If California poppy had not been designated as the California state flower, Matilija poppy, Romneya coulteri, might have been. It was nominated, but was less popular at the time, partly because it was familiar only to those near its native range in Southern California. Some of us know it as ‘fried egg flower’, because the big and floppy white flowers with yellow centers look like fried eggs.
Matilija poppy is a big and bold perennial, with flowers that are bigger than any other native species. They can get more than six inches wide! These flowers stand on top of lanky stems that might get taller than six feet. The somewhat sparse and light grayish foliage has a uniquely bristly but also slightly rubbery texture. Individual leaves might be longer than six inches, with elongated lobes.
As a native of dry chaparral regions, Matilija poppy is very resilient, and does not need watering once established. However, to survive long and dry summers, it starts to die back early, so should get cut to the ground later in summer or early in autumn before it becomes too unappealing. It then stays dormant through winter before regenerating in spring. Rhizomes can spread aggressively.
Tony Tomeo 
I wish mine would spread aggressively. It’s still a paltry clump.
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It might prefer warmer and drier weather.
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One of my favorite Texas natives, the white prickly poppy (Argemone albiflora) is similar in appearance. Both species are beautiful flowers, and ours were lush this year.
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Yes, that sounds familiar, because those who see Matilija poppy for the first time think that it is a prickly poppy. Are there more than one species of it?
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‘Aggressively’ is the word, so aggressive that it repeatedly comes up behind the skirting board into our library even though I keep digging up every scrap. It should come with a health warning.
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It does it only where happy though. Some appeared on the edge of a freeway near town only about twenty years ago, but have since conquered about half an acre. No one minds there, but I would not want them to do that in my garden. Those that I got the picture of have been confined for about as long, and have not yet escaped for an areas that is just a few feet wide.
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I love this poppy and had one in my back yard for a while, ten years ago when I still had a swimming pool and it was tucked into a dry corner by the decking. But it was a little too ragged looking, and so big! Seems like, to display its great features, it needs a lot of room and an appropriate context that I am sadly still not able to provide.
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Yes, that is the impression that I get. In refined landscapes, it sometimes gets pruned, and then does not bloom well. It really at its best where it can grow wild.
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It has been quite prolific this year in much of Southern California!
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I think that the season will be extended by the mild weather.
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I have it in my garden and it’s beautiful in the spring. Looks pretty awful the rest of the year, though. It’s beginning to express itself in other parts of the garden as predicted by you, but not too much of a problem so far.
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I think that like many potentially invasive plants, it might not be so bad if parts that migrate get dug and disposed of or given away annually after the main clump gets established. I know it is aggressive, but it seems to me that it gets overwhelming partly because no one wants to dig parts of it up after it puts on such an impressive show.
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