The stems of Euphorbia vandermerwei are usually four-angled. Paired spines angle outwards laterally on small spine shields that are spaced up the winged stem ridges. The spines are straight, hard and thin; in the photo dark at the tips, pale grey at the base. The stem colour, olive green in picture, may also be bright green in cultivation or comfort season, the spines reddish to brick brown.
This plant was photographed growing out in the open in the Walter Sisulu National Botanical Garden in June.