Yellow Cosmos

Sulphur cosmos is an annual and dies at the first frost. You cannot overwinter the plant itself. Leave a few faded seed heads standing so it sows itself, or collect the seeds, store them dry and cool, and sow them directly in the bed from mid-May (after the Ice Saints) or, started on the windowsill, from April.

In the home garden you propagate Cosmos sulphureus practically only by sowing. As a frost-tender annual, other methods are not worthwhile. For an early start, sow the seeds from March in seed trays, covered only thinly with soil (about 0.5 cm), warm at around 18 to 20 °C; they germinate after 1 to 2 weeks. Sulphur cosmos needs somewhat more warmth to germinate than the related garden cosmos (Cosmos bipinnatus), so keep it consistently warm and moist. After the last frosts (around mid-May, depending on region), plant the young plants in the bed 20 to 25 cm apart. Alternatively, sow directly outdoors from early/mid-May, once the soil has warmed up: rake the soil loose, sow the seeds thinly, press them down only lightly (do not bury them deep) and keep them evenly moist until the seedlings appear. In warm summers the plants like to self-sow: to encourage this, let a few flower heads ripen on the plant and the seeds fall; this thickens the stand the following year. A free-draining, rather lean soil promotes abundant flowering; on soils that are too rich the plant produces lush foliage rather than flowers. Choose a sunny site. Cuttings are possible in principle, but unusual and without advantage for an annual with plenty of viable seed.