How to Grow Nasturtiums in Australia

Nasturtium (Tropaeolum majus) is a fast-growing annual in the nasturtium family (Tropaeolaceae), grown for its bright edible flowers, peppery leaves and green seed pods. It is not watercress (Nasturtium officinale, cabbage family), and canary creeper, sold in the same range, is a related perennial climber, Tropaeolum peregrinum. Nasturtiums come quickly from seed and flower hardest in lean soil, so the usual mistake is feeding them.

Three mistakes that matter

1. Feeding them like vegetables. Manure, blood and bone or regular liquid feeds give lush leaves and few flowers. Grow nasturtiums in poor to average soil and skip fertiliser in the garden bed, apart from the light seedling feed in Care by growth stage.
2. Sowing into cold soil or before the last frost. Seed is slow to come up in cold soil, and frost blackens the plants overnight. Sow once the soil has warmed and the frosts are over, in the windows set out in When to plant.
3. Letting them escape. Nasturtium self-seeds freely and is an environmental weed in five states, spread by water and dumped garden waste. Remove seed pods near creeks and bushland, and bag or compost old plants rather than tipping them over the back fence.

When to plant

Seed comes up in 7 to 21 days once the soil holds about 16°C to 25°C, and frost kills the plants. In frost-free zones the cooler months suit them best, because hot, humid summers wear them down. One sowing can flower for months while you keep picking.

Site, soil and setup

Give nasturtiums full sun, with afternoon shade in hot northern and inland gardens. In heavy shade they grow leaves but few flowers. Any well-drained soil suits them, whether acid, neutral or alkaline, so there is no pH to correct.

Nasturtium sits outside the vegie rotation: grow it along bed edges, paths and fences, where its sprawl does no harm.

Container growing

Give nasturtiums a pot of at least 20 L, about 35 cm across, filled with fresh potting mix. The compact 'Tom Thumb Mix', 'Jewel Mix', 'Alaska Mixed' and 'Empress of India' suit pots. 'Top Night & Day' spills well from a hanging basket.

• Watering: water when the top of the mix feels dry. Potted nasturtiums do not tolerate drying out, and yellowing leaves are the first sign.
• Feeding: from about six weeks, worm casting liquid diluted to the colour of weak tea, fortnightly, and nothing stronger.
• Position: full sun, or morning sun and afternoon shade in hot zones.

Sowing Seeds

Sow nasturtium seed where it is to flower, and start it in pots only for an early start in frosty gardens.

Preparation

Nasturtium seed has a tough outer coat that slows germination. Soak the seed in water overnight, for about 12 hours, to soften the coat and help the seedling break through. Sow the soaked seed the same day, while it is still moist. The seeds are large and easy to handle, which makes them a good first sowing for children. See soaking and other seed treatments for the method.

Starting under cover

• Sow one soaked seed per small pot, 12 to 15 mm deep, 4 to 6 weeks before the last frost.
• Keep the mix just moist until seedlings appear.
• Harden off the seedlings (a week or so of longer spells outdoors each day, so they toughen to sun and wind).
• Plant out after the last frost, disturbing the roots as little as possible.

Sowing direct

• Sow two soaked seeds per spot, 12 to 15 mm deep, and cover them well, as the seed germinates in darkness.
• Space spots 20 to 30 cm apart for dwarf types and about 30 cm for trailing and climbing types.
• Keep the soil just moist, never soggy, until seedlings appear.
• Thin to the stronger seedling once the first true leaves open.

Care by growth stage

Water decides flowering more than food does, so keep the soil just moist and go easy on fertiliser. Weeks count from sowing.

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

Deadheading

Picking or snipping off faded flowers keeps plants flowering longer and stops seed forming where you do not want it.

• Dwarf types: pinch off each faded flower with its stalk, down to the leaves.
• Trailing types: cut back stems that smother neighbours or spill across paths.
• Climbing types: trim stems that outgrow the fence or trellis, and tie in new growth.
• For next year: collect seed from withered flowers, dry it well and store it in an airtight jar. Or leave a few pods to self-sow in a contained bed.

Companion planting

Nasturtium flowers feed hoverflies, ladybirds and bees, and the plants draw cabbage white butterflies and aphids that might otherwise settle on vegetables.

Grows well with

• Cabbage, kale and broccoli: planted along the edge of the bed, nasturtiums work as a trap crop for cabbage white butterfly.
• Fruit trees: a nasturtium ground cover beneath them shades the soil as a living mulch.
• Calendula: its pollen and nectar bring more beneficial insects to the edge of the vegie patch.

Keep apart

• Small, slow crops such as carrots and young seedlings: trailing stems can smother them.
• Bushland, creek banks and nature strips: seed and stem pieces spread into them and take hold.

Harvesting Flowers

Pick flowers just after they open, in the morning once the dew has dried; the first usually come 10 to 12 weeks from sowing.

• Snip flowers with their stalks, or pinch them off by hand.
• Pick small, young leaves, as older leaves turn bitter.
• For pickling, pick seed pods while they are still green.
• Eat only flowers from plants that have not been sprayed.

Flowers are best eaten the day they are picked. To hold them, lay them on a moist paper towel in a sealed container in the fridge for up to a few days. Dip them in cool water just before use.

Scatter flowers and young leaves through salads. Pickle green seed pods as a stand-in for capers only to a tested Australian recipe, as pods pickled without enough acid can carry botulism. If you have hay fever or asthma, eat a little first, as the pollen can trigger a reaction.

Common problems

Leaves but few flowers. Heavy shade or rich soil is the usual cause (see Site, soil and setup and Three mistakes that matter).

Blackened, collapsed plants. Frost is the cause, so in frosty zones treat nasturtium as a spring to autumn flower (see Three mistakes that matter).

Yellowing leaves in pots. The mix is drying out between waterings (see Container growing).

Wilting on hot afternoons. Plants flag in strong heat; afternoon shade and a deep watering in the morning help them recover.

Pests and diseases

Few pests trouble nasturtium, and the main ones are drawn by the same mustard oils that attract them to the cabbage family.

Cabbage white butterfly (Pieris rapae). The butterflies lay small yellow eggs, and the velvety green caterpillars, up to about 30 mm long, chew holes in the leaves. Read more

Aphids (family Aphididae). These small sap-suckers cluster on shoot tips and under leaves, and nasturtium is one of their favourite hosts. Read more

Cabbage leafminer (Liriomyza brassicae). Its larvae tunnel pale, winding trails through the leaves; it is widespread in Australia, and the damage is mostly cosmetic.

Prevention first

• Check the leaves weekly and rub off yellow eggs with your fingers.
• Pick off caterpillars and mined leaves, and squash small aphid clusters, by hand.
• Keep flowering plants nearby for hoverflies, whose larvae eat aphids.
• If you spray, use a product whose label names the plant and the pest, and follow it (see Harvesting Flowers).

Tips

1. Pull off any seed pods, then chop spent plants into the compost, where they break down quickly.
2. Grow the variegated 'Alaska Mixed' where the leaves will be seen, as they look good even between flushes of flowers.
3. Check with your local council about weed advice if your garden borders bushland or a creek.