Flower:

Flat or dome-shaped clusters (umbels) at branch tips and arising opposite the leaves on the upper half of a stem. Umbels are 1 to 2 inches (3 to 5 cm) across, made up of 6 to 16 groups (umbellets) of about 20 flowers each. Flowers are up to 1/8 inch (2 to 3 mm) across with 5 white petals notched at the tip, 5 long white stamens alternating with the petals, and 2 long white styles atop a disk-like, greenish-white base in the center.

Umbellets have up to 8 spreading to descending bracts at the base that are very narrow, taper to a pointed tip, and about as long as or somewhat longer than the flower stalks. Umbels usually lack bracts, or occasionally have 1. Stalks are light green, hairless and ridged.

Leaves and stems:

Leaves are alternate, the lowest leaves largest, twice compound (rarely thrice), generally triangular in outline, up to 12 inches long and 8 inches wide, becoming smaller and less divided as they ascend the stem. On largest leaves, the lowest branch pair commonly has 5 leaflets, the second lowest pair 3 leaflets, with 5 leaflets at the tip. Uppermost leaves may only have 3 leaflets. Leaflets are up to 2 inches long, generally lance to egg-shaped with pointed tips, toothed, sometimes with 1 or 2 shallow lobes, mostly hairless. The main stalk between leaflets is broad, ridged, and somewhat flattened.

The base of the leaf stalk becomes a sheath that clasps the stem. Stems are hollow, branched, hairless, ridged, erect to prostrate.

Plants can form dense colonies from horizontal, above ground stems (stolons) that root at the nodes. Stems are easily broken and fragments can take root where they touch the ground, increasing its chances to spread far and wide.

Fruit:

Fruit is oval to nearly round, about 2 mm long, hairless and ribbed, with the styles persisting at the tip. When mature, the fruit splits into 2 seeds.

Notes:

Water Celery, also commonly known as Java Water Dropwort, is a new introduction to Minnesota, first recorded in 2016 in the DNR-managed trout ponds of Le Sueur County. Two years later it was found establishing in the Minnesota River Valley floodplain in Hennepin County, from where it could easily spread downstream to the Mississippi River and beyond. Currently listed as a Prohibited species in Wisconsin, it is not yet on the Noxious Weed list for Minnesota but likely will in the future, due to its floodplain habitat with the ability to spread along waterways, plus its ability to reroot from stem fragments that contain nodes. The Le Sueur County populations are being managed, but as of this writing those in Hennepin County are not and pose the greatest risk. Seems to me it should be a candidate for “early detection, rapid response” but that horse may have already left the barn.

It most closely resembles (the also invasive) Goutweed (Aegopodium podagraria), which has leaflets all in groups of 3, and neither umbels nor umbellets have bracts at the base. A cultivar with variegated leaves is available in the nursery trade. I noticed one online nursery promoted this as “less invasive than Goutweed” which is not very comforting. This species has a long history of culinary and medicinal uses. It is also known to absorb excess nutrients and pollutants to improve water quality, though there are native plants that perform this function just as well if not better. In any case, I don't believe any of that is justification for letting it run free in the wild in Minnesota.

Note that there is a very similar native species, Pacific Water-dropwort (Oenanthe samentosa), native to the Pacific Coast from California to Alaska. The available descriptions do not note distinctions between it and O. javanica, nor can I find a key that includes both species, but from the images I've seen online, the umbellet bracts are a bit wider and flower clusters more dense, though they may not be consistent differences.