Q. There is a weird thread-like thing creeping over and growing into the stems of our English ivy groundcover that we have identified as a parasitic plant known as dodder or hellbine. It looks as if someone has thrown a yellow fishing net over our ivy, and the 'net' grows back very fast when it's removed. An Extension Bulletin from the University of Florida seems to say that all we can do is cut the ivy off close to the ground, and hope that the ivy grows back but not the dodder. Do you know anything better? And I haven't found any reference to its existence this far north. Should I be reporting it to somebody? It would wreak havoc with a cash crop!
- ---Sylvia; near Wilmington Delaware
But that's about the only affection involved with this pest. Dodder is a true parasite that lacks the ability to make its own chlorophyll, so it attaches itself to other plants to steal theirs. It begins life normally, as a seed that germinates in the soil. The brightly neon-colored stem that emerges will die quickly if it can't reach another plant. But if it does find a victim, it attaches itself to the poor plant, breaks away from its original root system, and digs into the host with tiny root-like structures called haustoria. Then it flourishes as it sucks moisture and nutrients from the host.
Although I've never seen it in person, it's apparently been observed just about everywhere. We ran a photo of it back when I was the editor of ORGANIC GARDENING magazine in the '90s, and over 400 readers wrote in to say that they had seen and/or battled it. We got reports from Quebec, upstate New York, Missouri, Alabama, North Carolina, Hawaii and Guam (where it was—and probably still is—a serious pest of farms and gardens). An Extension Agent from Cornell explained that there are three different species of the plant in the U. S. alone.
Getting rid of it by cutting the ivy back is, unfortunately, probably a little too optimistic at this point. Once a plant is infected with this vampire vine, you need to destroy the vine and the host plant immediately—before the dodder can produce its tiny white, pink or yellow flowers. Those flowers quickly produce seed pods—typically from late spring through early summer—and if the reddish brown seeds are allowed to develop and drop to the ground, you're in deep doo-doo, because more dodder will then sprout up right alongside any surviving ivy. The seeds aren't bothered by winter weather, and remain viable for many, many decades.
No matter what the timing, that existing ivy—like any plant dodder digs into—is a lost cause. As you quickly learned, pulling the vine off an affected plant doesn't help, as every little bit of haustoria that remains in the parasitized plant tissue can resprout. Again, when dodder appears, the best answer is to act quickly and remove and destroy the infected plant. If you're certain it didn't flower and set seed, you can replant right away. But the time of year you emailed us—mid-July—means that seeds have probably already formed and dropped.
My advice is to carefully mow the ivy down as low as you can get it. Don't spread the infected plants around with a weed whacker or the dodder will dig into other hosts. Collect every last piece of ivy, and dispose of it in the trash.
Then you have two choices. One is to spread corn gluten meal on the soil, wet it thoroughly and allow it to dry out; with any luck this natural pre-emergent herbicide will destroy the dodder seeds as they attempt to germinate. Your other option is to get a propane powered flame weeder and torch the soil several times on hot and dry days to destroy the seeds.
DON'T till the soil to remove the plants or to ready the ground for replanting—you'd be sure to plant some dodder seed.
The next move is up to you. I would personally install some sacrificial plants, water them well and wait to see if they get doddered before I replaced the ivy. If you feel sure you waged enough war against the wicked weed, you can try planting new ivy this fall.
Yes, this advice is extreme in the extreme, but you really have no other good options. Chemical herbicides would kill the ivy as well as the dodder—and might not be as deadly to the dodder. And you'd still have the seeds to deal with.
And listeners—let this be a warning to allayouse! Dodder can show up anywhere, at any time, and you must be prepared to act quickly and decisively when it does. If one of your plants suddenly appears to have been attacked by Spider-Man, carefully remove it and trash it. If you get to it before the flowers appear, you lose a plant. But when dodder is allowed to flower and set seed, you lose the season—and perhaps many seasons to come.