How Do I Start Annual Flowers Indoors from Seed?

Starting annual flowers from seed indoors gives you months of colour at a fraction of the cost of bought plants, and access to varieties no garden centre will stock. Half-hardy annuals — those that cannot survive frost but are sown early indoors to get a head start — include some of the most popular summer flowers: cosmos, marigolds, antirrhinums, petunias, zinnias, and begonias.

When to Sow Annual Flowers Indoors

Most half-hardy annuals are sown 6–8 weeks before your last frost date. In the UK this means sowing in late February to March; in the US, February to March for most zones. Sowing too early produces large plants stuck in pots for weeks; sowing at the right time gives compact seedlings ready to harden off and plant out precisely when frost risk passes.

Some exceptions: begonias and antirrhinums benefit from a 10–12 week head start as they are slow-growing. Cosmos and zinnias grow so fast that 4–6 weeks is plenty — sow too early and they become potbound before planting-out time.

Germination Conditions by Flower Type

• Cosmos: 18–22 °C; germinates in 5–10 days; sow just 3–4 mm deep
• Marigolds (Tagetes): 18–24 °C; fast at 5–7 days; sow 5 mm deep
• Antirrhinums (snapdragons): 18–21 °C; surface sow (need light); slow at 14–21 days
• Petunias: 21–24 °C; surface sow (need light); 10–14 days
• Zinnias: 21–27 °C; very fast at 4–7 days; sow 5–6 mm deep
• Begonias: 24 °C; surface sow; slow at 14–21 days; need extra warmth

Handling Very Fine Flower Seeds

Many flower seeds are extremely small — antirrhinums, petunias, and begonias especially. Mix them with dry silver sand in a piece of folded paper and tap gently to scatter thinly. Sow onto the surface of moist, fine-grade seed compost. Press lightly to make contact. Cover with a sheet of glass or clear polythene and place in a warm propagator. Do not cover these fine seeds with compost — they need light.

Pinching Out for Bushy Plants

Once seedlings have 4–6 leaves, pinching out the growing tip — removing the top pair of leaves between your finger and thumb — encourages side-shooting and produces much bushier, more floriferous plants. This is especially valuable for cosmos, antirrhinums, and sweet peas. It feels counterintuitive to remove growth, but the resulting plants produce far more flowers.

Planting Out

After hardening off for 10–14 days, plant out once all frost risk has passed. Space generously — 25–30 cm for most annuals — as crowded plants are more prone to disease and produce fewer flowers. Water in well and deadhead regularly through summer to encourage continuous flowering.