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A Year in the Life of a Giant Hogweed
title graphic

by Bruce A. Bohm
hogwood
A relative of hogweed (Heracleum mantegazzianum), Cowparsnip (Heracleum lanatum) is the only member of the genus that is native to North America.

Everyone knows what a weed is: it’s a plant growing in the wrong place. Whether it’s in an aquarium, in a potted plant, in a garden, or widespread in a field, it’s not supposed to be there. Weeds vary from the green algal slime on the walls of an untended fish tank, bind-weed (Convolvulus sp.) in a row of pole beans, dandelion or cat’s ear (Hypochaeris radiata) in a lawn, or silk oak (Grevillea robusta) colonizing a slope on Kaua`i. And, as those examples suggest, weeds can vary drastically in size from the unicellular alga to the 10 m tall tree. A single definition hardly fits such an array of organisms. Instead of a technical definition term to define weediness, it is most useful to look at some of the features of plants that enable them to be weeds. Comprehensive discussions of weediness can be found in two classic treatments of the subject by the late University of California, Berkeley botanist, Herbert G. Baker (Baker 1965, 1974).

 No single feature distinguishes a weed. Weedy species, in general, have the capacity to grow in a wide variety of soils, particularly where there has been disturbance through human activity, such as construction sites or in agriculture. Many are wind pollinated (especially the grasses), or if insects are required, the floral display is an open landing area with no specialized features (no species-specific pollination). There is also a capacity to reproduce through vegetative propagation. Weedy species tend to grow quickly, reach reproductive maturity sooner than non-weedy plants, and produce large seed crops over extended periods in their lives, all of which serve to maximize availability of next generation recruits. Many weedy species are particularly well adapted physiologically to their newly occupied territory in that they out compete local species for resources, particularly water, or alter the soil in such a way—change the nitrogen balance in the soil, for example—that natural vegetation is at a disadvantage.

Although the plant featured here, the giant hogweed, does not fit the currently accepted term ‘super weed,’ it is useful to include a brief comment on this comparatively new phenomenon. The term has entered the language over the last generation or so in reference to weedy species that have become resistant to herbicides. Theoretically, overuse of any herbicide can lead to resistance, as is the case with bacterial strains over time becoming resistant to antibiotics. Much of the problem, however, revolves around the use of Roundupä one of the most effective of the glyphosate-based weed killers. These herbicides are used extensively to control weeds in fields of genetically modified crops (e.g., cotton, corn, soybeans) with several applications per growing season not unusual. Weedy species can undergo mutations that render the plants resistant to herbicides resulting in the need to treat with larger and larger doses. In some cases total resistance has been observed.

 Are all weedy species bad? If they are invading otherwise useful areas, or are poisonous to humans or farm animals, or pose special risk as to increase the possibility of fire, then the answer is probably yes. However, let’s not rule out the possibility that a concerted effort to manage a weedy invasive species might be beneficial. I am thinking in particular about an invasive species that lives in my garden, a plant that I did not put there on purpose, and one that requires some effort to ‘control.’ I live with the extremely invasive Himalayan blackberry, Rubus armeniacus. (N.B. There is some debate about the correct name, but the name used here is in agreement with the currently recognized E-flora of British Columbia). However, with assiduous—even merciless—pruning it can be kept under control so that an annual crop of fruit is assured. Tip rooting is a problem but with a wary eye it is possible to enjoy the wonderful fruits of this uninvited guest. I am not so sanguine about the subject plant, however.

Heracleum mantegazzianum, commonly known as giant hogweed, is a native of the Caucasus that was introduced into North America as a garden curiosity owing to its impressive size, both in height and floral display. It can reach 6m in height and has a compound umbel flower head that can span at least a meter. The genus Heracleum, with about 65 north temperate species, is a member of the carrot family, Umbelliferae in the older literature, Apiaceae in most current floras. The only member of the genus native to North America is the cow parsnip, H. lanatum(see above image), which is common in wet meadows and, stream banks. This species can grow to two meters but even the largest ones are dwarfed by its Eurasian cousin.

Many species of plants produce toxic chemicals that serve as a defense against both potentially infectious micro-organisms as well as larger herbivores; the umbels are no exception. One such set of compounds belong to a class of polyphenolic derivatives called furanocoumarins. These compounds consist of coumarin molecule fused to a furan ring. The feature of these compounds that is important in this discussion is their capacity to bind to DNA a reaction catalyzed catalyzed by ultraviolet light. Of particular danger is the capacity to cross-link two strands of DNA thus disrupting their normal activity. Furanocoumarins are also capable of reacting with proteins, a process that can also lead to skin rashes.

Skin rashes can occur when weedy areas are cleared and protective clothing and gloves are not worn. Because the stems of hogweed and its smaller cousins, e.g. cow parsnip, are hollow they are known to have been used by children to make super pea-shooters, etc. This behavior often results in skin rash and blistering on and around the lips. Cow parsnips are a lesser problem, but hogweed toys can be quite dangerous.

 We first encountered giant hogweeds many years ago when a neighbor, removed a number of these plants from his garden. We took one as a novelty so that my wife and I, being botanists, could observe the giant creature a little more closely. As I told a colleague recently, I’m probably the only person in the neighborhood who keeps an aggressive alien plant as a pet. In an effort to prevent spread from my plant—Parks personnel have removed hogweed from all public places in Vancouver—I remove all new recruits in the spring, should they appear, and destroy the seed crop of the current year, with the exception of a few fruits that are planted for the next year’s pet plant.

hogwood
The series of illustrations that follow show some of the features of this plant. Even as a young individual, the array of leaves is quite impressive (Hogweed.jpg).
hogwood
Although not nice to touch, the stems have interesting coloring (Hogweed1.jpg) and can be photographed from a safe distance.
hogwood
New leaves, which are enclosed within a bundle, expand rapidly
hogwood
The terminal bundle consists of the flowering heads tightly packed together.
hogwood
Opening of the terminal bundle is followed by rapid expansion of the terminal floral array and elongation of the satellite stalks and expansion of the satellite flower heads.
hogwood
The terminal flower head and the five maturing satellite heads are seen from above in the next illustration.
hogwood
A close-up view of a fully opened flower head shows the asymmetrical nature of the floral arrangement with flowers in the center of the head characterized by equal length petals while peripheral flowers have distinctly asymmetrical petals with asymmetry increasing centrifugally. That picture also shows a pollinating visitor.
hogwood
The first images were taken during the summer of 2012 with the plant reaching a height of about 11’ 6” by the beginning of July.
hogwood
Owing to the apple tree interference, it was necessary to remove the terminal floral head for closer examination.
hogwood
A photograph of one of the individual flower heads shows a range of maturing fruits.
hogwood
This close-up shows additional detail of the fruits.

  The 2013 terminal head was removed and its parts counted in order to get some idea of the seed production potential of the plant. The terminal head consisted of 58 ‘spokes’ (analogous to the ribs of an umbrella). At the ends of these spokes are the terminal umbels which consist of the pedicels, i.e., the individual flower stems themselves. Eleven terminal umbels were selected at random and their pedicel number determined. With a range of 33 to 65, the average number of pedicels per secondary umbel was 49.4. The seed yield would be 58 x 49.4, or approximately 2,865, if all flowers in the terminal flower head produced fruits. Counting spokes in the four satellite heads yielded 51, for the lowest satellite, and 48, 52, and 53 for the three upper ones. The number of pedicels arising from each of these was estimated to be approximately the same as the terminal head (rough counts showed this to be the case). Theoretically, then, all five of the flower heads, together, could yield a crop amounting to more than 14,000 fruits. The actual yield of fruit was nowhere near that number, but even if yield was only 10% of theoretical, one sees the potential threat of such production of an unwanted alien.

Literature cited…

Baker, H. G. 1965. Characteristics and modes of origin of weeds. In The Genetics of Colonizing Species. H. G. Baker, G. L. Stebbins. eds. Academic Press, New York, pp. 147-172.

Baker, H. G. 1974. The Evolution of Weeds. Annual Review of Ecology and Systematics. 5: 1-24.

August 7, 2013



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