Parasitic plants need their hosts to survive

Tribune – Review / Pittsburgh Tribune – Review, Jul 23, 2010 | by Jessica Walliser

Question: A string-like weed is choking and killing flowers in our garden. Once it gets started, it kills the plants it entwines. We cannot find where it goes into the ground, but assume it must somewhere. The strands appear as tan or straw color, are about the thickness of string or a thin spaghetti strand, and are a similar color. Help — we need your advice on a remedy.

Answer: The weed you are describing is called dodder. It is very unusual plant in that it’s a parasite, feeding on carbohydrates stolen from its host rather than creating its own food through photosynthesis. Dodder has no leaves and only a very minuscule amount of chlorophyll and relies exclusively on its host for survival. A dodder seed germinates only when a suitable host is present; it then attaches itself to the host by inserting small root- like appendages into the plant’s tissue and begins to draw out carbohydrates. Once this attachment is secured, the soil connection shrivels away, which would explain why you could not find where it goes into the ground.

Because dodder needs this host plant to survive, it seldom kills the plants it attaches to, except in cases of extreme infestation. It does, however, weaken the host plant and affect its overall growth, making it more susceptible to attacks from other pests and diseases.

According to the USDA, there are about 47 species of dodder in North America, some of which are native and others introduced. Dodders are annuals that spread by seed (yes, they do produce a flower) so religious removal of this year’s “crop” will help reduce next year’s population. There are no herbicides that are effective against dodder, but dodder can be prevented with the use of pre- emergent herbicides in the garden in early spring before the seeds germinate. These products prevent all seeds from germinating, so don’t use it in garden areas where you want things to grow from seed.

The best control is obtained by judicious removal of the parasite; hand-pulling is the only thing that will rid you of it once it has become established.

It’s actually a very fascinating plant. Dodder recently has been put into its own family (it used to be included in the morning glory family), because it is so unique. Interesting common names include devil’s guts, devil’s hair and strangleweed. It’s not too often that a common name is so utterly appropriate — sorry you had to learn that firsthand.

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