The Parasitic Morning Glory

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Dodders (Cuscuta spp.) are among the most fascinating parasitic plants imaginable. They form stringy, twisting stems, apparently leafless (although it actually does have tiny, well-spaced leaves in the form of scales), that wrap around their host plant’s stems and steal their nutrients. Most have lost almost any trace of green chlorophyl (or lose it shortly after germination) and so come in gaudy colors like yellow, orange, red or pink. They look like a mass attack of very thin spaghetti.

You would likely assume dodders are fungi, but you’d be wrong. They are, as members of the Convolvulaceae, essentially a sort of morning glory that took a surprising evolutionary turn. Not content with wrapping around its host plant with twining stems in order to reach the sun, the most obvious characteristic of a morning glory (various species of Ipomoea, Convolvulus, Calystegia and others), well known as climbing plants, dodder began producing short aerial roots called haustoria that worked their way into the host, stealing its sugar, moisture and minerals. Dodder will even attack its own brethren: morning glories! Yes, it can be a cannibal!

Dodders are found all over the world, from temperate climates, where they grow as annuals and tend to be fairly small, to subtropical and tropical ones, where they’re perennial and sometimes grow to cover entire trees. There are some 200 species worldwide and they’re not easy to tell apart.

From Seed to Weed

Dodder seed capsules usually produce numerous hard-coated seeds that drop to the ground in the fall. Capable of surviving for 20 years or more, they germinate best on the surface of the soil or when just barely covered by it. Temperate species germinate in spring when temperatures rise above 50?F (10?C). 

Even the seedling is essentially leafless: its cotyledons are only vestigial, so its sprouts already looking like a strand of vegetable spaghetti. And dodder doesn’t dodder: it’s in a hurry. It has only 5 to 10 days to hook up with a host plant and it grows at lightning speed, for a plant that is, up to 3 inches (7.5 cm) a day, even 6 inches (15 cm) for some tropical species. 

It actively hunts for a host. First, the tendril circumnutates, that is, it spins round anti-clockwise about once per hour, lengthening as it goes, until it runs into a stem, then it starts to wrap around it. It can also smell its host and will grow toward ones it likes while ignoring others. If it comes into contact with a dead stem or an unsuitable host plant (temperate species tend to prefer thin-stemmed flowering plants), it will abandon it and move onto another. 

Once it reaches its host, it quickly produces the above-mentioned haustoria (modified aerial roots) that sink into the stem like the fangs of a vampire and begin “drinking the plant’s life blood.” It also loses its rather weak root at this point and becomes entirely epiphytic. And it will leap from plant to plant as it grows, forming the stringy masses mentioned. 

In summer or early fall, depending on the species, clusters of small white, pink or yellow bell-shaped flowers form, then pea-sized seed capsules produce copious amounts of seed (each plant is capable of producing several thousand seeds) that will soon launch a new generation of dodders. Seeds readily cling to clothes and tools as well as birds and animals and are thus easily transported to new sites. Seeds of species found in aquatic environments may also be carried by water.

Then, in temperate areas, the mother plant dies.

Unloved

As you can imagine, dodder is not a great friend of the farmer or gardener, as it damages its host and can lead to crop failure. This has led to all sorts of common names with negative connotations: devil’s guts, strangleweed, scaldweed, fireweed, beggarweed and witch’s hair. 

Besides stealing minerals, carbohydrates and water, dodder can transmit viruses, bacteria and other diseases from one plant to another.

In agriculture fields, dodder has proven very susceptible to herbicides, both preemergent and non-selective, and is becoming rare in some areas. 

In the home garden, dodder is best removed by culling dodder seedlings as soon as you see them. Once they’ve attached to their host plant, they can be difficult to eradicate, as they can regrow from haustoria left on the plant. It’s often necessary to cut the host plant back severely to control dodder. Soil solarisation has proven ineffective against dodder. However, replacing host plants with species it can’t live on (grasses, for example), works very well. 

Do not put dodder in the compost pile, at least not when seed formation has started. Put it out with the trash in a sealed plastic bag.

Plants Susceptible to Annual Dodder 

The following plants have proven susceptible to dodder species in temperate climates. The list would be much longer in tropical areas where shrubs and trees area also affected. And to give dodder it’s due, it also does attack and help suppress species considered to be weeds, like nettle and ragweed.

*Some tomato varieties are resistant to dodder.

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Dodder: it may be a nasty weed, but it’s truly a fascinating plant!