Nasturtium

Light and exposure

Nasturtiums want full sun, at least six hours a day. It is in full sun that they flower most and that their colours, bright orange, deep red or yellow, stay truest. In part shade they still grow but put their energy into leaves rather than flowers.

They are happy almost anywhere, a tub on a south-facing terrace, the foot of a trellis, a balcony planter or a dry slope in the garden where little else grows easily. It is precisely in poor, sun-baked corners that they perform best.

Watering

Nasturtiums cope well with drought once established and dislike being overwatered. Water moderately after sowing to help germination, then space out the watering: soil that stays a little dry between waterings pushes the plant to flower rather than to bulk up its leaves.

In a pot or hanging basket the compost dries out faster and needs a bit more attention in high summer, but keep a light hand. Too much water combined with rich soil is exactly the combination that gives you a nasturtium all leaves and no flowers.

Soil and repotting

This is the point that surprises people most: nasturtiums prefer poor, well-drained soil to rich ground. Soil that is too fertile, loaded with nitrogen, pushes the plant into lush foliage at the expense of flowers. Never feed a nasturtium, even in a pot: no fertiliser, no fresh compost, at most a little ordinary potting mix at sowing time.

A sandy, stony or simply unfertile corner of the garden, the spot where nothing else wants to grow, often makes the ideal home. It is one of the few flowers in a Belgian garden that does better in poverty than in comfort.

Temperature and humidity

Nasturtiums are frost-tender at every stage. Seeds only germinate from around 12 to 15 degrees, and a frost, even a light one, blackens the foliage and ends the flowering overnight.

In Belgium you never sow or plant before mid-May, once the ice saints have passed. After that the plant grows without any trouble all summer, even in a heatwave, and carries on until the first frosts of October or November, which always bring the season to an abrupt end.

Pruning and upkeep

Nasturtiums need no structural pruning, but two simple habits clearly extend the flowering. Deadhead the spent flowers as you go: the plant then stops putting its energy into seed and keeps producing new blooms.

On climbing varieties, pinch out the tips of young stems to force them to branch and cover a trellis or netting more thickly. On trailing varieties in a planter, let them run freely, it is exactly that spilling habit that gives the effect you want.

Pests and diseases

Black aphids are the nasturtium’s most faithful visitor, and largely by design: the plant is used as a trap crop in the vegetable garden, planted near broad beans or cabbages to draw the colonies onto it instead of onto the crop. A nasturtium well covered in black aphids is doing its job, as long as the infestation does not weaken it too much; otherwise a firm jet of water or cutting back the worst-affected stems is enough to get it going again.

Cabbage white caterpillars sometimes riddle the round leaves with holes, especially in summer near a cabbage patch. Apart from these two visitors, the nasturtium is a tough plant, rarely troubled by the usual garden fungal diseases.

Propagation

Direct sowing is by far the simplest method, and it is the one nasturtiums prefer. Sow the large round seeds straight into the ground in mid-May, about two centimetres deep, in stations of two or three seeds spaced roughly twenty centimetres apart. Germination takes one to two weeks.

The seeds are big and easy to handle, which makes this one of the very first plants worth letting children sow themselves. Nasturtiums also often self-seed from one year to the next: do not be surprised to see new plants coming up in the same spot the following year, without you having sown a thing.

Month-by-month care calendar

Green Hub tips

• Never feed a nasturtium: poor soil gives you flowers, nitrogen-rich soil gives you leaves and almost nothing else.
• Plant it near broad beans or cabbages in the vegetable garden: it draws black aphids onto itself and works as a trap crop.
• Flowers, leaves and young seeds are all edible, with a peppery taste close to watercress, lovely for lifting a salad.
• Sow straight into open ground in mid-May, about two centimetres deep: the large seeds need no pricking out at all.