Photo by Jim Jung. All rights reserved.

Dodder

Dodder (genus Cuscuta) is in the Morning Glory family and has an interesting life history. Its common names make it a good choice for a feature in the month of Halloween since it's also known as strangleweed, witches' shoelaces, devil's guts, and hellbine.

It's a parasite, a plant which makes its living robbing water and nutrients from other plants. As such dodder produces no leaves, has no chlorophyl and is a sickly yellow color. Like all plants it begins life as a seed, putting down roots and producing a shoot. But within days the shoot attaches itself to another plant, producing rootlike organs called "haustoria" that penetrate the vascular system of the host plant and siphon off its nutrients for the use of the dodder, like a permanently attached vampire or leech, sucking the blood of its victim. Dodder then severs its connection to the soil as its roots atrophy and wither away. In due time its leafless orange or yellow stems produce dainty white flowers, which produce the seed that will repeat the cycle.

There are ten species of Dodder in Illinois, and they all look and behave pretty much alike. I first encountered dodder when I began working at a nursery in Carbondale in the early 70s. A customer had made a frantic call that her garden was being taken over by yellow string. My boss at the time made a house call and brought some pieces back to show us. It was too early in the year for it to bloom, so it did indeed look like string.

It's not uncommon for someone to live in our area for years and never notice dodder, although I see it most frequently along the shores of Crab Orchard Lake and Pine Hills, where its dense, stringy mats cover the shoreline vegetation. It also turns up now and then in people's flower and vegetable gardens.

Some species of Dodder occurs in 49 states (it's absent from Alaska). Its seed is small and difficult to clean from desirable seed and it can stunt important crops that it finds especially yummy (such as alfalfa or cranberries). Because of this, the USDA lists dodder as a federal noxious weed and it is similarly proscribed by at least 14 states, although Maine and Florida exclude native species from the proscribed list (the USDA also looks the other way at some specified species.)

Control of dodder is two-fold.

1. Remove it when you see it. You can try to prune it out of the infected plant, but cut well below the visible dodder as its haustoria penetrate into the tissues of its host. It is most reliable to destroy the plants it infects, or to kill both the dodder and the host plant with any commonly used herbicide. Dodder is an annual, but it is possible for a sort of natural root division to occur when the dodder plant dies but the haustoria inside the host plant survive the winter and then resume vigorous growth the following spring.
2. Apply a pre-emergent herbicide such as DCPA( Dacthal) the following Spring to prevent any seeds that may be present in your soil from developing into new plants. Dodder germinates in April and May in USDA zones 6 & 7. Either repeat this for several years or keep a close watch on the infected area, for dodder seeds can lie dormant for at least five years.

If it comes back anyway, plant a dodder-resistant crop like wheat and/or something "disposable" such as tomatoes from seed that will entice the dodder once it germinates and that you can sacrifice along with the new dodder until your area is clean.

I've never had to deal with dodder in my own garden (although one year Jim tried to establish a colony in our yard. It stayed small in our grass (not its favorite host), lasted for three years and eventually died out.) I'd always assumed that the seedlings just sort of fell over another plant and glommed onto it. At most, I thought that maybe, like many vines, they grew toward the darkness cast by their future host, be it a blade of grass or the stem of a bush. But recent research conducted at Pennsylvania State University has shown that dodder has a more interesting behavior. It seems to smell out its prey.

Researchers* sowed dodder near possible things for it to be attracted to. They discovered that, rather than flailing about randomly until runs into something, it can locate its victims. In the first part of the experiment, they discovered that if dodder germinates near a tomato plant, 80% of the seedlings will head for the tomato. When germinated near a vial of tomato extract and a control vial, they headed for the tomato extract. The seedlings would grow toward wheat in less striking numbers, and if given a choice between wheat and tomatoes, would grow toward the tomatoes, and further research revealed that wheat gives off a chemical that repels dodder. This in turn suggests that we may be able to protect valuable crops from infestation by dodder by either breeding them (or engineering them) to produce dodder-repelling compounds, or by spraying them with the appropriate scent.

Sort of like hanging up garlic to repel vampires?

*The research on dodder "smelling out" its victim was done by Justin Runyon, a graduate student working with Consuelo De Moraes and Mark C. Mescher, as reported in Science in Sept. 2006. I was made aware of it by an AP report by Randolph E. Schmid, "Parasitic weed seems to smell its prey."

Offsite Links:

• Go to Science Magazine's homepage and search for "dodder" for abstracts on the research refered to above, specifically "Parasitic Weed uses chemical cues to find host" by Elizabeth Pennisi.
• Cuscuta L. entry in the PLANTS database of the USDA Natural Resources Conservation Services
• USDA noxious weeds program
• The Dodder page on the Study of Northern Virginia Ecology website presents information about the plant and provides excellent pictures.