Wildflowers: Like little calendulas

These flowers appeared in my yard a couple of years ago and seem to be very aggressive. The plants grow 12 to 18 inches tall and produce vivid, ¾", orange-yellow blooms all year except during the winter. The plant smells somewhat like calendula. They seem drought-tolerant and grow in full sun.

Does anyone know what they are?

 

It looks like pot marigold. Calenendula officinalis!

 

I bought Calendula officianalis plants last year from, I believe, a reputable garden catalog. The flowers are very similar to my photo, but they are 3 top 4 inches in diameter.

 

The originals were probably a cultivar selected for large flowers; the seedlings have come up with normal-size flowers ;-)

 

2 cm (3/4") is rather much less than normal size :-)

 

Thank you, Everyone. I'm going to believe these are some sort of naturalized calendula. They are too similar to deny it.