When to sow marigolds

French and African marigolds are tender annuals: the first frost kills them. The usual way is to sow them indoors in March or April, about eight weeks before they can go outside, and plant them out once frost is over, from mid-May in Germany, Austria and Switzerland and from late May in much of the UK.

You can also sow straight into the bed from May, once the soil has warmed to about 64°F. Direct-sown plants flower later, and in a cold spring the seedlings are an easy meal for slugs, so raising plants in pots is the safer route.

A word on names, because 'marigold' means two different plants. In the UK, calendula (Calendula officinalis) is also called marigold, pot marigold or common marigold. This guide is about Tagetes: French marigolds (Tagetes patula), African marigolds (Tagetes erecta) and signet marigolds (Tagetes tenuifolia). If a seed packet only says 'marigold', check the botanical name.

Sow thinly in trays or small pots and cover the seed with only a thin layer of compost. Keep the tray warm, ideally at about 68°F. Prick them out into their own pots once they have their first true leaves.

Harden the plants off for a week before planting out, and choose a sunny spot. Marigolds are not fussy about soil as long as it drains.

How far apart depends very much on the type, so go by the packet. Low French and signet marigolds can sit fairly close as an edging; tall African marigolds need a lot more room. If you are growing marigolds as a dense cover against nematodes (see below), plant no more than about 7 in apart.

Slugs are the main enemy. Marigolds attract them, which has given them a reputation as a slug trap: some gardeners plant a few to lure slugs away from other crops and then collect them. If you try that, use sturdy pot-grown plants and pick the slugs off often, because they will eat the marigolds too. Small, freshly planted marigolds can vanish overnight.

Remove faded flowers regularly. It keeps new ones coming and helps prevent disease on old blooms. With regular deadheading, marigolds flower from June until the first frost.

French marigolds (Tagetes patula) are compact, about 6–12 in tall, with single or double flowers in yellow, orange, red and bicolours.

African marigolds (Tagetes erecta) are tall and upright, up to about 35–47 in, with large flowers. They make good cut flowers, and some tall orange types are grown as dye plants for wool and fabric.

Signet marigolds (Tagetes tenuifolia) stay under about 6 in, form neat mounds of fine leaves and small single flowers, and are the ones grown for eating: the flowers are edible, and the spicy leaves can go into pastes and pesto. Grow French and African marigolds for show, not for the plate.

Partly, and this is one of the few companion-planting claims with real research behind it. Marigold roots release a compound called alpha-terthienyl that is highly toxic to some plant-parasitic nematodes, the microscopic worms that damage roots. French marigolds work against the widest range, mainly root-knot and root-lesion nematodes. In Germany, Tagetes is used as a cover crop for exactly this on vegetable and strawberry land.

The catch is how you grow them. The effect comes from a dense stand of marigolds, planted no more than about 7 in apart and grown for at least two months before the crop that needs protecting. In trials, marigolds interplanted between vegetables did not reduce nematodes. The variety matters too: French marigolds such as 'Single Gold' and 'Tangerine' did well in trials, while some others barely worked.

So if a bed has a nematode problem, sow or plant a solid block of French marigolds there for a summer and grow your crop in that spot afterwards. As edging for a vegetable bed, marigolds look cheerful, but don't expect them to protect the plants inside.

French marigolds are the classic partner for tomatoes, said to keep whitefly away. A glasshouse study backs this up to a point: marigolds planted among tomatoes from the start slowed the build-up of whitefly, mainly through limonene, the same scent found in citrus peel. Brought in after whitefly had taken hold, they did little.

The same study found that other plants that whitefly doesn't feed on, such as basil and nasturtium, worked about as well. So the benefit seems to come from mixing tomatoes with other plants, not from marigolds alone, and it was measured under glass rather than outdoors.

Marigolds belong to the daisy family, Asteraceae, with lettuce, sunflowers and calendula. As a summer flower they sit outside the vegetable rotation, unless you are using them as a nematode cover, in which case they take the bed for a season.