How to Grow Zinnias in Australia

Zinnia (Zinnia elegans) is a warm-season annual in the daisy family, Asteraceae, grown for garden colour, pots and cut flowers. It revels in Australian summer heat and can flower for months from one sowing. The usual mistake is to stop picking, because a plant left to set seed soon stops making flowers.

Three mistakes that matter

1. Sowing into cold soil. Zinnias are frost tender, and seed sitting in cold, wet soil comes up slowly or rots before it shoots. Wait for the soil temperature in When to plant, or start seed in deep cells under cover.
2. Crowding plants and wetting the leaves. Powdery mildew spreads fastest through crowded plants with damp leaves and still air. Give each plant the full spacing in Sowing Seeds, and water the soil early in the day so leaves are dry by nightfall.
3. Leaving spent flowers on the plant. A zinnia that sets seed has done its job, and flowering often tails off by late summer. Cut for the vase and remove faded heads every few days, as set out in Deadheading.

When to plant

Sow once the frosts are over and the soil holds at least 21°C. Seed germinates best between 22 and 25°C. For a longer run of flowers, sow a second batch about two weeks after the first.

Where spring is slow to warm, start seed in cells under cover about four weeks before the last frost. Late sowings keep flowering until the cold nights of autumn stop them.

Site, soil and setup

Choose a spot with at least six hours of direct sun a day, sheltered from strong wind. Zinnias need free-draining soil with compost through it, and do best at a pH of about 6.3 to 6.8.

Where you can, grow zinnias in a different bed each year. The leaf spot fungus described in Pests and diseases can survive in old zinnia debris for up to two years.

Container growing

Compact types such as 'Thumbelina Dwarf Mix', 'Formula Mix' and 'Envy' suit pots, and 'Dream' grows well in a large tub. A dwarf type needs a pot at least 20 cm wide and deep, about 6 L of fresh potting mix. Tall types need at least 40 L and a pot 40 cm across.

• Watering: pots dry out quickly in summer heat, so check them daily and water whenever the surface feels dry.
• Feeding: water in fish and seaweed blend at 25 mL per 9 L every fortnight. From the first flowers, switch to liquid potash at the label rate, fortnightly.
• Position: a hot, sunny patio or balcony out of strong wind.

Sowing Seeds

Sow zinnias direct where they will flower, because they resent root disturbance, and start them in cells only where spring is cool.

Sowing direct

• Sow seed 5 to 7 mm deep, with a few seeds at each spot and the spots 30 to 45 cm apart.
• Keep the soil damp but never soggy; seedlings usually appear in 7 to 14 days.
• Thin to the strongest seedling at each spot once true leaves show.
• Mulch with sugar cane or pea straw once seedlings stand about 5 cm tall.

Starting under cover

• Sow one or two seeds per deep cell or small pot of seed raising mix, 5 to 7 mm deep.
• Keep the tray warm and bright; a seedling heat mat helps hold the germination temperature.
• Plant out after about four weeks, once seedlings have true leaves and stand 5 to 10 cm tall, before roots circle the cell.
• Harden off seedlings over one to two weeks first (moving them outdoors for longer each day so they adjust to sun and wind).
• Set each seedling at the same depth it grew in the cell, then water it in well.

Tall doubles give the longest stems for cutting.

Care by growth stage

Warmth, steady water and a feed that shifts to potassium once buds form keep zinnias flowering. Weeks count from sowing.

If you prefer a conventional feed: mix a controlled release all-purpose granular (NPK around 14-5-10) into the soil at planting. Water in an all-purpose soluble (NPK around 23-4-18 to 27-5-9) fortnightly during leafy growth. From the first buds, switch to a flowering soluble (NPK around 15-4-26 to 15-5-20) fortnightly.

Deadheading

Cutting and deadheading keep a zinnia making flowers instead of seed, and one pinch early on gives more stems to cut.

• Pinch once: at about 30 cm tall, take off the top few centimetres just above a pair of leaves with clean, sharp snips.
• Deadhead often: every few days, cut each faded flower back to a leafy joint, where the plant branches, rather than leaving a bare stalk.
• Cut for the vase: a long stem taken back to a leaf pair does the same job and pushes fresh side shoots.
• Dwarf bedding types: snip off spent heads as they brown; compact types such as 'Thumbelina Dwarf Mix' hide many under fresh growth.
• End of season: bin spent plants and fallen leaves rather than composting them, as leaf spot and mildew fungi survive on debris.

Companion planting

Zinnias pull bees, butterflies and hoverflies into the garden, so the best partners are plants that gain from those visitors.

Grows well with

• Sweet alyssum (Lobularia maritima): a low edging in front that draws hoverflies, whose larvae eat aphids.
• Cosmos (Cosmos bipinnatus): wants the same sun and heat, and also brings in hoverflies.
• Zucchini, cucumber and other summer vegetables: zinnias beside the bed keep bees busy nearby.

Keep apart

• Short plants on the south side of tall types: zinnias up to 1 m tall shade them for much of the day.
• Pumpkins and squash sprawling into the bed: their big leaves smother the airflow that keeps mildew down.

Harvesting Blooms

Cut when a flower is fully or almost fully open and the stem just below it is firm, because zinnias open little further once picked.

• Cut in the early morning or cool of the evening, since stems cut in midday heat wilt quickly.
• Give each stem the wiggle test: shake it gently, and cut only if the stem stays straight.
• Strip leaves that would sit below the waterline, and stand stems in clean water straight away.

In a clean vase, stems usually last 7 to 10 days. To hold them for a day or two, keep them cool at 4 to 10°C and no colder, as zinnias are sensitive to cold damage. Tall types fill a jug on their own, while button types suit a small posy.

Common problems

Seedlings collapse at soil level. Damping off (Pythium species and Rhizoctonia solani) rots young stems, most often in wet, cold seed raising mix or soil. Read more

Transplants sit still for weeks. Zinnias hate root disturbance, so seedlings moved from shallow punnets often stall while direct-sown plants catch up.

Double flowers open single. Stress such as transplant shock can make double types carry single blooms for a period afterwards.

Tall plants flop in storms. An unpinched plant on one tall stalk, heavy with flowers, can blow over in summer wind and rain.

Pests and diseases

Leaf diseases rise with humidity, so a muggy Brisbane summer and the tropical wet season are the hardest times for zinnias.

Powdery mildew (Golovinomyces species). A white, floury film spreads across the leaves in warm, humid spells and weakens the plant. Caught early, a spray of 1 part full-cream milk to 10 parts water can slow it.

Alternaria leaf spot (Alternaria zinniae). Reddish-brown spots with greyish-white centres grow and merge until leaves brown and dry, and petals can spot too.

Aphids (family Aphididae). These small sap-suckers gather on new shoots and under leaves, which can wilt or discolour and turn sticky with honeydew. Read more

Two-spotted mite (Tetranychus urticae). Tiny mites speckle leaves pale and spin fine webbing underneath, and numbers build fast in hot, dry, dusty weather. Read more

Prevention first

• Thin crowded seedlings and pull weeds, so air moves freely between plants.
• Choose varieties listed with good mildew tolerance, such as 'Formula Mix' and 'Tetra State Fair'.
• Hose shoot tips and leaf undersides with a firm jet to knock off aphids and mites.
• Pick off spotted leaves and put them in the bin, not the compost.

Tips

1. In cool zones with a short summer, choose quicker, lower types such as 'Formula Mix' or the early-blooming 'Thumbelina Dwarf Mix'.
2. Pinch only half the plants in a row: pinched stems flower a little later, so the display lasts longer.
3. For bees and butterflies, grow some single-flowered types, as an open centre is easier to reach than a tight pompon.
4. Zinnias have become weeds in some parts of Australia, so check your council's advice before letting plants drop seed near bushland.