Every once in a while I pick up a plant on a whim because it seems interesting without expecting much from it and it ends up becoming a garden staple for me that I rely on to fill in bare spots, provide color when little else looks good, or some other such use. The pink evening primrose (Oenothera speciosa) is one such plant. I had never seen one flowering and new very little about the plant but at some point five or six years ago I bought a small, flowerless specimen. Since I had a vague notion that it was a plant that occurs in arid habitats, I planted it in a bed at the south side of a group of blue spruces (Picea pungens) that was very dry and sunny and which I had therefore planted with such species as eastern prickly pear cactus (Opuntia humifusa) and sedum (Hylotelephium spectabile). It did not appear to do much during that first summer but in the following spring it emerged in a clump several times the size of the original plant and produced an abundance of light pink, wide-open flowers approximately 2" (ca. 5cm) across from late May through most of June. Over the subsequent years the clump kept spreading, weaving its way through much of the border and forming an effective ground cover, while always being easy to pull up when it got too close to other plants. When my parents moved to their current home two years ago, I only managed to transplant a few shoots with barely any roots, and that, too, in the worst heat of midsummer. Nevertheless, the following summer brought a sizeable clump bearing plenty of flowers, and this year this remarkable little plant has filled in most of the empty space in that bed with its charming flowers.
Close-up of the flowers of Oenothera speciosa
Origin: Oenothera speciosa is native to approximately the southern two-thirds of the continental United States as well as northern Mexico.
USDA Hardiness Zone: The pink evening primrose is listed as hardy to Zone 5a.
Pink evening primrose and lavender (Lavandula angustifolia) in the frontyard of my family's suburban Michigan home (Zone 6a)
Size: Oenothera speciosa shoots vary somewhat in height but they will usually be between 5" (ca. 12.5cm) and 1' (ca. 30cm). The horizontal spread of the plant is practically unlimited, since it spreads by underground stolons that can travel quite far.
Flowering Time: The flowering time of pink evening primroses will vary considerably based on the local climate; here in southeastern Michigan they generally begin blooming at the end of May and keep on going through June.
Light Requirements: The pink evening primrose requires full sun.
Colony in my parents' front yard, grown in one year from one small plantlet
Soil Requirements: This plant is not picky about the soil, provided there is decent drainage.
Siting in the Garden: Oenothera speciosa should be planted in a sunny, well-drained spot with plenty of space to spread. The shoots can come up through turf, though mowing should get rid of them. Nevertheless, if this will bother you the plant should be sited at a good distance from any lawns. Also, since the plant is of relatively short stature, it should not be planted too close to any tall, vigorous plants that might crowd it out.
Care: A resilient and vigorous wild flower adapted to a huge geographic range, pink evening primrose is virtually carefree in the garden. They can be planted anytime from spring until mid-summer and should be watered regularly until established. A light mulch of shredded bark or wood is beneficial as well, though by no means necessary. Once established, the only care required is to pull up shoots that are coming up in unwanted locations, which is quite easy since the individual stems are quite delicate. Over the years, colonies may not only spread but also shift somewhat, with shoots coming up more densely in one area one year and in another the next. Unless the plants are used in a fairly relaxed planting, this may necessitate some moving around in the long run.
Pink evening primrose flowering in my family's garden last week
Propagation: The easiest way to propagate this plant is by separating off shoots with as many roots as possible and planting them in the new location. Keep them well-watered until established, as indicated by new growth and the absence of drooping in hot sunshine. This can be done any time from the onset of warm weather in spring to the middle of summer. Propagation from seed is presumably possible as well, and the dry seed pods can be collected from the plants in late summer.
Use in the Garden: Oenothera speciosa is a great ground cover or plant for the front of a border in a dry and sunny location. I find it easy to control and have not had issues with it crowding out other plants but its growth habit nonetheless means that it is more suited to naturalistic and lush plantings than to formality and excessive order. In other words, if you prefer a garden that consists of clumps of hosta separated by two feet of immaculate mulch, this is not the plant to use. Otherwise, it is a great plant to perk up spots that are otherwise hard to manage without constant irrigation, and since it retains its foliage long after flowering ends, a carpet of pink evening primroses usually continues to look neat for much of the summer and even into fall, at least here in Michigan.