How to Sow Cosmos Seed: Depth, Warmth, Spacing and Thinning

Sow a quarter inch deep into warm soil

Cosmos seed is a thin dark spindle about half an inch (12 mm) long, tapering to a point at one end. It is large enough to place individually, so there is no reason to scatter it. Press each seed a quarter inch (6 mm) into the compost and firm the surface lightly. Sown much deeper than half an inch (12 mm), a fair share of seedlings run out of reserves before they reach the light, and the tray comes up thin for no obvious reason.

The seed needs warmth rather than light to germinate, and a covering of a quarter inch is exactly right. Below 60F (15C) it sits and sulks and often rots in wet compost. At 70 to 75F (21 to 24C) the seedling loops appear in five to ten days, usually nearer five.

Cosmos seed keeps well. A packet stored in a cool dry drawer stays usable for three years, with germination falling off gradually rather than collapsing. Sow an old packet thickly rather than throwing it away.

1. Fill and firm before you sow Level the compost and press it with a flat board until it sits half an inch below the rim. Seed sown into loose fluffy compost sinks deeper than you intended the first time you water.
2. Place the seed by hand Two inches (5 cm) apart in a tray, or two seeds to a three inch (7.5 cm) pot. The seed is easy to handle individually, and a crowded tray of cosmos becomes drawn within ten days.
3. Water from below the first time Stand the tray in an inch of water until the surface darkens, then lift it out. Watering hard from above buries the seed and washes it into the corners of the tray.

Do not start too early

This is the mistake that costs most gardeners a good cosmos year. The plants grow extremely fast, and a seedling started ten weeks before the last frost becomes a tall soft plant sprawling across a windowsill with nowhere to go. It goes into the ground checked, leggy and root bound, and it never makes a good shape.

Four to six weeks indoors before your last expected frost is plenty. That gives a stocky plant with four to six true leaves at planting time, which is exactly the size that establishes fastest. If your light is poor, sow four weeks before rather than six: a small firm plant overtakes a tall pale one within a month of going out.

Direct sowing outdoors works very well and is often the better option. Once the soil holds above 60F (15C) and all frost has passed, sow where the plants are to flower. A direct sown cosmos never suffers a transplant check, roots deeply, and stands up to wind far better than a transplanted one. It flowers only a week or two behind an indoor sown plant.

Sowing two batches three weeks apart is the single easiest way to have cosmos in flower right through to frost, because the first batch inevitably goes leggy and tired by late summer.

Sow into poor soil on purpose

Cosmos comes from dry, lean, sunny ground in Mexico, and it flowers best when the soil is unimproved. Do not dig compost or manure into a cosmos bed and do not sow it where you have just spread a rich mulch. Rich ground gives you an enormous leafy plant with almost no flowers, which is the most common cosmos complaint of all.

The most useful thing you can do at sowing time is choose the leanest, sunniest, best drained spot you have. A gravelly border, the poor end of a bed, or ground that has already had a hungry crop taken off it will all produce shorter, sturdier, far more floriferous plants than the good soil next to it.

The same applies to seed-starting mix. Sow into an ordinary low nutrient seed-starting mix rather than a rich multipurpose one. Seedlings raised in rich compost stretch fast and go soft, and soft cosmos seedlings collapse at the base.

Thin early, and to the full spacing

Cosmos seedlings put on height quickly and a crowded stand goes from healthy to drawn in about ten days. Thin as soon as the first pair of true leaves is open, usually two weeks after germination, and thin to the final spacing rather than in stages. Crowded plants stretch, and a cosmos stem that stretched as a seedling stays thin and floppy for the rest of its life.

The tall types need twelve to eighteen inches (30 to 45 cm) between plants, and the shorter bushy ones nine to twelve inches (23 to 30 cm). That looks absurdly generous when you are looking at inch high seedlings, and it looks obviously correct by midsummer when each plant is three feet wide.

Snip the surplus off at soil level with scissors instead of pulling. Cosmos roots go straight down fast and knit together, and pulling one seedling lifts the neighbor with it. If you want the thinnings, prick them out at the seed leaf stage while the root is still a single unbranched thread.

Write down the sowing date and the date you first see seedlings. Two seasons of that record tells you exactly how early you can start on your own windowsill without producing drawn plants, which no seed packet can tell you.