sow in new seed raising mix and a scrubbed tray.
• Water trays from below, and beds at the roots in the morning.
• Clear away the damp daytime shelter where snails hide.
• Check leaf undersides once a week in hot, dry spells.

Tips

1. In cool and alpine gardens, Melbourne Cup weekend in early November is the traditional marker for planting out frost-tender flowers such as lobelia.
2. Keep a punnet of spare clumps growing on, ready to fill gaps in an edging.
3. In a lined wire basket, push 'Cascade Mixed' clumps through the sides as well as planting the top, so the flowers cover it all round.