PlantSowGrow
How to grow Cornflower (Blue Boy)
Centaurea cyanus
Vivid true-blue flowers loved by bees; a hardy annual you can sow outdoors in spring for flowers all summer.
At a glance: Sow mid March to mid June.
When to sow, plant and harvest
Timed for a typical UK garden (last frost mid-April). The app uses your own town's frost dates.
Growing Cornflower (Blue Boy): what it needs
- Light
- Full sun · 6+ hours a day
- Watering
- Water in dry spells; pots daily in summer
- Soil
- Any well-drained soil
- Spacing
- 25 cm
- Height
- 75 cm
- In pots
- Grows well in containers
- Feeding
- Liquid feed every two weeks once flowering starts
- Hardiness
- USDA zones 2–11
- Difficulty
- Easy
Step by step
- Mid March – mid June · 4 weeks before the last spring frost
Sow Seeds Outdoors
Sow thinly where they are to flower, in a sunny spot, into raked, weed-free soil.
- 7–14 days after “Sow Seeds Indoors” · Late March – early April
Check Germination
Seedlings should appear in 7–14 days. Keep the soil moist. If sown outdoors, thin them to 25 cm apart once they have two true leaves.
- 14 days after “Plant Out” · Early April
Add Support
Push in canes or twiggy sticks and tie in stems before they flop.
- 61–91 days after “Check Germination” · Late May – late June
First Flowers — Deadhead Regularly
Snip off faded flowers every few days to keep new ones coming for months. Take a photo for your journal!
- Mid October – early November · around the first autumn frost
Clear or Collect Seed
When frost blackens the plants, pull them up for the compost heap. Leave a few seedheads first if you want to save seed.
