Answer: Oenothera speciosa, Mexican Evening Primrose is a perennial, native to the southwestern U. S. and Mexico. Plants grow about 1' tall and 3' wide and spread by underground rhizomes. It makes a good groundcover for dry slopes or parking strips but can become invasive where soil is regularly watered and growth is not checked. I'd compare it more to California poppy, in that it will spread in clumps and pop up where you least expect it. You can control the plants by cutting the roots on a annual basis (slip a shovel into the ground around the plant to sever roots), or by placing an underground barrier such as corregated fiberglass (much like you would control running bamboo). Oenothera won't overtake a yard in a single year, but the plants do require some regular control or they will get out of hand.
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