This section is from the book "The Gardener's Monthly And Horticulturist V23", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
I have noticed in the Monthly inquiries about the dying out of the Centaurea gymnocarpa. I believe the principal cause of the Centaurea dying in the summer is its being planted out when it is too much pot bound. I may be mistaken. I have had the same trouble, but never with the following treatment:
In taking my cuttings, I do so late, after most other bedding plants are killed with frost. I strike them in the Azalea house, either in a box or in the bench, never at any time keeping the sand too wet, just damp enough to prevent flagging, letting them stand in the bench until the beginning of February. In this way ninety per cent, of them will have rooted. Then pot in two-inch pots, and when once established and growing freely, cut them back to two or at most three eyes. This prevents flowering. If punched higher they are liable to still run to seed. By the first week in April they will have filled the pots with roots, and made nice bushy little plants. They should then be shifted into lour or five-inch pots. I have planted out plants treated in this way; also those left in the two-inch pots. Those planted from the two-inch pots dying in patches and looking ragged all the season; those from the four-inch remaining without a break until long after frost, and pulled up in splendid condition when the beds were cleaned.