This past week I received several inquiries about dodder control and so, felt this would be an appropriate time to describe this plant and how to control it.
Dodder or strangle-weed is an annual parasitic plant. There are 8 species found in Iowa. All are native, separated only by subtle flower and fruit differences. Because it lacks chlorophyll, the plant is a yellowish color. The lack of chlorophyll also means it cannot produce any food, thus it produces small suckers that attach to a host plant in order to take what nutrition (food) it needs for growth. It appears to be a stringy mess because of its narrow, twining stems and leaves that have been reduced to thread- like scales. Numerous, compact, white flowers appear in late summer. The 2-celled fruit capsules burst open to release 2 to 4 seeds, which is the only way the plant reestablishes itself each year. Seeds germinate on the soil surface in early spring. The resulting plant is a 2 - 4 inch long thread-like plant with a small root system. Once this plant attaches itself to a (host) plant the root system disappears and the dodder becomes completely dependent on its host.
Unfortunately, there is little that can be done once this plant has wrapped itself around its host plant, especially if its your prized flower or some other desirable ornamental or herb. Pulling the dodder from its host is often futile and usually results in the stripping of the green leaves, branches, and stem tips of the ornamental. However, if you want to prevent seed production, this type of control is necessary and must be done early (before flowering).
Chemical control is extremely limited because of dodder's parasitic nature. Only Dacthal is labeled for the preemergence control of dodder. This herbicide is labeled for weed control with a number of woody and herbaceous ornamental species, but there are some sensitive species (i.e. pansy), so make sure you read and follow the label before making an application. Note: this article was written in 1992. Dacthal may no longer be available.
Dodder seeds are fairly long-lived in the soil, so do not expect elimination of this weed in one year, unless you prevent seed production the first year it infests an area. A few weeds in a landscape is often tolerated, but if it's one like dodder, relying on your ornamentals for food, it needs to be controlled.
This article originally appeared in the August 12, 1992 issue, pp. 138-139.