How to Grow Four O'Clocks in Australia

Four o'clock (Mirabilis jalapa), also called marvel of Peru, is a bushy perennial in the family Nyctaginaceae. It is grown for fragrant, trumpet-shaped flowers that open late in the afternoon, often in several colours on one plant. It thrives in hot summers and forms a tuber that regrows where winters are mild. Frost cuts it down, so cold districts grow it as an annual. The mistake most gardeners make is letting it seed freely, because it is poisonous and can escape the garden.

Three mistakes that matter

1. Letting the seed fall. Four o'clock self-seeds, and its tuber sends up new shoots, so one plant can become a patch. It has escaped into bushland and creek banks in several states. Pick off seed before it drops and dig out strays.
2. Sowing into cold soil. Seed sits idle or rots in cold ground, and frost kills young plants outright. Wait until the frosts are over, about when tomatoes go in, or start trays under cover.
3. Leaving the tuber in frozen ground. The tuber (a swollen, food-storing root) survives light frost under mulch but can die in hard freezes. In frosty inland and highland gardens, lift it in autumn, as under Harvesting Blooms.

When to plant

Four o'clock seed germinates when the soil holds 20 to 22°C, so sow once the soil has warmed and frosts have passed. In frosty districts, start trays under cover 6 to 8 weeks before the last frost. One sowing is enough, because plants flower for months and the tubers can return.

Sowings made early in a window flower longest before the cold. A seedling heat mat holds a tray at the right warmth in a slow spring.

Site, soil and setup

Four o'clock is a perennial, so it needs no rotation. Choose its bed with the weed record below in mind: it is a minor environmental weed in several states, although no state declares it.

Rules change, so check with your council or state biosecurity agency before planting near bushland.

Container growing

A pot keeps the tuber contained and saves lifting it for winter. Give one plant a pot of at least 10 L, about 28 cm across, and use premium potting mix. Set it by a doorway or window, where the evening scent carries indoors.

• Watering. Water when the mix is dry a finger's depth down; see watering pots through summer.
• Feeding. Give fish and seaweed blend at 25 mL per 9 L once a fortnight from planting until the first flowers.
• Position. Full sun, sheltered from strong wind.
• Winter. In frosty gardens, move the pot to a dry, frost-free shed once the tops die back.

Sowing Seeds

Start four o'clocks in cells or small pots under cover, or sow them straight into a warm bed after the last frost.

Starting under cover

• Soak the seed in water overnight before sowing, which can speed germination.
• Fill cells or 8 cm pots with fresh seed raising mix and water it first.
• Sow two seeds per cell at the depth in the table, and firm the mix over them.
• Keep the mix moist but never wet, at the temperature under When to plant.
• Once both seeds come up, snip out the weaker one at soil level.
• Grow seedlings on in full light so they stay short and sturdy.

Sowing direct

Sow where plants are to flower once the soil is warm and frost is over. Soak the seed overnight first, as for trays. Set two seeds per spot at the spacing in the table, then keep the soil moist until seedlings show. Thin each spot to the strongest seedling, and protect them from snails for the first few weeks.

Transplanting

Plant out seedlings that stand 5 to 10 cm tall with true leaves, after the last frost. First harden them off (a week of gradually longer outdoor stays, so leaves toughen to sun and wind), as in hardening off seedlings. Water each in with seaweed solution at 30 mL per 9 L.

Care by growth stage

Warmth decides how well four o'clock grows: once settled, it takes heat and dry spells with little help and flowers from summer into autumn.

Week ranges run from the sowing date. If growth stalls in poor sand, a light dose of all-purpose controlled release granular (NPK around 14-5-10), scratched in once in spring, can help.

Cutting Back

Pinching and trimming keep four o'clock dense and tidy, and removing seed stops it spreading.

• Pinch out the growing tips of young plants for denser, bushier growth.
• Snap off faded flowers every few days, because they cling to the plant.
• When flowering finishes, cut stems back to just above a pair of lower leaves.
• After frost kills the tops, cut the stems to ground level.
• Pull self-sown seedlings while small, and dig out suckers with their roots.
• Bin unwanted tubers rather than composting them, because they can resprout.

Companion planting

Four o'clock belongs at the back of a sunny border or along a path, where daytime flowers carry the colour while its blooms stay shut.

Grows well with

• Zinnia, a warm-season flower sown at the same time into the same sunny beds.
• Cosmos, whose airy daytime flowers fill the hours before four o'clocks open.
• Salvia, which shares its liking for sun and heat.
• Asiatic lilies, whose large flowers contrast with the small trumpets.

Keep apart

• Low plants such as violas and alyssum, which a bushy 90 cm plant shades out.

Harvesting Blooms

Each flower opens in the late afternoon, sooner on cloudy days, and wilts by the next morning, so dusk is the time to enjoy them.

• Visit at dusk, when fresh flowers open and release their scent.
• Snap off wilted flowers in the morning, as under Cutting Back.
• Collect seed once the fruits turn black and wrinkled, before they drop.
• Wear gloves to handle seed, and to dig or lift roots.
• In frosty districts, dig tubers before the first frost and shake off the soil.

Dry collected seed on a tray for a week, then store it in a labelled paper envelope, somewhere cool, dry and out of children's reach. Store lifted tubers in a dry, cool, frost-free shed until spring, then replant after the last frost.

Common problems

No seedlings. Cold soil is the usual cause (see Three mistakes that matter); seed can also lie dormant until the soil warms. Read more

Thin, leaning seedlings. Too little light makes seedlings stretch, so move trays into full light. Read more

Flowers never seem to open. They open late in the day and close overnight, so look again at dusk.

Leaves droop in the afternoon. Heat makes leaves wilt by day, and they recover as the evening cools; water only when the soil is dry 5 cm down.

Stems snap or sprawl. The stems are weak and brittle, so give plants shelter from wind and pinch young tips as under Cutting Back.

No regrowth in spring. Frost much below minus 5°C can kill the tuber in the ground; see Three mistakes that matter.

Pests and diseases

Four o'clock has few serious enemies, and most trouble strikes soft young growth.

Green peach aphid (Myzus persicae). Clusters of small aphids suck sap from young shoots and leave sticky honeydew. Hose small colonies off, or spray insecticidal soap on a cool day. Read more

Common garden snail (Cornu aspersum). Snails feed on tender leaves and shoots at night and leave slimy trails. Hand-pick them by torchlight and remove their daytime hiding spots. Read more

Damping off (Pythium species and Rhizoctonia solani). Seedlings collapse at soil level, rotted by common soil fungi and water moulds that thrive in wet mix. Read more

Prevention first

• Use fresh seed raising mix and clean pots for every sowing.
• Water trays and beds so the mix stays moist but never soggy.
• Check new shoots weekly, and leave early aphids for ladybirds and hoverflies.
• Clear boards, pots and weeds where snails shelter near seedlings.

Tips

1. Plant Tall Mix at the closer spacing, 30 cm, for a summer hedge or screen along a fence.
2. Adelaide gardeners grew four o'clocks soon after the colony was founded. Some forty years later, a local newspaper called them too vulgar for a genteel garden.
3. Label lifted tubers clearly in the shed, because the poisonous root looks like an edible one and has been mistaken for a herb.
4. Move a plant in its first year if it must move at all. Where winters are mild, an old tuber can grow 30 cm or more long and weigh up to 18 kg.