A strong growing plant with alternate leaves. There is one leaf per node along the stem and the leaves are extended at the base to encircle the node, so that the stem apparently passes through it (perfoliate).

Other rare Bupleurum species may turn up - B falcatum, B baldense and B subovatum have all been recorded in VC55.  B subovatum is an occasional bird-seed alien and is grown in gardens; it is similar to and often mistaken for B rotundifolium. 

Yellow flowers; leaves simple and entire and less than twice as long as wide (B subovatum leaves are mostly more than twice as long as wide).  Fruits are smooth between ridges and umbels have 4-8 rays.  B subovatum fruits are papillose (with small nodules or pimples) and umbels have 2-3 rays. 

A photo of the plant in habitat, plus details of leaves, and flowers/fruits

Thorow-wax was formerly an arable weed of chalk and limestone soils, but it is now a rare bird-seed casual or ornamental escape.

To follow

Annual.

Uncommon with only occasional scattered records in England and Wales.

Rare in Leicestershire and Rutland where there are few recent records. 

In the VC55 Checklist (Jeeves, 2011) it is listed as Alien, archaeophyte; recorded from 20 localities.

It was on the 2011 VC55 Rare Plant Register (Jeeves, 2011) but is not on the current RPR (Hall & Woodward, 2022)