Centaurea nigra - Kew Pollination Collection

Direct sow knapweed seeds outdoors where they are to grow from March to April. Choose a position in full sun on well drained soil which has been raked to a fine tilth. Sow seeds thinly, at a depth of 3mm (1/8") in drills spaced 30cm (12") apart. Water the ground regularly, especially during dry periods. Germination usually takes 14-21 days. When cornflower seedlings are large enough to handle, thin them out to 15cm (6") apart.

Alternatively, grow knapweed indoors from August to September for early summer flowering next year. Sow on the surface of a good quality seed compost at a temperature of 15-20C (59-68F) and cover with a very fine sprinkling of compost or vermiculite. Do not exclude light as this helps germination. When seedlings are large enough to handle, transplant and grow them on in cooler conditions until large enough to move outdoors. Overwinter plants in cool, light, frost-free conditions before planting out the following spring.

Water knapweed plants regularly until fully established. Knapweed is drought tolerant once established and will happily tolerate almost any soil, except those that are excessively waterlogged.

This plant will self-seed freely. If seedlings are not wanted, deadhead stems of knapweed flowers as the blooms fade. In autumn, cut back old stems to just above ground level to make way for new growth in the spring.

Seeds and garden supplies will normally be delivered within the time period stated against each product as detailed above. Plants, bulbs, corms, tubers, shrubs, trees, potatoes, etc. are delivered at the appropriate time for planting or potting on. Delivery times will be stated on the product page above, or in your order acknowledgement page and email.

Orders for packets of seed incur a delivery charge of £2.99.

Orders which include any other products will incur a delivery charge of £6.99.

Where an order includes both packets of seeds and other products, a maximum delivery charge of £7.99 will apply - regardless of the number of items ordered.

Large items may incur a higher delivery charge - this will be displayed in your shopping basket.

Please see our Delivery page for further details, and more information on different charges that may apply to certain destinations.

For more information on how we send your plants please visit our Helpful Guide on plant sizes.