How to Grow Rudbeckia in Australia

Rudbeckia, the black-eyed Susan or gloriosa daisy (Rudbeckia hirta), is a member of the daisy family, Asteraceae. It is grown for bold summer and autumn cut flowers in yellow, orange, bronze and mahogany. Botanically it is a biennial or short-lived perennial, but it flowers in its first year from seed, so it is usually grown as an annual. It shrugs off heat once settled, and the usual mistake is burying the small seed, which needs light to germinate.

Three mistakes that matter

1. Burying the seed. Light helps rudbeckia seed germinate, so seed covered like a vegetable often never comes up. Press it onto the surface and barely cover it, no deeper than 1 mm, as set out in Sowing Seeds.
2. Rich soil and heavy feeding. Moderate fertility gives the best show of flowers, while extra nitrogen from manure or strong feeds pushes leaves at the expense of blooms. Keep feeding light and follow Care by growth stage.
3. Soggy ground. Rudbeckia tolerates most soils except poorly drained, wet ones, and plants in clay that stays sodden after rain often fail. Choose a free-draining or raised bed, and water only once the top 5 cm has dried.

When to plant

Sow once the soil holds 20 to 22°C, and keep young plants away from frost, which they do not tolerate. Day length decides flowering: plants flower freely once days reach about 14 hours, so long southern summers suit them best. One sowing per window is usually enough, because deadheaded plants keep flowering from summer into autumn.

In frosty gardens, Melbourne Cup weekend is a safe marker for planting out tray-raised seedlings. Finish autumn sowings by about Anzac Day, so seedlings are settled before the cold arrives.

Site, soil and setup

Give rudbeckia full sun, or some afternoon shade in hot northern and inland gardens. It grows in clay, loam or sand across a wide pH range, as long as water drains away.

Rotate cut-flower beds so rudbeckia does not follow itself or other daisy-family flowers, which can share powdery mildew.

Container growing

'Autumn Colours', at about 55 cm, suits pots better than the taller gloriosa mix, which soon outgrows small containers. Use a pot at least 30 cm across, holding about 13.5 L, with drainage holes and fresh potting mix. The gloriosa mix needs a 40 cm pot of about 28 L. Bagged mix can carry Legionella bacteria. Put on a mask and gloves before opening it, dampen it to keep the dust down, and wash your hands afterwards.

• Watering: once the mix is dry 5 cm down, soak it until water runs from the base.
• Feeding: from the first buds, water on fish and seaweed blend at 25 mL per 9 L each fortnight.
• Position: full sun, or a spot with afternoon shade in hot northern and inland gardens.

Sowing Seeds

Raise rudbeckia in trays for the most control over its small seed, or sow it straight into a fine, weed-free seedbed once frosts are over.

Starting in trays

• Sow 5 to 7 weeks before planting out, into punnets or cells of fresh seed raising mix.
• Press the seed onto the surface and barely cover it, no deeper than 1 mm. The sand trick helps spread it thinly.
• Water from below or with a fine mist, so the seed is not washed under.
• Expect seedlings in about 7 to 14 days in a warm spot.
• Sow 'Autumn Colours' one seed per cell, so no seedling is lost to thinning.
• Plant out at 5 to 10 cm tall, 30 to 45 cm apart. First harden them off (a week or two of gradually longer time outdoors).

Sowing direct

• Rake the bed fine and level, and scatter the seed thinly over the surface.
• Press it in with the back of the rake or a board, covering it by no more than 1 mm.
• Water with a fine rose and keep the surface damp until seedlings show.
• Thin to 30 to 45 cm apart once the first true leaves open.

Both varieties are sown and grown the same way, but they differ in height and in how they are best used.

Care by growth stage

Rudbeckia lives mostly on the compost in its bed. Steady water while young and regular cutting decide how long it flowers, and established plants cope with heat and dry spells.

If growth stalls in thin, hungry soil, scratch in all-purpose controlled release granules (NPK around 14-5-10) at the label rate.

Deadheading

Each flower left to ripen seed slows the next flush, so regular deadheading keeps rudbeckia blooming from summer into autumn.

• Every few days: cut faded flowers just above a set of leaves with sharp snips.
• Picking for the vase: counts as deadheading, so cut often.
• Late autumn: leave a few seed heads if you want self-sown seedlings, or remove them all if you do not.
• End of flowering: pull spent plants, or in mild gardens cut them back to just above the ground and wait for regrowth.
• Spring, on plants that return: trim off the dead leaves once new ones appear.

Companion planting

Rudbeckia fits a sunny cutting bed with flowers that share its sowing months and lean soil.

Grows well with

• Zinnias: sown in the same warm months at the same 30 to 45 cm spacing, for mixed summer vases.
• Marigolds: French types stay low enough to edge the front of a rudbeckia bed.
• Celosia: another full-sun cut flower for drying beside rudbeckia.
• Vegetable beds: the flowers feed pollinators and draw beneficial insects into the garden.

Keep apart

• Sunflowers on the sunny side: cut-flower types reach 1.2 to 2.5 m and shade rudbeckia for much of the day.
• Lawn edges: lawn fertiliser adds nitrogen, which gives leafy plants with fewer flowers.

Harvesting Blooms

Cut stems for the vase when the flowers have opened but before they are completely open.

• Take the longest stem you can, snipping just above a set of leaves.
• Strip any leaves that would sit under water, and stand stems in a clean bucket of water straight away.
• For drying, leave the flowers on the plant until completely open.
• Wear gloves for a big picking, because the leaves and stems carry short, stiff hairs.

To dry flowers, hang small bunches upside down in a dry, dark, airy room for a couple of weeks. Keep them away from the damp air of a kitchen, laundry or bathroom. The petals shrink and fade a little as they dry. Use fresh stems in summer vases and the dried ones in autumn and winter arrangements.

Common problems

Bare trays or rows. Seed buried too deep, a surface left to dry out, or cold soil usually explains it. The first of the Three mistakes that matter gives the fix. Read more

Seedlings topple overnight. Damping off (a stem rot of young seedlings, mostly from Pythium species and Rhizoctonia solani) is worst where mix or soil stays wet. Read more

Leafy rosettes but no flower stems. Days shorter than about 14 hours delay flowering, and the longest days in Darwin reach only about 12.5 hours. Autumn sowings in the north often wait for spring, and tropical plants often flower more lightly.

Short stems late in the season. As autumn days shorten, plants keep flowering but on shorter stems with smaller blooms, which is normal.

Pests and diseases

Serious trouble is uncommon: snails go for young plants, and mildew for crowded ones.

Garden snail (Cornu aspersum) and grey field slug (Deroceras reticulatum). Both feed at night and chew seedlings and soft leaves, leaving silvery trails. Go out with a torch about two hours after sunset, especially after rain, and collect them. Read more

Aphids (family Aphididae). These small sap-suckers crowd new shoots and leaf undersides and leave sticky honeydew. Knock them off with a strong jet of water, or spray insecticidal soap onto the undersides. Read more

Powdery mildew (order Erysiphales). Grey-white, powdery patches spread over the leaves in warm, humid weather, most often on crowded plants. Remove badly affected leaves and spray 1 part full-cream milk to 10 parts water. Read more

Prevention first

• Keep the full spacing from Sowing Seeds, so leaves dry quickly after rain.
• Water at the roots early in the day, leaving the foliage dry.
• Start every tray sowing in fresh mix and a scrubbed tray.
• Check new shoots and seedlings once a week, and after rain for snails.

Tips

1. Check the botanical name on the packet. The black-eyed Susan vine (Thunbergia alata) is an unrelated climber, naturalised along the Queensland and New South Wales coast.
2. Grow 'Autumn Colours' at the front of a bed and the gloriosa mix behind it, so every flower gets full sun.
3. Where self-sown seedlings appear in spring, lift the best with a trowel and move them to where you want them.