Gardening Weeds

The Parasitic Morning Glory

Ill.: creazilla.com, pikping & pngitem.com

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.

Common dodder (Cuscuta gronovii)
Dodders wrap around and parasite their host plant. Photo: HLLindsay, deviantart.com

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 IpomoeaConvolvulusCalystegia 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!

Japanese dodder (Cuscuta japonica) covering a tree
Japanese dodder (Cuscuta japonica), a subtropical species, can cover trees. It has become invasive outside of Asia, including in California. Photo: Yuan-Min Shen, Taichung District Agricultural Research and Extension Station, Bugwood.org

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. 

Dodder seedlings reaching for a young tomato plant.
Dodder seedlings reaching for a young tomato plant. Photo: Moran Farbi, plb.ucdavis.edu

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. 

Haustoria starting to pierce a plant stem
Haustoria starting to pierce a plant stem. Photo: OpenCitrusExperience

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. 

Flowers of five-angled dodder
Flowers of five-angled dodder (Cuscuta pentagona). Photo: Mason Brock, Wikimedia Commons

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.

Field infested with golden dodder
Field infested with golden dodder (Cuscuta campestris). Photo: Alexey Sergeev, asergeev.com

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 

Dodder on a coleus
Dodder on a coleus (Coleus scutellarioides). Photo: H R. Wick, UMass

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.

Vegetables and Herbs

Asparagus
Beet
Carrot
Eggplant (aubergine)
Fennel
Garlic
Marjoram
Mint
Melon
Onion
Pea
Pepper
Potato
Sweet potato
Summer savory
Tomato*

Crop Plants


Alfalfa
Bush clover
Chickpea
Clover
Flax
Lentil
Mung bean
Sesame
Soybean
Sugar beet

Ornamentals


Aster
Cattail
Chrysanthemum
Coleus
Dahlia
English ivy 
Ferns
Goldenrod
Helenium
Hyacinth bean
Impatiens 
Mimulus
Morning glory
Periwinkle
Petunia
Trumpet vine

*Some tomato varieties are resistant to dodder.

_________________________

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

Larry Hodgson is one of Canada’s best-known garden communicators. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He has written for many garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 60 other titles in English and French. He is a past president of the Garden Writers Association (now Garden Communicators International) and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. He resides in Quebec City, Quebec, Canada.

8 comments on “The Parasitic Morning Glory

  1. Ann Mackay

    Scary spaghetti! I’m glad I haven’t come across it!

  2. Christine Lemieux

    New to me…..WOW!

  3. Through my entire career, I have encountered this only a few times. All encounters were icky. I found it in the wild south of Saratoga. I could not see how far into the distance it had migrated. However, that was many years ago, and I have not seen it in the region since then. Weirdly, I found it in bedding plants, within a bed that was regularly replanted. I suspect that it came in from the outside, but I can not help but wonder if it came in with the bedding plants. Others have found it in nursery stock too. That is strange because nursery stock should be clean.

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