Dodder Cultivation

Cuscuta europaea on a sad Coleus, with flowers and mature fruits.

Dodders (genus Cuscuta) are twining parasitic plants in the morning glory family (Convolvulaceae). Cuscuta plants are holoparasites, which have non-functional scale leaves and have lost the ability to make chlorophyll, and thus are completely dependent upon host plants for all of their nutritional needs. Cuscuta europaea is an annual species that thrives on a wide variety of hosts, and is one of the easiest parasitic plants to cultivate indoors or in a greenhouse.

Cuscuta europaea seeds and one week seedlings.

Dodder has small seeds, and the nearly rootless, filamentous seedlings must attach to a host plant soon after germination, or they will quickly exhaust their resources and die. It seems to be beneficial to scarify the seeds by rubbing them lightly on sandpaper; otherwise germination can be poor. The seeds can be sown on the soil surface next to an appropriate host plant. I've used Coleus hybrids with good results, but C. europaea isn't picky and a wide variety of common garden plants work, including Impatiens, sunflowers and tomatoes. Cuscuta seedlings latch onto smaller, tender shoots more easily than to older host growth, so freshly established Coleus cuttings are a good starting point.

Cuscuta europaea, young seedlings establishing on Coleus.

Only a fairly low percentage of dodder seedlings manage to wrap around a host stem and form haustoria--the nutrient-absorbing connections to host vascular tissues. If successful, the peg-like vestigial root and lower parts of the dodder seedling wither away, while the portion in contact with the host remains alive. The live portion of an establishing seedling spends a week or so as a tiny yellowish ring around the host stem, not putting on much obvious growth, perhaps while the haustoria are working their way into position. After they get organized, though, young dodder plants explode into growth, sending branching stems in every direction and establishing new haustorial connections to their host plant wherever they make contact, and eventually reaching outwards to infest other compatible plants within reach.

Cuscuta europaea, one month old and spreading.

Cuscuta europaea reaches maturity within a month or two of germination, producing clusters of small, pale, non-showy flowers. These are self-pollinating, and almost all of them yield papery fruits containing several viable seeds, even in the absence of any pollination agents. This particular dodder species is short-lived, and begins to slowly decline as the seeds start ripening and the host plant loses vigor. The plants will hang on for six months to a year before dying out completely, assuming they don't kill their host outright, but need to be restarted from seed periodically. Dodder stems trained onto fresh hosts can be severed from the original plant after they have formed haustoria, and these cuttings often seem to be reinvigorated, at least temporarily. Dodders are potential invasives, so old plants with seeds should be disposed of in a way that the seeds will be destroyed or not potentially spread into the environment.

Ironically, C. europaea is itself easy pickings for common greenhouse insect pests like aphids and mealybugs. It's possible to control these with the usual insecticidal soap or appropriate pesticides, but often it seems to make more sense to just start over with a fresh sowing of seeds, given how quickly C. europaea grows. Many aphid species can be controlled with a minute parasitic (technically a parasitoid, since it invariably kills its host. Like Aliens.) wasp, Aphidius spp. Aphidius themselves are sometimes subject to infection with hyperparasite wasps such as Dendrocerus. Parasites on parasites on parasites on parasitic plants on hapless green plants; an appropriate vignette of how the natural world works, to think about during this Darwin Day season.

Memorial Day Bog Walk

Iris versicolor, Northern Blue Flag.

Memorial Day is usually a good time to catch some of the more impressive native wildflowers here in New England. This year, I checked out an unnamed bog at an undisclosed location in Willington, Connecticut. Walking to the bog there were some richer swampy spots where the wild geraniums and blue flag iris were in full bloom.

Cyprepedium acuale, Pink Lady-slipper (this photo actually from Mansfield Hollow State Park).

Closer to the bog, the vegetation changed to indicate the presence of nutrient-poor, acidic, bog-friendly soil. Sugar Maples and White Ash were replaced by pines and oaks as the dominant trees, and the understory started to include Pink Lady-slipper orchids in flower, and one little patch of Epigea repens (Trailing Arbutus).

Ledum groenlandicum, Labrador Tea, in the Willington bog.

After a short but slightly hairy time navigating the inevitable marginal ring of open water and peaty muck at the edge, I emerged into a sunny sphagnum bog. This is a floating bog, though the open parts are mostly dense enough with moss and scrub to provide secure footing. It is still irresistible to bounce up and down a little bit, and watch the dwarf trees within a 20 foot radius sway back and forth.

Ledum, or Labrador Tea, was in full bloom. Labrador Tea is mainly a boreal plant, though it hangs on here and there in bogs in the more temperate parts of the eastern US. The leaves have a distinctive brown fuzzy underside, and can in fact be used to make a somewhat wintergreen-tasting tea. The Willington bog is the only local spot I know of where it grows, and I had never caught it in flower before.

Sarracenia purpurea var. purpurea, the Purple Pitcher Plant.

The pitcher plant flowers weren't quite open yet for the most part. There aren't a huge number of Sarracenia in this bog, though the population seems pretty stable. There aren't any sundews in this bog at all, which is strange; it looks perfect for Round-leaved Sundews at least, and the other area bogs have one or both of the local Drosera species. It's doubly odd because there is an area of seeps not half a mile away where both sundews grow in large numbers.

Picea mariana, Black Spruce, about 1 m tall, with female cones.

Another interesting feature of this bog is a large population of Black Spruce trees, stunted into natural bonsai. Black Spruce, like Ledum, is primarily a boreal plant, which persists in boggy spots and the occasional cold mountain slope in southern New England. There is a tiny mistletoe, Arceuthobium pusillum, that grows on the spruce trees in the Willington bog, but I couldn't find it this time. I seem to recall seeing it once before, years ago when I first visited this location, in later summer, but that time the mistletoe was pointed out by the late Les Mehrhoff, who knew the local flora twice as well as anyone else. It is a shame that Les won't be guiding any more bog walks.

Viscum minimum: a Dwarf African Mistletoe

Viscum minimum fruit emerging from its host.

Parasitic plants, which obtain some or all of their water and nutrients by attaching to a free-living host plant, seem to be unusually well represented in the floras of desert regions. Mistletoes (genus Viscum), probably the most familiar of vegetable parasites, may be encountered in arid places (and elsewhere) in the old and new worlds. Mistletoes are usually hemiparasitic, meaning that they extract water and minerals from their host, but are green and can manufacture some of their own nutritional needs via photosynthesis, like ordinary plants.

Viscum minimum is a minuscule mistletoe endemic to South Africa, where it grows inside of the succulent stems of euphorbias. Most of the time, V. minimum grows internally, with no indication of its presence on the exterior of the host plant. However, when it reproduces, the mistletoe’s greenish flowers and showy orange-red berries break through the euphorbia’s skin, in order for pollination and seed dispersal to take place.

Old plant of Euphorbia polygona ‘Snowflake.’

Viscum minimum is one of the few types of mistletoe that is ever cultivated outside of its native land. The host plant, usually Euphorbia polygona, is an easy to grow and attractive addition to a greenhouse or windowsill succulent collection. The mistletoe, which can be acquired either in the form of an infected euphorbia cutting or by smearing fresh, sticky seeds onto the surface of a suitable host, doesn’t cause significant harm to its host plant, and will produce crops of colorful fruit in time for the holidays year after year.