How to Grow Zinnia and Cosmos from Seed: Sowing Months, Care
Zinnia and cosmos are the two summer flowers that laugh at Punjab heat. To grow zinnia from seed, press seeds about half a centimetre deep into a sunny pot in February to March or again in July, keep the soil just moist, and you will usually see sprouts within a week and the first flowers roughly two months later. Cosmos follows the same calendar, asks for even less, and both keep blooming for months if you cut the spent flowers.
The short version
- Sow, plains: mid February to March (spring), and July (monsoon) for an autumn show
- Sow, Karachi: almost year round except the hottest, wettest weeks; October to March is easiest
- Sow, hills: April to June after frost
- Pot: 8 to 10 litres (about 25 cm) holds one zinnia or two dwarf cosmos
- Sun: full sun, 6 hours or more; zinnias get leaf spot in shade
- Sprouting: about 3 to 7 days in warm soil
- Life cycle: both are annuals: one season, then seed
Prefer to listen? This post as a 7 min video · Watch the video
Zinnia vs cosmos: which suits your space?
| Zinnia | Cosmos | |
|---|---|---|
| Look | Bold, stiff, button to dahlia shaped flowers | Airy, daisy like flowers on feathery leaves |
| Soil | Likes compost | Flowers best in poorer soil; rich soil makes leaves |
| Height | Dwarf 20 to 30 cm; tall up to about a metre | Dwarf to over a metre; tall ones flop in wind |
| Weak point | Powdery mildew in humid weather | Falls over on windy roofs |
| Best for | Pots, cut flowers, colour | Borders, butterflies, self sown patches |
Yellow and orange cosmos (Cosmos sulphureus) handles the fiercest heat and spreads itself by seed every year. Pink and white cosmos (Cosmos bipinnatus) is taller and prefers spring and autumn. Both pull in butterflies and bees; our sibling page on a butterfly and bee garden pairs them with other desi plants.
When to sow zinnia seeds in Pakistan
| Zone | Spring sowing | Second sowing | Skip |
|---|---|---|---|
| Karachi, coastal | February to March | September to November | Heavy monsoon weeks |
| Multan, Sukkur, Bahawalpur | February | July to August | Sowing into 45°C May |
| Lahore, Faisalabad, Delhi | mid February to March | July | November to January frost |
| Islamabad, Peshawar | March to April | July | Winter |
| Hills | April to June | None | Anything near frost |
Established plants handle 45°C if watered, but seedlings sown into peak heat often scorch, so sow before it arrives. These are not winter flowers: frost kills them.
Seed or nursery seedlings: what should a beginner buy?
Seed, honestly. Zinnia and cosmos seeds are large, sprout fast and grow quickly, so they are among the easiest flowers to start yourself. Nursery seedlings save you about three weeks, but they are often root bound in tiny bags, and zinnias in particular sulk after a rough transplant. Buy seedlings only if you are late for the season, and pick short, sturdy plants without flowers.
How to sow, step by step
- Fill a pot with a free draining mix, roughly two parts garden soil to one part compost and one part sand or cocopeat (see our potting mix guide). For cosmos, go lighter on compost.
- Water the mix and let it drain.
- Press zinnia seeds about 0.5 cm deep; scatter cosmos and cover with a thin layer of soil.
- Keep the surface moist, not soggy, in bright light. Warm soil gives sprouts in 3 to 7 days.
- Thin to one zinnia per 25 cm pot, or space them 25 to 30 cm apart in a bed. Cosmos can sit a little closer.
- If you start in trays, move seedlings at two to four true leaves, keeping the root ball intact. Our seed starting guide covers trays and hardening off.
How do you care for zinnia in pots?
Water at the base when the top 2 to 3 cm dries: every two to three days in spring, often daily for pots in May and June. Wet leaves in the evening invite mildew, so water in the morning.
Feed zinnias lightly every three to four weeks with compost or a balanced feed. Skip extra feeding for cosmos; it grows tall and leafy with fewer flowers when spoiled.
Pinch zinnias once when they are about 20 to 30 cm tall: snip the top bud above a pair of leaves. The plant branches and gives more, longer stemmed flowers.
The weekly deadhead, and the last flush trick
Once a week, cut every faded flower back to a leaf pair. The plant then puts its energy into new buds instead of seed, and blooming carries on for months. Near the end of the season, stop cutting and let the final flush dry on the plant: that becomes next year’s free seed, and cosmos will often sow itself in the same spot.
How long does zinnia take to flower?
Usually about two months from sowing, a little less for dwarf types in warm weather and a little more for tall, large flowered kinds. Cosmos is similar. A February sowing in Lahore typically flowers from April through the summer, and a July sowing carries colour into October and November.
Why is my zinnia not flowering?
- Too little sun: under six hours, plants stretch and flower poorly.
- Too much nitrogen: lush leaves, few buds; stop feeding, especially cosmos.
- Too young: give it the full two months.
- Pot too small: a root bound seedling in a cup stalls.
- Extreme heat: buds blast in hot dry wind; light afternoon shade helps in Multan.
Pests and problems
Powdery mildew (white dust on leaves) is the main zinnia complaint, worst in humid, mild spells with damp leaves. Space plants for airflow, water at the base in the morning, remove the worst leaves, and choose mildew resistant varieties where seed packets mention it. Leaf spots come more often in shade. Aphids and caterpillars can be picked or hosed off; spray soapy water only if needed, never on open flowers where bees forage.
How to save zinnia and cosmos seeds
Leave a few flowers to dry fully brown on the plant. For zinnia, pull the dry head apart: the viable seeds are flat, arrow shaped and have a slight bump in the centre. Cosmos seeds are slim, dark needles that come away easily. Dry them indoors for a week, then store in a labelled paper envelope in a cool, dry place. Hybrid colours may not come true. Our sibling guide on flower seed saving goes deeper.
Zinnia and cosmos are not known as dangerous garden plants, but no ornamental flower should be eaten unless you know it is an edible variety grown without sprays. If you ever use a fungicide or insecticide, read the full label, wear gloves, spray in the evening when bees are not flying, and keep children and pets away until it dries. Anyone with hay fever may find daisy family flowers bother them at close range; see a doctor for wheezing or breathing trouble.
Find more flower guides in Phool aur Paudhe.
Zinnia and cosmos questions
How do you grow zinnia from seed?
Press seeds about half a centimetre deep in a sunny pot of free draining mix, keep it moist, thin to one plant per 25 cm pot, and pinch once for branching. Flowers follow in roughly two months.
When should I sow zinnia seeds in Pakistan?
Mid February to March in Punjab and Islamabad, with a second sowing in July. Karachi can sow from autumn through spring, and the hills sow after the last frost.
Is zinnia annual or perennial?
The garden zinnia is an annual: it grows, flowers and sets seed in one season, then dies. Save seed to grow it again.
Can zinnias grow in pots?
Yes, very well. Use a pot of at least 8 to 10 litres in full sun; dwarf types suit balconies best.
Why are my zinnia leaves turning white?
That white dust is powdery mildew. Water at the base in the morning, give plants more space and remove badly affected leaves.
Do zinnias need full sun?
Yes, at least six hours. In shade they flower less and suffer more leaf diseases.
Does cosmos come back every year?
Cosmos is an annual, but it often sows itself, so new plants appear in the same spot the next season if you let some flowers set seed.
Should I pinch my zinnias?
Yes, once, when plants are about 20 to 30 cm tall. Pinching the top bud gives a bushier plant with more flowers.
Why is my cosmos tall but not flowering?
Usually too rich soil or too much fertiliser. Stop feeding, give full sun, and pinch the tips to encourage buds.