Nasturtiums as an Edible-Garden Companion: What the Evidence Actually Shows

Aphids and cabbage white butterflies really do use nasturtiums: black bean aphid feeds on them alongside beans, and cabbage white caterpillars feed on them alongside brassicasThe cabbage family: cabbage, kale, broccoli, cauliflower, Brussels sprouts, kohlrabi, swede, turnip and more, sharing the same pests, diseases and rotation rules. Full definition →. What is not shown is that planting nasturtiums near your crops draws pests away from them. Grow nasturtiums because they are easy and edible from leaf to seed pod, and keep netting, fleeceA lightweight, breathable fabric laid or draped over plants to trap a little warmth and keep off frost, wind and some flying pests. Full definition → or hand-picking as the real defence.

Why it is a real host plant

Nasturtium (Tropaeolum majus) is among the summer host plants for black bean aphid, with beans, poppies and dahlia (RHS). Black bean aphid overwinters as eggs on shrubs such as common spindle, viburnum and philadelphus, then a generation of winged females flies off in search of summer hosts. Nasturtium is one of those destinations. Keep spindle, viburnum and philadelphus out of the bean bed, or well away from it, so the eggs do not overwinterKeeping a plant alive and intact through the cold months, whether left outdoors, moved under cover, or stored dormant. Full definition → beside next year's crop. A later Scottish spring probably means a later aphid flight, so have nasturtiums in place by then.

On the caterpillar side, nasturtiums are among the plants cabbage caterpillars feed on, alongside cabbages and other brassicas. Cabbage white lays eggs on nasturtium, and caterpillars that hatch on it develop normally. But caterpillars moved from cabbage to nasturtium refused to feed and starved, because nasturtium foliage contains strong antifeedants, with chlorogenic acid the most prominent (Renwick et al., Journal of Chemical Ecology). So a nasturtium can attract egg-laying adults, but it will not rescue a bed where caterpillars are already feeding on cabbage.

What the evidence supports

Gardeners plant nasturtiums near runner beans and tomatoes in the hope of an effect (RHS). Garden Organic's companion planting page states that nasturtiums pull blackfly away from beans and that Chinese cabbage draws cabbage whites from brassicas, but gives no trial, and says elsewhere that companion planting often has unproven benefits. For carrot fly it notes that trials gave inconsistent results. Gardeners' World says large and small white caterpillars feed on nasturtium leaves, luring them away from brassicas. None of the three cites a trial. Even if a trap crop works, it concentrates pests rather than removing them. For control, Garden Organic advises washing off aphids and picking off caterpillars, and cutting plants back if an attack is severe.

Growing nasturtiums

Nasturtiums do best on poor soil: skip feeding, rich compost and fertiliser (Garden Organic).

• Soil and site. Poor, well-drained soil in full sun. If the soil is too rich, nasturtiums make foliage at the expense of flowers. The RHS record lists chalk, loam and sand.
• Type. Bushy types suit containers, hanging baskets and the front of a border. Climbing types, which reach 1.5 to 2.5m, can be trained up wigwams or obelisks.
• Sowing. Sow indoors from March to April and outdoors and plant out in May (Garden Organic), 1.5cm deep, thinned or transplanted to 20cm apart. Gardeners' World gives March to May, thinning to 30cm, and says a late sowing gives flowers until the first frosts. Keep plants well watered in dry weather.
• Hardiness. RHS rates nasturtium H3The RHS's H1a–H7 scale for how much cold a plant can survive outdoors, H1a being tender indoor-only and H7 hardy anywhere in the UK. Full definition →: hardy in coastal and relatively mild parts of the UK, down to between −5 and 1°C. It is an annual, so a hard frost ends it. Wait until the local frost risk has passed, or start seed indoors and harden plants off.
• Pests. It may be susceptible to glasshouse whitefly, glasshouse red spider mite and virus diseases, as well as the aphids and caterpillars above. Slugs and snails are well suited to the damp, mild UK climate (RHS), and seedlings and soft young shoots are the targets: transplant sturdy plantlets rather than young seedlings, and protect them with clochesA small, portable clear cover placed over an individual plant or short row to trap warmth and keep off wind, rain and pests. Full definition →.

Eating nasturtiums

The petals, leaves and seed pods are all edible.

• Flowers. A spicy-peppery taste. Pick them just after they open fully, and keep picking and remove seed heads to encourage more flowers.
• Leaves. A taste similar to cress, good in salads, pasta dishes and vinaigrettes.
• Seed pods. The fat green, unripe pods can be pickled and used as an alternative to capers, sometimes called poor man's capers.

Verdict

Grow nasturtium for what is certain: an easy annual that asks for neglect and gives leaves, flowers and pickled pods, and is tagged by RHS as a plant for pollinators.

Common questions

Sources