Getting it to bloom

Why won't my Sulphur Cosmos bloom? (and how to make it flower)

Also called Yellow cosmos, Sulphur cosmos (Cosmos sulphureus).

More about sulphur cosmos

About Sulphur Cosmos

Cosmos sulphureus · also called Yellow cosmos, Sulphur cosmos · flowering

Sulphur cosmos is a heat-loving annual with bright orange, gold and yellow semi-double flowers above coarser, less feathery foliage than C. bipinnatus. Vigorous and drought-tolerant, it flowers prolifically through summer in full sun and poor soil, attracting bees and butterflies. Sow direct after frost and deadhead to extend the long show.

Plant type: flowering

Watch for — Aphids: Gather on soft tips and buds. Dislodge with a water jet or leave for natural predators; rarely serious.

The reasons sulphur cosmos isn't blooming

Almost every non-blooming sulphur cosmos traces back to one of these, roughly in order of how common they are:

Too little sun — most of these need full sun (or very bright light) to flower well; shade gives leaves, not blooms.
Too much nitrogen feed, driving lush foliage at the expense of flowers (very common with general or lawn feeds).
The plant has not been deadheaded, so it stops flowering once it sets seed.
Irregular watering — drought or waterlogging at the budding stage makes buds abort.
It is still too young or was checked by a transplant and is rebuilding before flowering.

Feeding sulphur cosmos a high-nitrogen general feed and growing it in too little sun — you get a big leafy plant and almost no flowers.

The fix — how to get sulphur cosmos to flower

Maximise sun. Give sulphur cosmos the sunniest spot you have — for most bedding and fruiting plants, more direct light directly means more flowers.
Switch the feed. Move off high-nitrogen feeds and use a higher-potassium "bloom" or tomato-type feed as it comes into flower.
Deadhead regularly. Remove spent flowers often to keep it producing more rather than stopping to set seed.
Water consistently. Keep moisture even through budding and flowering — drought-then-flood swings make buds drop.

Light and feeding do most of the heavy lifting here. Dial in the spot with the light guide for sulphur cosmos and get the feeding right with the sulphur cosmos fertilising schedule — the wrong feed (too much nitrogen) is one of the most common silent reasons a healthy plant makes leaves instead of flowers.

Bloom season and what to expect

Sulphur Cosmos flowers across its growing season (mostly summer) and, kept fed and deadheaded, can bloom for many weeks or right up to frost.

Post-bloom care so it flowers again

Deadhead, keep feeding lightly, and many will rebloom; collect seed from the best plants at the end of the season if you want to grow them again.

For everything else this plant needs day to day, see the full sulphur cosmos care brief and its watering schedule — a stressed, badly watered plant rarely has the energy to flower at all.

Sulphur Cosmos blooming — frequently asked questions

Why won't my sulphur cosmos flower?

Sulphur Cosmos blooms on the season's growth given enough sun, warmth and the right feed — there is no cold or photoperiod trick, just good growing conditions and a bloom-leaning feed. The most common reason it is not happening: Too little sun — most of these need full sun (or very bright light) to flower well; shade gives leaves, not blooms.

How do I make sulphur cosmos bloom?

Give sulphur cosmos the sunniest spot you have — for most bedding and fruiting plants, more direct light directly means more flowers. Move off high-nitrogen feeds and use a higher-potassium "bloom" or tomato-type feed as it comes into flower.

When does sulphur cosmos normally bloom?