Published in the Contra Costa Times on July 22, 2006

Q. I have a fennel plant that is being smothered with yellow thready stuff that looks kind of like silly string.  How do I get rid of it?

A. You are describing Dodder, a parasitic annual plant that infects many different kinds of plants. There are over 150 species of Dodder, Cuscuta spp.,  found all around the world, and all of them are plant  parasites.  There are a least eight species of Dodder native to our region, however a new form from Japan has been found in the county, and it has a much larger host range. Dodder is spread by people when soil is moved, including soil on shoes, tools and tires. Each plant can produces thousands of seeds, only a bout 5% of which will germinate in a year. The seeds have a hard thick coat that enables them to survive summer heat.  The thread-like wiry stems emerges from one of  the germinating seeds  in the spring, and rapidly twines itself around a host plant. There are no visible roots or leaves. The stems live on resources stored in the cotyledons ( seed leaves) until it has penetrated the tissue of the host plant and has access to the nutrients and water stored within. Dodders have limited photosynthesis ability, as all the resources they need are taken from the host plant. As dodder plants grow, they continually reattach to the host. When other suitable hosts are nearby, dodder shoots spread from host plant to host plant often forming a dense mat.

 You want to remove all plants showing any sign of dodder infestation. It is not currently recommended to compost the dodder infested host, even if you have a hot compost pile. Double bag the pulled plants in plastic, and leave in the full sun for up to a week to kill any forming seeds. Check your garden regularly, pruning off any sign of infested plant material The following is a list  of host plants susceptible to the native dodder species. The new Japanese Dodder can use most broad-leafed plants as hosts..

Some Common Ornamental and Vegetable Plants Susceptible to Dodder Infestation.

asparagus

beet

carrot

eggplant

garlic

melons

onion

pepper

potato

sweet potato

tomato

chrysanthemum

English ivy

fennel

impatiens

marjoram

mint

morning glory

periwinkle

petunia

summer savory