Carrot family (Apiaceae)
Description: This biennial or short-lived perennial plant is 3-6' tall, branching occasionally. The stout stems are terete, glabrous, and sometimes glaucous; they are pale green, pink, or reddish purple, often with prominent longitudinal veins. The lower portion of the central stem is hollow. The compound leaves are odd-pinnate or doubly odd-pinnate; they alternate along the stems. The lower compound leaves are up to 1½' long and ¾' across; the upper compound leaves are much smaller. Each division of a compound leaf typically has 3-7 leaflets. The bases of petioles are partially enclosed by their sheaths; otherwise they are similar to the stems in appearance, although more slender. The glabrous leaflets are 1½-4" long and ½-1¼" across; they are oblong-elliptic with wedge-shaped bottoms, tapered tips, and dentate margins. Sometimes the leaflets fold upward along the length of their central veins. Leaflet venation is pinnate. The lateral veins of leaflets extend to the notches between the teeth, rather than to their tips, along the leaflet margins.
The upper stems occasionally produce compound umbels of small white flowers. These compound umbels are up to 6" across and consist of 10-20 umbellets. Individual umbels are dome-shaped on top, rather than flat. Individual umbellets have about 12-15 flowers that are clustered together. Each flower is about 1/8" across, consisting of 5 white petals, an insignificant calyx, 5 white stamens, and a divided style. The tiny petals are constricted at their bases, and they have notched tips. The blooming period occurs during mid-summer, lasting about 1 month. The flowers have a slight fragrance that is sometimes detectable. Afterwards, each flower is replaced by a small angular fruit containing a pair of seeds. The root system consists of several fleshy roots at the base of the plant; they are ovoid or oblongoid in shape. These fleshy roots are exceptionally poisonous; the stems and foliage are somewhat less poisonous. This plant spreads by reseeding itself into neighboring areas.
Cultivation: The preference is full to partial sun and wet to moist conditions. Some standing water is tolerated, if it is temporary. Either loamy or sandy soil is acceptable to this plant; it should contain some organic material to retain moisture. Foliar disease isn't a significant problem for healthy plants in an appropriate location.
Range & Habitat: The native Water Hemlock has been observed in nearly all counties of Illinois (see Distribution Map); it is occasional to locally common. Habitats include moist open woodlands, swamps, wet prairies, prairie swales, marshes, seeps, and roadside ditches. Water Hemlock prefers moister locations than the introduced Conium maculatum (Poison Hemlock), and so these two species rarely compete with each other for the same ecological niche. It is not uncommon to find Water Hemlock growing where Iris virginica shrevei (Blue Flag Iris) also occurs.
Faunal Associations: The exposed nectar of the flowers attract primarily insects with short mouthparts. These floral visitors include leafcutter bees (Megachile spp.), Halictid bees, cuckoo bees (Sphecodes spp.), plasterer bees (Colletes spp.), masked bees (Hylaeus spp.), Sphecid wasps, Vespid wasps, Tiphiid wasps, spider wasps (Pompilidae), velvet ants (Mutillidae), cuckoo wasps (Chrysididae), Eucoilid wasps, Braconid wasps, soldier flies (Stratiomyidae), Syrphid flies, thick-headed flies (Conopidae), Tachinid flies, flesh flies (Sarcophagidae), Muscid flies, and miscellaneous beetles (Robertson, 1929). The larvae of a butterfly, Papilio polyxenes (Black Swallowtail), feed on the foliage of Water Hemlock, while the larvae of a moth, Epermenia cicutaella, feed on the flowers and immature fruits. Several aphids suck plant juices from this plant, including Cavariella aegopodii (Carrot-Willow Aphid), Cavariella pastinacae (Parsnip-Willow Aphid), and Hyadaphis foeniculi (Honeysuckle-Fennel Aphid). Other insect feeders include Apion pensylvanicum (a weevil), Orthops scutellatus (Carrot Plant Bug), Paroxyna atlantica (Atlantic Grasshopper), and Paroxyna clavuliger (Olive-Green Swamp Grasshopper); see Pepper (1965), Blackman & Eastop (2013), Majka et al. (2007), Wheeler et al. (1983), and Harms & Grodowitz (2009). The toxic foliage and roots are usually left undisturbed by mammalian herbivores, although cattle and other livestock sometimes eat this plant with dire results. The fleshy roots are especially toxic: just a small piece can be fatal.
Photographic Location: The photographs were taken of plants in a wet prairie along an abandoned railroad in Champaign County, Illinois. There was 2 ft. of standing water around the base of the plants as the result of a recent heavy rainfall, although at other times the site is merely moist.
Comments: Water Hemlock is a reasonably attractive and eloquent plant, while the flowers provide nectar to many beneficial insects. It is fairly easy to distinguish Water Hemlock from other members of the Carrot family because of its double compound leaves and rather large leaflets that are rarely lobed. Many other members of the Carrot family have only simple compound leaves, or their leaflets are much smaller in size, or their leaflets are deeply lobed. On Water Hemlock, the lateral veins of the leaflets extend to the notches between the teeth, rather than to their tips, along the leaf margins. Apparently, no other member of the Carrot family in Illinois has this characteristic.