How to Sow Calendula Seed: Depth, Spacing and Autumn Sowing

The seed is unmistakable, and it wants darkness

Calendula seed looks like nothing else in the seed box: a curved, ridged gray hook, sometimes almost a full ring, rough to the touch and a quarter to half an inch (6 to 12 mm) long. That size makes it easy to place one seed at a time, which is worth doing because scattered calendula comes up in clumps and then has to be thinned twice. This is the pot marigold, Calendula officinalis, and it is not the same plant as the French or African marigolds sold as bedding; those are Tagetes, they smell strongly, and their petals are not the ones used in the kitchen.

Cover the seed properly. Calendula germinates in darkness and seed left on the surface dries out or gets taken by birds rather than sprouting. Half an inch (1.5 cm) deep is right, firmed down with the flat of the hand so the soil is in contact with the seed all round. Sown at the right depth in soil around 60F (15C) it is up in five to ten days, and every seedling that appears will flower.

1. Sow direct where it is to stand Calendula has a fine branching root and does transplant, but a direct sown plant is stockier and flowers a week earlier. Sow modules only if slugs make outdoor sowing hopeless.
2. Place the seed, do not broadcast Push a finger in to the first joint, drop one hook in, close the soil. Broadcast calendula germinates in a mat that has to be thinned twice and stretches while you do it.
3. Two seeds per station Put two seeds an inch (2.5 cm) apart at each spot you want a plant and pull the weaker at two true leaves. Germination is good but a few seeds always fail.
4. Water the row once, then wait Soak after sowing and leave it unless the surface bakes hard. Calendula seed rots in cold ground that is kept permanently wet.

Do not soak, chill or scarify calendula seed. It needs no treatment at all, and soaked seed sticks to wet fingers and ends up sown in clumps.

Cool soil is fine, and that is the point of the plant

Calendula is a hardy annual that germinates well below the temperature most flowers need, which is why it can go in outdoors weeks before anything tender. It will come up at 45F (7C), slowly, and the seedlings shrug off a light frost once they have their first true leaves. That cold tolerance is the whole reason it flowers early and carries on flowering into weather that stops everything else.

Heat is the limit rather than cold. Above about 80F (27C) germination falls off and the seedlings that do appear come up thin and stretched. A midsummer sowing in a hot climate is better made in light shade, or postponed until late summer when the ground has cooled and the plants will flower through autumn.

Spacing decides whether you get mildew in autumn

A well grown calendula reaches eighteen to twenty four inches (45 to 60 cm) tall and branches into a mound almost as wide once you start deadheading. Sown at the six inches the packet often suggests, the row closes into a solid block by midsummer, air stops moving through it, and powdery mildew takes the lower half in late summer. Wider spacing does not reduce the crop, because each plant simply branches more.

Twelve inches (30 cm) between plants is the working standard in a bed, with rows eighteen inches (45 cm) apart so you can reach in to pick. For cut flowers on long stems, go to nine inches (23 cm) and accept that the plants will draw each other up, which is exactly what you want for stem length. In a container, one plant to an eight inch (20 cm) pot or three to a twelve inch (30 cm) pot is the limit.

Calendula is often sown between brassicas and beans to bring in hoverflies. Keep it a full foot clear of the crop row, because a mature plant leans over and shades young seedlings badly.

Thinning, and telling the seedling apart

The seed leaves are long, narrow and pale, more like a strap than a spade, and the first true leaves are soft, slightly sticky to the touch and faintly aromatic when rubbed. That stickiness is the identification, and it separates calendula seedlings from the fat, smooth cotyledons of most weeds that come up in a herb bed at the same time.

Thin at the two to three true leaf stage. Pull the surplus rather than cutting it, in damp soil, so the whole root comes away without disturbing the neighbor. Keep the stocky, dark, short jointed seedlings and pull anything that is racing ahead in height, because a stretched seedling makes a floppy plant that needs support all season.

1. Thin in two passes Take the row to six inches (15 cm) first, then to twelve inches (30 cm) a week later. Slug losses in between then do not leave gaps you cannot fill.
2. Move the thinnings rather than binning them Unlike most direct sown annuals, calendula thinnings lift and replant well at three or four leaves. Water them in and they catch up within a fortnight.
3. Keep short and stocky, not tall Height at the seedling stage is stretch, not vigor. The squat seedling with wide leaves makes the better branched plant.
4. Firm the soil back after pulling Pulling one seedling loosens its neighbor. Press the soil back around the plants you have kept and water once.

Autumn sowing, successions and what to write down

In any climate where winter lows stay above about 20F (minus 7C), a late summer sowing overwinters as a flat rosette and flowers four to six weeks earlier than anything sown in spring, on a bigger and better branched plant. It is the single best trick with this crop and costs one packet of seed. In colder gardens a spring sowing as soon as the soil is workable does the same job a month later.

Two spring sowings four weeks apart, plus the autumn sowing, give flowers from earliest summer through to frost. Record the sowing date, the emergence date, and the date of the first open flower for each batch. After two seasons those three numbers tell you exactly how many days your ground needs at each point in the year, which is the only way to time successions properly. Mark the position of each sowing on a sketch of the bed as well, because calendula self seeds heavily and next spring you need to know which seedlings are volunteers.