Fruit bodies erect, tough, pliant, clavarioid in shape, usually branched near the top, occasionally simple, up to 8 cm tall by 3-5 mm broad, often flattened in cross section above, rounded below; the base dark brown to black, often tomentose, branch tips white from asexual spores (conidia) or concolorous with the base and minutely pimpled with perithecial pores.
Ascospores 10-14 X 4-6 µm, black, smooth, kidney shaped. Asexual spores hyaline, smooth, elliptical to elongated.
Scattered to gregarious to clustered on rotting wood.
Xylaria hypoxylon can be found year round in California on downed wood. Fruiting bodies are blackish, thin, wiry and branched with white tips. The white tips consist of masses of asexual spores (conidia). As the fruiting body matures, it thickens, becomes all black, and sexual (ascospores) are produced in embedded perithecia. The latter form tiny pores on the surface of the fruiting body. Critical study of this species is needed in California, as there may be a number of species of Xylaria masquerading under this name, or the species may be extremely variable.