PlantFiles: Ornamental Knapweed, Perennial Cornflower
Centaurea montana 'Coerulea'

This is one of the weediest plants I've ever had in my garden. I deeply regret the purchase of two of these that became parents-of-millions. They grow in my lawn, they grow in the neighbours' yards, they've crowded out my rock garden plants... and they think Round-Up is Miracle Grow. I'm going to be cutting down and digging out for years.

This was planted in my garden in 2008 given to me by a friend so not sure of the exact cultivator but it is growing with great success. I have it in both full sun and part-shade and all are doing well with many blooms.. Today is May 27th and the plant is in full bloom with many more blooms to come! This plant transplants easy so you can give small ones away to friends and at times there are many to give away! After quite a few years of growing this plant it has done exceptionally well!! I find small plants here and there more then not from seeds being dropped by the birds. I have shared this plant with so many people who also love it!! One down fall of this plant is when it gets tall it will have the tendency to fall over after a while especially when there is no rain and not enough water.
Secets are:
1)Dead head!! You will get many more blooms come up and the bloom time will last quite a bit longer.
2)After second set of blooms allow the seed heads to sit on top and dry naturally, you will then witness tons of finches all over your plants.
3)After the plant looks really terrible and falling over watch for new growth at the base, when you see the new growth go ahead and cut away the old growth. I have not gotten another bloom from the new growth till the following year.

Bushy plant in the spring, peony-like as one commenter said. I'm not a fan of the foliage come mid-summer. I wouldn't say it's ratty, but it sort of withers. Self-sows easily and reblooms if cut back. I'd love the plant if the foliage stayed pretty.

Love this plant. some get to be as big and full as peony bushes, self sows alot, easy to move around in early spring..

I also experience problems with mildew. The answer for me is to cut them back hard when about through blooming (end of June). They fill back in quickly and give me a second bloom in September. And, yes, they reseed like crazy, but are easy to pull out where unwanted.

Mine are not ratty looking and I have no problem with moldew or mold in my area. They add pretty blue/purple color to my garden.

These are weedy and very hard to get rid of, with deep roots and the ability to regrow from a fragment. They reseed like crazy. Also, they get mildewy and ratty looking in mid summer.

On the plus side, they do look pretty when blooming in late spring.