How to Grow Sweet Peas in Australia

Sweet pea (Lathyrus odoratus) is a scented climbing annual in the pea family, Fabaceae, grown for posies and cut flowers. Across most of Australia it is a cool-season flower: autumn sowings bloom from late winter into spring. Most failures come down to timing, with seed sown into warm, wet soil or a summer-flowering type sown where winters are mild.

Three mistakes that matter

1. Sowing into warm or soggy soil. Seed sown while autumn soil is still hot, or kept wet, often rots before it comes up. Wait until the soil cools below about 20°C, water once at sowing, then only lightly.
2. Growing the wrong type for your winter. Summer-flowering types need long days, so in mild-winter gardens they climb for months and meet the heat before flowering. There, choose the winter and early types in the table under Sowing Seeds.
3. Letting pods form. Once a vine sets seed it slows, then stops flowering. Pick every few days and pull off any pods you missed, as set out in Deadheading.

When to plant

Seed germinates best in cool soil of about 13 to 18°C, and germination drops sharply above 25°C. Southern gardens traditionally start on St Patrick's Day in mid-March; warmer gardens wait for the soil to cool, usually from April. One sowing flowers for months if you keep picking, and a second sowing of another flowering type stretches the season.

At the early edge of a window, test the soil at sowing depth in the morning and wait if it still feels warm. In very cold gardens, skip autumn and sow in spring, because harsh frost damages the flowers.

Site, soil and setup

Give sweet peas at least six hours of direct sun and shelter from strong wind. They want free-draining soil around pH 6.0 to 7.5, and they collapse in ground that stays wet.

Rotate the bed each year, because soil-borne diseases build up where sweet peas grow in the same spot. Leafy greens or root vegetables make a good next crop.

Container growing

Climbers need a big pot with a teepee or trellis fixed in it; the dwarf types need no support at all.

Bagged potting mix can carry Legionella bacteria: dampen it before opening, wear a mask and gloves, and wash your hands afterwards.

• Watering: water at the base whenever the top 5 cm of mix is dry, and never let pots dry out once buds form.
• Feeding: from the first buds, give liquid potash at the label rate every fortnight.
• Position: full sun, out of strong wind. In Brisbane and Perth, light afternoon shade in late spring helps.

Sowing Seeds

Sow sweet peas where they are to flower, along the base of their support, or one to a deep pot.

Preparing the seed

The hard seed coat slows germination, so give it a head start. Soak the seed overnight in water at room temperature and sow the seeds that have swollen. Nick any that stay small and hard: file or cut a shallow notch in the coat on the side opposite the small scar (the eye). Sow soaked or nicked seed the same day, into moist soil.

Sowing direct

• Sow 15 to 25 mm deep and 15 to 20 cm apart along the base of the support.
• Water in once, then hold off for a few days, because seed in wet soil can rot.
• Seedlings usually appear in 10 to 21 days in soil of 13 to 18°C.
• Guard seedlings from snails, birds and mice with wire covers or bottomless pots until they are about 10 cm tall.

Starting in pots

• Sow one seed per deep pot or tube, because the seedlings quickly form deep roots that dislike disturbance.
• Stand the pots outdoors in full light, not in a greenhouse, where seedlings grow soft and leggy.
• Harden off seedlings raised under cover (a week or two of more time outdoors each day, from a few hours to full days).
• Plant out with the root ball whole and water in with seaweed solution at half strength, 15 mL per 9 L.

The store's varieties fall into groups set by day length. Winter types flower on about 10 hours of daylight, spring types on about 11 and summer types on about 12.

Care by growth stage

Sweet peas are legumes that make some of their own nitrogen, so rich feeding gives leaves rather than flowers. Steady moisture and potassium from the first buds matter most.

If growth stalls in poor soil, a light dose of all-purpose controlled release granular (NPK around 14-5-10) at the label rate can help.

Deadheading

Every flower left to fade becomes a pod, and pods tell the vine its work is done. Pinch early and pick often.

• At 10 to 15 cm: pinch out the tip above the second or third leaf joint, so the plant branches from the base.
• In flower: cut open stems every few days, and snip off faded flowers and pods.
• End of the season: cut the vines at the base, leave the roots in the soil, and chop the tops for mulch.

Companion planting

Low, cool-season flowers sown in the same autumn weeks suit the base of a sweet pea trellis.

Grows well with

• Violas: sown in the same months, they edge the bed without climbing.
• Calendula and cornflowers: on the same autumn sowing list, they fill out a cutting bed.
• Sunflowers and marigolds: sown after the vines finish, they use the nitrogen the roots leave.

Keep apart

• Garden peas: they share pea aphid and powdery mildew, and their seed is easily mixed up with poisonous sweet pea seed.
• Corn on the north side: tall stalks shade a vine that needs six hours of sun.

Harvesting Blooms

Cut a stem when its lowest flowers are open and at least two buds at the tip are still closed.

• Pick in the cool of the morning.
• Cut the longest stems you can with clean snips.
• Strip the lower leaves and stand the stems in water straight away.

Sweet peas usually last 4 to 5 days in a vase, and a little sugar or flower preservative in fresh water adds a few more. For seed, leave a few late pods until they turn brown and dry. Store the seed in a labelled paper envelope somewhere cool and dry.

Common problems

Seed does not come up. Seed rotting in warm or waterlogged soil is the usual cause; see the first point in Three mistakes that matter. Read more

Seedlings collapse at soil level. Damping off (a seedling rot caused mostly by Pythium species and Rhizoctonia solani) strikes in wet soil. Read more

Lush vines, few flowers. Too much nitrogen from manure or leafy-plant feeds pushes leaves over flowers, so switch to potash.

Buds drop before opening. Dry spells, heat, low light and over-feeding all cause it; keep the soil evenly moist once buds form.

Flowering stops early. Pods forming or the first hot weather ends the display; see the third point in Three mistakes that matter.

Pests and diseases

Snails take the seedlings, and aphids, mildew and mites follow the vines into spring.

Garden snail (Cornu aspersum) and grey field slug (Deroceras reticulatum). They are among the worst threats to young sweet peas, especially on damp nights. Hand-pick them by torchlight about two hours after sunset, set beer traps and put copper bands on pots. Read more

Pea aphid (Acyrthosiphon pisum). Aphids cluster on soft shoot tips and weaken the vines. Hose them off with a firm jet, use insecticidal soap on what remains, and let ladybirds and lacewings finish the job. Read more

Powdery mildew (Erysiphe species). White, powdery spots spread over the leaves in humid weather, often in late spring, and the leaves yellow. Remove the worst leaves and spray 1 part full-cream milk to 10 parts water. Read more

Two-spotted mite (Tetranychus urticae). These tiny sap-suckers leave yellow, stippled leaves and build up fast in hot, dry weather. Hose the leaf undersides, then use insecticidal soap there if they persist. Read more

Prevention first

• Keep to the spacing in Sowing Seeds, so air moves through the vines.
• Water the soil, not the leaves, and do it in the morning.
• Clear boards, pots and debris where snails shelter by day.
• Check shoot tips weekly for aphids, before colonies build.

Tips

1. Plant 'High Scent' beside a door, a gate or a path, where the scent is caught in passing.
2. Hang 'Little Sweetheart' in a basket at eye level, close to where you sit.
3. Sow an early type and a summer type in the same autumn week, so the second picks up as the first fades.