When to sow nasturtiums

Nasturtiums come from the Andes and are killed by the first frost, so outdoors they go in only once frost is over: in Germany, Austria and Switzerland from early to mid-May, in the UK usually from May. You can keep sowing into late June for flowers into autumn.

For an earlier start, sow in pots indoors or in a greenhouse in March or April and plant out after the last frost. They flower from June until frost cuts them down in autumn.

The seeds are big enough to handle one by one. Push each one about 0.4 in deep and cover it well: nasturtiums germinate in the dark. Soaking the seed in water for a few hours or overnight beforehand can speed things up. Expect seedlings in one to two weeks, and don't worry if they come up unevenly.

Nasturtiums dislike root disturbance, so if you raise them indoors, sow two or three seeds per small pot rather than in a tray, and plant the whole pot's contents out together without breaking up the root ball.

Space plants 10–12 in apart, more for trailing types that will ramble. One or two plants at the edge of a raised bed, spilling over the side, are plenty; they will cover a lot of ground by late summer.

The most common mistake is too much kindness. On rich, well-fed soil nasturtiums grow huge leaves and flower late and sparsely. Plant them in ordinary or poor, free-draining soil and don't feed them.

Give them sun: in shade they flower less. They do need water in long dry spells, though. Heat and drought turn the leaves yellow and make the plants a magnet for aphids.

Nasturtiums also do well in pots, window boxes and hanging baskets, where the trailing types can hang down.

The whole plant above ground is edible. Leaves and flowers taste peppery and cress-like thanks to mustard oils, the leaves more strongly than the flowers. The flowers make a bright addition to salads.

The green buds and young, still-soft seeds can be pickled in vinegar or brine as a substitute for capers. Pick them while they are green and firm, before they harden.

Pick flowers and leaves as you need them from June to the first frost; picking keeps the plant producing.

Nasturtiums come in two habits. Compact, bushy varieties stay low and neat, good for bed edges and pots. Trailing and climbing varieties send out long shoots that scramble over the ground, up a fence or down from a basket.

Colours run from cream and yellow through orange to deep red, and some varieties, such as the 'Alaska' types, have cream-marbled leaves.

Aphids are the main pest, especially on plants stressed by heat and drought. Rub small colonies off by hand or pinch out the worst shoots.

Caterpillars of the large and small cabbage whites eat nasturtiums as readily as cabbage and can strip a plant in days. Check under the leaves from midsummer and pick off eggs and caterpillars. Leaf miners leave pale trails in the leaves but rarely matter.

Nasturtiums belong to their own small family, Tropaeolaceae. They contain mustard oils like cabbages do, and the same caterpillars eat them. As a flower they sit outside the vegetable rotation, but don't count on them to keep cabbage whites off your brassicas.

Nasturtiums are widely recommended as a sacrificial crop that lures blackfly away from beans and cabbage whites away from brassicas, or as a flower that draws helpful insects to beans and tomatoes. Aphids and caterpillars certainly do visit nasturtiums, but we found no trial showing that this protects the crops next to them. It is tradition, planted in hope.

One glasshouse study found that nasturtiums planted among tomatoes reduced whitefly about as much as French marigolds did. That was under glass and against one pest, so treat it as a promising hint rather than a rule.