Because Common Evening-primrose (Oenothera biennis) is a biennial, in late summer it is possible to find first-year leafy rosettes growing near tall second-year flowering plants. Just as with Wild Carrot, it's the first-year plants that provide a tasty taproot to knowledgeable foragers. The taproots can be gathered from late summer through spring (so long as the ground is not frozen) -- they go out of season when the plant begins to produce an aerial shoot (which also happens to be edible). Given the number of vegetables Common Evening-primrose provides (flowers, buds, taproots, shoots, etc.), she is well worth befriending.
Over the years I've introduced several wild edibles into my yard. For example, last year, I scattered some Wild Carrot (Daucus carota) seeds in a few garden beds, covered them lightly with leaf mulch and walked away. A couple weeks ago, I noticed that the beds were looking lush with green growth and figured it was time to get my hands in the soil to check on the roots.
Compared with shovel-dug Wild Carrots that I've unearthed from more compacted ground, these decently-sized, pale yellow taproots came up with a steady tool-free tug. Freshly dug Wild Carrots may lack the crunch and orange-color of typical cultivated carrots, but their flavor and aroma is similarly pleasant. I chopped up the roots and added them to soups and stews with much satisfaction.
Note: In Foraging Wild Shoots: Wild Carrot, I point out that Wild Carrot is closely related to some seriously poisonous plants. Please read that post, and be sure you are 100% confident in your identification skills before gathering and eating any part of this plant.