Oenothera Biennial, or Evening Primrose to the rest of us is a lovely plant that has beautiful yellow flowers that appear in the evening and are slightly fragrant too. Some gardeners regard then as weeds and remove them, but I think they make a great addition to the summer flowers in the garden. They need little or no attention apart from perhaps staking of the flower spike as it gets tall. After the flowering has ceased then just remove the plant. If you leave it till late then it will have seeded already and next years plants will appear quite quickly.
The foliage is a simple rosette of leaves and the crown in the center is where the flower spike will appear from in the early summer, the flower spike will grow up to around 2 meters in height. In the autumn the seed pods will open and sow seeds around the area of the plant, so once you have one, and as long as you let it seed, you will have some every year.