Pretty soon, we will start seeing fields filled with winter annual weeds, and one weed that you may encounter over the course of the next several weeks is cutleaf evening-primrose (Oenothera laciniata Hill).
Cutleaf evening-primrose is a winter annual or sometimes a biennial that is native to the U.S. and can be found throughout Missouri. I believe this weed has become especially prevalent in no-till corn and soybean fields in Missouri over the past several seasons. This plant derives its name from the tendency of most members of this family to close their flowers during the day but open them during the evening and on into the night.
Cutleaf evening-primrose seedlings have cotyledons that are kidney- or egg-shaped in outline with very short hypocotyls, which are the stems that occur below the cotyledons (Figure 1). Seedlings initially develop into a basal rosette of leaves (Figure 2). Young leaves have margins that are untoothed, but subsequent leaves have deeply toothed margins. Leaves often have hairs on top but not on the leaf undersides. Mature plants have leaves that are lanceolate in outline, are relatively narrow with a white midvein, and have deeply toothed margins. Cutleaf evening-primrose can either take on a prostrate or upright growth habit and at most will grow to about 3 feet in height (Figure 3).
The stems are often reddish in color, hairy, and can be either simple or branched from the base. Leaves are arranged alternately along the flowering stems. Mature plants produce flowers that occur singly in the leaf axils, which is the region where the leaves attach to the stems. Individual flowers consist of four yellow or yellowish-red petals that are approximately ½ to 1 ¼ inches in diameter and are fused at their base, forming a long narrow tube (Figure 4). Individual flowers are attached directly to the stems (sessile), although because of the long fused tube it may not appear that way. As mentioned, flowers usually open only in low light situations (evening or night), and petals often fall off the plant within 24 hours of exposure to strong sunlight. The fruit is a capsule that is about ¾ to 1 ½ inches long and can be straight or curved (Figure 5). Capsules are hairy at first but become smooth with age. When the capsule matures, it splits open to expel the seeds within it. Research has shown that cutleaf evening-primrose seed can remain viable in the soil for several decades.
When applied alone, glyphosate (sold as Roundup, Touchdown, and a variety of other trade names) provides only moderate control of cutleaf evening-primrose, which may be one reason why this weed has become more prevalent in no-till crop production fields in Missouri. Effective control of this weed will only be achieved when glyphosate is mixed with an effective tank-mix partner like dicamba (sold as Clarity and a variety of other trade names, 2, 4-D, saflufenacil (in Sharpen, Op-till, and Verdict), and flumioxazin (in Valor, Valor XLT, and Envive). Other research has also shown that paraquat (Gramoxone Inteon) plus 2, 4-D or dicamba will also provide good control of this species. Applications should be targeted to plants that are in the rosette stage of growth, as plants become much more difficult to control as they mature and produce flowers.
REVISED: October 2, 2015