This month we on the FCPA EDRR team are on the lookout for non-native invasive Java Water Dropwort (Oenanthe javanica). Based on information from Pennsylvania agencies, B and I thought that we had relocated some extensive populations in a creek floodplain in Herndon, and we documented as such (for example, 166611987). However, other Invader Detectives have identified our plants as Water-hemlock (Cicuta maculata), and I am tending to agree.
I reviewed the descriptions of the two plants in Flora of Virginia. Since they’re in different genera, there’s no straightforward dichotomous key for the pair.
One clue that we had the native C. maculata is that the plants were coming into flower on 10 June. The Flora has May to August for Water-hemlock and July to August for Java Water Dropwort—but that’s hardly definitive for a plant that’s just getting established in the Commonwealth and with voucher specimens for only Fairfax and Arlington Counties. There are some described differences between the two plants with regard to the inflorescence and fruit, but how do you catch the invasive before it flowers?
So we have to go back to descriptions of the stem and leaves. Here’s C. maculata:
Perennial 6-18 dm stout, erect, caulescent, branched, glabrous. Leaves 10-30 × 8-26 cm, 2 or 3 × pinnately compound, ovate in general outline; leaflets 2-12 × 0.6-3 cm, lanceolate, acute to acuminate; petioles 0.4-3 dm [4-30 cm], sheathing.
Comments: Stems often mottled below with purple. [emphasis added]
And here’s O. javanica:
Perennial with fibrous roots. Stems 3-12 dm, decumbent [reclining on the ground but with an erect or ascending tip], rooting at lower nodes. Leaves alternate, sometimes basal with petioles 5-10 cm, blades 3-20 cm long, ovate to triangular, 1 or 2 × pinnately compound, reduced upward, ultimately becoming sessile on expanded sheaths; leaflets 10-50 × 5-20 mm wide, ovate or rhombic-ovate, rounded, narrowed or tapered at base, serrate.
In other words, they’re both pretty typical members of the Carrot Family. Overall, the Water-hemlock is taller, with larger leaves, but there’s a lot of overlap.
Two things that we overlooked in our survey of the Herndon site:
- O. javanica stems are decumbent, whereas the plants in our observations are erect.
- “Brittle stems of java waterdropwort are jointed and hollow and can easily break off and take root.” I don’t believe that we checked our specimens for this character.
Since O. javanica is reported to be edible (it has common names like Water Celery and Vietnamese Parsley), it’s a good thing that we didn’t give our plants a taste!