Growing Signet marigold

The signet marigold is a true all-rounder in the vegetable garden. Originally from Mexico and Central America, it has been valued for centuries in cooking and as a medicinal plant. In our climate it's an annual and sadly dies off at around 4 °C. But from early summer right through to the first frost, it has an awful lot to offer.

What makes the signet marigold (Tagetes tenuifolia) so special: its roots produce sulphur-containing compounds called terthiophenes, which kill free-living nematodes in the soil. At the same time, the essential oils in its leaves repel whitefly and cabbage root fly. The single flowers attract honeybees, wild bees and hoverflies, whose larvae in turn devour aphids. A real all-in-one solution.

Compared to its relatives T. patula and T. erecta, the signet marigold has the advantage that slugs and snails largely leave it alone.

On top of that, the flowers and leaves are edible and taste of lemon, tangerine or blood orange depending on the variety. In Mexico they're still used as a spice and for tea, and in the Caucasus a related species even serves as a saffron substitute.

You can start signet marigolds indoors from February or March on a windowsill. The key thing: they need light to germinate. Scatter the seeds onto the surface of the compost, press them down lightly and cover with no more than a whisper of substrate (5 mm maximum). At room temperature of 18 to 20 °C, seedlings appear after about 10 to 14 days, sometimes up to three weeks.

Once the first feathered true leaves appear and the seedlings are 5 to 8 cm tall, it's time to prick them out. Use a dibber to protect the delicate roots. Then move the seedlings into more nutrient-rich compost.

Signet marigolds shouldn't go outdoors until after the last frosts in mid-May, as they can't cope with frost at all.

Direct sowing into the bed is possible from late April. If you want to get ahead, you can start as early as January and have flowering plants from June. In the first few days after planting out, a temporary slug barrier doesn't hurt, even though T. tenuifolia is rarely nibbled later on.

Signet marigolds love sun. They'll tolerate partial shade, but flowering and flavour suffer noticeably.

The signet marigold is barely fussy about soil. It thrives in pretty much any garden soil, as long as it drains well and doesn't tend towards waterlogging. It likes consistent moisture but really dislikes drying out completely or sitting in wet soil.

As a light feeder, a dose of compost before planting is all the signet marigold needs. Too much nitrogen leads to lush foliage at the expense of flowers. It works brilliantly in pots, whether in a window box, a large container or as a low bed edging. In containers the compost dries out faster, so check more often.

Among the signet marigold varieties, the so-called Gem series stand out. 'Lemon Gem' with pale yellow flowers and a distinct lemon aroma is probably the best-known and most frequently recommended variety. 'Tangerine Gem' and 'Orange Gem' bring a tangerine flavour.

If you're after a range of colours, go for 'Starfire', a mix of various yellow and red shades. 'Luna Orange' stands out with its cushion-like, heavily branched growth and works brilliantly as ground cover. 'Red Gem' adds a striking accent with its crimson flowers.

If your main goal is nematode control, plant Tagetes patula 'Single Gold' as well. This variety is apparently very efficient at tackling Pratylenchus nematodes. Ideally, combine Tagetes tenuifolia for flavour and Tagetes patula for maximum soil hygiene.

The most important job is to deadhead faded flowers regularly. This encourages the plant to keep producing new buds and can extend the flowering period by weeks.

When watering, direct the water straight at the root area. Moisture on leaves and flowers encourages grey mould.

A quick note on skin protection: all Tagetes species contain phototoxic substances that can cause contact dermatitis in sensitive people when combined with sunlight. If you're working with marigolds for a longer stretch, gloves are a good idea.

Grey mould (Botrytis cinerea) is the most common fungal disease in signet marigolds. It appears as a grey, fuzzy coating on flowers and leaves, especially in damp weather. Remove affected parts straight away and always water at the base. Good air circulation helps too.

Powdery mildew can appear in unfavourable weather, but aphids and spider mites are rarely an issue outdoors. Waterlogging can lead to wilt diseases. Overall, though, signet marigolds are robust plants that are seldom troubled by disease.