Isn’t that pretty?

It’s also deadly.

Umbellifers can be tasty, and they can be poisonous. And they can be both. The deadliest plant in North America is a native umbellifer, and the notorious European native pictured here is widespread in the Maryland piedmont. This specimen was right on the Billy Goat C trail.

This is Conium maculatum, aka poison hemlock. It’s found throughout the US (except Mississippi and Florida) and most of southern Canada, and is on the noxious weed list in eight states. It likes moist soils in disturbed areas and can grow to eight feet tall in its second year (it’s a biennial). In young plants all you’ll see is a basal rosette of leaves.

Most of the brown material in the above photo is poison hemlock plants that are dead or dying after setting seed (there’s a still-flowering plant on the left side, but you have to zoom way in to see it).

There are five related alkaloids present in poison hemlock, but the one that occurs in greatest concentration is coniine, a peripheral nervous system disrupter. It doesn’t take much coniine to kill an adult human (The LD₅₀  (mice) is in the range of 10 milligrams per kilogram of body weight). Remember that umbellifers often have hollow stems? There are documented instances of children dying after blowing across the tops of cut poison hemlock stems.

North America’s deadliest plant is Cicuta maculata, the spotted water-hemlock. It grows in wet areas all over the US and Canada, including the Maryland piedmont. The plant looks similar to Conium maculatum but usually the foliage is not as finely cut, and it doesn’t get quite as tall. Despite being a native it’s on the noxious weed list in Nevada. There are three other species of Cicuta native to North America; of these C. bulbifera, bulblet-bearing water hemlock, has been recorded in Montgomery and Cecil counties, but it is listed as Endangered/S1 (highly state rare). The poisonous compound in Cicuta species is called cicutoxin. It’s a central nervous system disrupter that usually causes death by respiratory failure. The LD₅₀ (mice) is 2.8 mg/kg.

(By the way, the specific epithet “maculata” or “maculatum” means spotted.)

There are a few other species of umbellifers found in parts of the US that have similar toxicity, but none of those are found in Maryland.

For the most part the poisonous compounds found in umbellifers fall into two broad categories: systemic poisons and phototoxins. When a phototoxin – say the sap from an umbellifer – gets on the skin, nothing happens. Until the skin is exposed to sunlight. Then there is a reaction that renders the skin highly photosensitive, so that sun exposure causes burns. Second degree burns, even. And the resulting skin discoloration and scarring can last for months or be permanent.

Wondering which plants have phototoxic compounds? It’s an interesting list, and includes species in a number of families, including the citrus family. Depending on which authority you consult, most of the vegetable and spice umbellifers at least have the potential to cause such a reaction. This includes anise, carrots, celery, chervil, coriander, dill, fennel, lovage, parsley, and parsnip. Usually the reaction isn’t bad, but it’s a good idea to wear gloves and long sleeves when handling them in the garden.

Interestingly, the celery reaction only happens when the celery has been infected with a particular species of fungus.

Among the phototoxic umbellifers, none looms as large as the giant hogweed,  Heracleum mantegazzianum. I mean that both literally and figuratively. Literally, because this Caucasian native can grow to 18 feet tall. It’s a biennial, so that means it grows to 18 feet in a single season. And figuratively, because the sap is exceptionally dangerous. Crews in New York and British Columbia tasked with removing it suit up head to toe to prevent the sap from touching them. This video has a rather silly intro but basically good information. There’s a lot of good information on the New York State Department of Environmental Conservation website.

Giant hogweed is on the federal noxious weed list and on 13 state lists. It’s found in a handful of states in the Pacific northwest, upper Midwest, New England and mid-Atlantic, and North Carolina. The Maryland Biodiversity Project has records for it in Garret, Baltimore, and Harford counties.

Giant hogweed has two cousins present in the US, H. maximum (native) and H. sphondylium (alien), both of which are commonly called cow parsnip. The former is present in the Maryland piedmont, but there are no records of the latter in Maryland. Both species contain phototoxins, but are not nearly as dangerous as H. mantegazzianum.

next time: more flower pix