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Plant Care

How to Grow Cosmos

Cosmos are easy to grow from seed, they bloom in as little as 12 weeks and are perfect for adding vibrant colours to borders and pots. For a prolonged flowering season, sow seeds indoors in March or April, and once sprouted with two pairs of leaves, transplant them into individual 7cm pots to grow under cover.

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Grow Cosmos From Seed

To maximise the flowering season of Cosmos, start by sowing seeds indoors in March or April. Fill a seed tray with fine, high-quality, peat-free compost and sow seeds on the surface, ideally one per cell. Cover with a 3mm layer of compost or vermiculite, moisten, and place in a heated propagator at 18-25ºC or cover with a clear bag. Once sprouted with two pairs of leaves, transplant into 7cm pots. In May, harden them off for a week in an open cold frame or patio before planting in their final location, and feed fortnightly with liquid tomato fertiliser once buds appear.

For bushiness, pinch cosmos tips after 2-3 true leaves grow.

What Cosmos Needs

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Pruning: Deadhead spent cosmos flowers to promote new blooms. Position: Cosmos prefer full sun and thrive in poor soil with good drainage. Protection: Tie in or stake Cosmos in exposed spots when they grow +1m.

Growing Cosmos Across the Season

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Sow Cosmos seeds undercover in mid-March, then transplant seedlings or sow directly post-frost.

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Deadhead spent blooms, water frequently and feed bi-weekly.

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Stake tall cosmos to prevent toppling and collect seed heads season-end.

Cosmos S.O.S.

Cosmos flourish in sunny spots with well-draining, low-nutrient soil and moderate watering. However, they may suffer from fusarium fungal infections or bacterial wilt, causing wilting and leaf discolouration, necessitating plant disposal. Powdery mildew can occur in shade, leading to leaf yellowing, which can be managed with proper air circulation, light, and horticultural fungicide. Leafhoppers may spread aster yellows, causing yellow mottling and distorted flowers, also warranting a plant pull.

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Give plants lots of sun for bountiful blooms.

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