Marigolds are annuals, not perennials, whatever their reappearance each spring suggests. Sow seed 6 weeks before your last frost, expect germination in 5 to 7 days, and space plants 10 inches apart. They are the garden's most-planted companion flower, though the evidence is narrower than the reputation.
Are marigolds perennial?
No. They are annuals everywhere, and the plant dies with the first hard frost.
The reason so many people believe otherwise is that marigolds self-seed prolifically. A plant left to set seed drops it around itself, and the following spring seedlings appear in the same patch of ground. The bed looks like it came back. What actually happened is that a new generation germinated where the old one died.
That is worth knowing because it changes what you do: if you want marigolds in the same place next year, leave a few plants to finish flowering and set seed rather than deadheading everything. If you do not, pull them before the seed drops.
Marigold companion plants
Marigolds are planted beside more vegetables than any other flower, so it is worth separating what is supported from what is repeated.
The well-supported effect is on root-knot nematodes. Certain Tagetes varieties release compounds from their roots that suppress these soil-dwelling pests. The important detail, which most advice omits: this works when marigolds are grown as a dense stand across the ground for a full season and then dug in — effectively as a cover crop. A few plants dotted between tomatoes do not deliver it.
The weaker claim is scent repelling flying pests. Marigolds are widely said to deter whitefly, aphids and beetles by smell. Support for this is thin, and where it exists the effect is modest. Whether companion planting actually works sets out how to weigh claims like this.
The genuinely reliable benefit is flowers. Marigolds bloom heavily over a long season and draw hoverflies, parasitic wasps and other predators of aphids and caterpillars. That is a real mechanism, it is not specific to marigolds, and it is probably the main reason gardens with them do better.
Good neighbours, and why.
Tomatoes — the classic pairing, and the one where the nematode effect is most relevant since tomatoes are badly affected by root-knot nematodes. Peppers and aubergines share the same nematode susceptibility. Cucumbers and squash benefit mainly from the pollinator draw, since both need insect pollination to set fruit at all. Bush beans sit happily alongside them.
Bad neighbours. There are effectively none. The frequently repeated claim that marigolds inhibit beans has no support worth citing, and it contradicts the equally common advice to plant them together — we do not carry it.
Sowing marigold seed
Marigolds are among the easiest seeds in the garden and germinate fast.
Sow a quarter of an inch deep in moist compost. Seeds are long, thin and easy to handle, and they can be spaced individually rather than scattered.
Germination takes 5 to 7 days in warm compost — noticeably quicker than most flowers, which is why they are a good first sowing for a child. Keep the surface damp but not wet; damping off is the one thing that reliably kills marigold seedlings.
Start about 6 weeks before your last frost for early flowers, or sow direct once the frost date has passed. They grow quickly enough that direct sowing loses very little time, so there is no need to start them indoors unless you want colour early. The planting date calculator works the dates from your own frost date.
Marigolds are frost-tender. A late frost kills seedlings outright rather than setting them back, so do not plant out early on a warm week in spring.
Spacing
10 inches between plants, 12 inches between rows for French marigolds.
African marigolds are much larger and want more — around 18 inches. If you are growing them as a nematode cover crop rather than for display, plant much more densely than either figure, because the effect depends on root density across the whole area.
The plant spacing calculator will work out numbers for a bed.
Growing in containers
Marigolds suit pots well and are often the plant used to fill gaps in mixed containers. Allow 6 to 8 inches of depth and one plant per 6 inches of width.
The container caveat is the same as the ground one: rich potting mix gives leaf rather than flower. A standard mix with no added feed is right.
Soil, water and feeding
Poor soil suits them. This is the counterintuitive part and the commonest growing mistake: marigolds flower best on unremarkable ground. Rich soil, or a generous nitrogen feed, gives a large dark-green leafy plant with few flowers. How to add nitrogen to soil covers why excess nitrogen produces leaf at the expense of bloom — marigolds are a clear case of it.
Full sun. Water about an inch a week; they tolerate dry spells better than most bedding plants.
Deadhead regularly. Removing spent flowers before they set seed keeps the plant flowering for months — unless you want them to self-seed, in which case leave the last flush.
Common problems
Lots of leaves, few flowers. Soil too rich or feeding too generous. Stop feeding; the plant recovers within a few weeks.
Seedlings collapsing at soil level. Damping off, from compost kept too wet or poor air movement. Sow thinly, water from below, and let the surface dry slightly between waterings.
Slugs on young plants. Marigold seedlings are a favourite. They are much less vulnerable once established, so protection matters mainly in the first fortnight.
Spider mites in hot, dry spells. Fine webbing and stippled leaves, usually on plants that have been dry and sheltered. Watering and airflow help more than treatment.
Marigolds are most often planted alongside tomatoes and peppers, both of which are susceptible to the nematodes they suppress.