The short version: December suits heat-loving flowers in every zone, though our calendar gives the tropics, into the wet, just one: celosia. Zinnia, sunflower, celosia and portulaca go into warm soil everywhere else, and cool and alpine gardens have the longest December list, with December the last month for most of it. Against hot soil, water the row first, shade it and keep it damp.
December is when a seed row can turn against you. In most gardens the soil is warm enough for every summer flower, yet a bare drill in full sun can dry to a crust by lunchtime, which can kill seed that has just started to grow.
Our sowing calendar by zone runs from a single December flower for tropical gardens to its longest list for cool and alpine ones.
Get seed up in hot soil: water first, then shade the row
Hot bare soil reduces emergence, our sunflower guide notes, so keep the top centimetre cool and damp.
- Soak the bed the evening before, so it is moist well down.
- Sow late in the afternoon or on a mild, overcast day, but not in a heatwave.
- Cover with fine compost or vermiculite at the depth in the grow guide, not soil that crusts. Portulaca, ageratum, salvia, rudbeckia, alyssum, busy Lizzie, Swan River daisy and billy buttons need light to germinate, so press them onto the surface with 1 mm of cover at most.
- Shade the row with 30% to 50% shade cloth on low hoops. A board suits buried seed such as sunflower and zinnia, not light-needing seed; lift it as seedlings show.
- Keep the surface damp, watering twice a day if needed. Run the hose until the water is cool, as water left in a hose in the sun can scald.
Weeks to first flowers, from our grow guides: portulaca 6 to 7, sunflower and marigold 8 to 12, zinnia 10 to 13, celosia and ageratum 12 to 14. So celosia and ageratum sown in mid-December flower in March.
What to sow in December, zone by zone
The six zones are broad groupings, so if your garden sits between two, or up high, let your own first and last frost dates decide.
Tropical (Darwin in the Northern Territory, Cairns and Townsville in Queensland, Broome in Western Australia). The wet usually sets in across the north in December, and celosia is the one flower our grow guides give for it, in pots or raised beds, with crested types spaced so heads dry between downpours. Zinnia, cosmos, marigold and sunflower wait for the dry season. See our Top End build-up guide.
Subtropical (Brisbane and south-east Queensland, northern New South Wales). Sunflower, zinnia, celosia, portulaca, globe amaranth and amaranth all go in, with impatiens for shade. December is the last month for vinca (Catharanthus roseus) and aster, though aster sown this late flowers in humid autumn, when mildew rises. Our cosmos guide pauses sowing here until January.
Arid and semi-arid (Alice Springs in the Northern Territory, Broken Hill in New South Wales, Mildura in Victoria, Kalgoorlie in Western Australia). Zinnia, sunflower, celosia, portulaca, globe amaranth and amaranth suit the heat; our calendar closes cosmos, marigold and vinca in November. Shade seed rows, then mulch deeply once seedlings are 5 cm tall.
Temperate (Sydney and the New South Wales coast, Melbourne in Victoria). Zinnia, sunflower, marigold, globe amaranth, amaranth, portulaca, verbena and ageratum run into January. December closes the warm-season window for cosmos, celosia, vinca, aster, impatiens, annual phlox, salvia, rudbeckia and the native Swan River daisy (Brachyscome).
Mediterranean (Perth and southern Western Australia, Adelaide in South Australia). Zinnia, sunflower, globe amaranth and amaranth run into January, and December is the final month for cosmos, celosia, marigold, portulaca, verbena, vinca, aster and ageratum. Where sand repels water, apply a wetting agent, at the rate on its label, before sowing.
Cool and alpine (Tasmania, Canberra in the ACT, Ballarat in Victoria, Orange and Armidale in New South Wales, Stanthorpe in Queensland). The longest list, and the last call for most of it: sunflower, zinnia, marigold, cosmos, portulaca, celosia, salvia, vinca, phlox, verbena, rudbeckia, Californian poppy, schizanthus and Swan River daisy all close when December ends. Alyssum and the native billy buttons carry on into January. Sow early and choose quick, dwarf types to flower before autumn frosts.
Flowers that take heat and dry spells
- Portulaca shrugs off heat and dry spells if the soil drains freely.
- Globe amaranth can flower from summer until frost from one sowing.
- Vinca handles heat and dry spells well.
- Zinnia and amaranth revel in heat with deep watering at the base when the top few centimetres dry.
- Native everlasting daisies, sown in autumn or spring, flower through December in temperate, Mediterranean and cool gardens.
Celosia is the odd one out: it loves heat but not drought, so give it about 25 mm of water a week, counting rain.
Cut flowers for summer vases
Pick soon after sunrise, straight into a bucket of water, as stems cut in the heat wilt fast. See our flower conditioning guide.
- Zinnias: cut fully open, as they open little further in the vase.
- Sunflowers: cut as the outer petals lift. Single-stem types give one bloom each, so sow a few seeds every two to four weeks.
- Celosia, globe amaranth, amaranth and billy buttons: use fresh, or hang in bunches to dry.
Keep December flowers going: deadhead, compost, cut back
Sweet peas are in their last weeks in temperate gardens and flower into February in cool ones. Spring-sown zinnias, cosmos and annual phlox are flowering in temperate and Mediterranean gardens, and dahlias have started in subtropical and arid ones, with southern plants following in January.
Weekly, cut anything fading back to a leaf joint or side shoot; pick sweet peas every few days, as ripening pods wind the vine down. Compost spent flowers, except diseased ones and weedy seed heads, since a home heap rarely gets hot enough to kill every seed.
Perennial clumps such as perennial phlox, echinacea and black-eyed Susan (rudbeckia) can be cut back by about a third before buds form, to flower later on shorter, sturdier stems. Catmint is different: shear it by a third after its first flush for a second round.
Daisy flowers bring in the pest eaters
Spring-sown calendula, marigold and zinnia earn a place beside the vegetables: adult hoverflies, lacewings and tiny parasitic wasps feed on nectar and pollen, while their young eat or parasitise aphids and other soft-bodied pests. Hoverflies have short mouthparts and feed at open-centred flowers, so choose single marigolds and zinnias over tight pompon doubles. NSW Local Land Services notes that plantings with overlapping flowering periods strengthen populations of the natural enemies of crop pests. See attracting predatory insects.
A caution on climbers, poisonous seed and garden escapes
Cardinal climber, morning glory and moonflower (Ipomoea alba) are December sowings on our calendar outside the tropics and can cover a fence by autumn, but their seeds are poisonous if eaten. So is every part of vinca. Keep packets and saved pods out of reach of children and pets. If someone swallows any part, phone the Poisons Information Centre on 13 11 26; for a pet, ring your vet.
NSW WeedWise, from the NSW Department of Primary Industries and Regional Development, records that under the North Coast regional weed plan a person should not buy, sell, move, carry or release moonflower into the environment, so choose another climber from Port Macquarie to the Tweed and on Lord Howe Island. Moonflower also regrows from stem fragments, so bin prunings. Celosia, vinca, ageratum and Californian poppy have gone wild in parts of Queensland and elsewhere, so near bushland, deadhead them before seed ripens and pick climber pods while still closed.
Frequently asked questions
Is December too late to sow flowers?
Not outside the tropics. Cool and alpine gardens should sow early, as December closes most of their windows; elsewhere zinnia, sunflower and globe amaranth run into January.
Can I sow any flowers in the tropics in December?
Celosia, in pots or raised beds that drain fast. Most tropical flower sowing waits until the wet eases, from about April to August.
Why did my December seeds not come up?
Usually the surface dried or overheated as the seed began to swell, or light-needing seed was buried. Resow at the right depth into a soaked bed, shade the row and keep it damp.
Can I still sow cosmos in December?
Yes in temperate, Mediterranean and cool gardens, where it is the last month. Our calendar skips December in subtropical gardens and closes the arid window in November.
A shaded seed row and a bucket of water at dawn carry a flower bed a long way into summer.
Related growing guides
- Pinching out, the Chelsea chop and the summer cut-back
- Flowers to sow in January, zone by zone
- Flowers to sow in November, zone by zone
- What to sow in December: vegetables for every zone
- Herbs to sow in December, zone by zone
- Growing zinnias from seed
- Sunflowers from seed
- Growing cosmos from seed
- Deadheading: where to cut, plant by plant
Browse zinnia seeds for summer picking, or globe amaranth seeds for heads that dry.