The young fruit bodies just prior to opening are found near the surface of the ground, and are rounded, with or without an umbo, and attached to the substrate with a basal mycelial tuft or cord.

The fornicate exoperidium (outer peridium) has the upper, arched part (fibrous and pseudoparenchymatous layers) split to about the half-way point or more into 4–7 rays.

The mycelial layer has a beige brown to somewhat yellowish-brown, felted to tufted outer surface, darkening to reddish-brown if moist.

The endoperidium (inner peridium) is more or less smooth to the unaided eye, but densely protruding light to grayish-brown hyphal tips are present.

Scanning electron microscopy has revealed that the spore surface is covered with column-like processes, up to 0.45 μm high, that may be more or less confluent.

[6] Geastrum welwitschii is morphologically similar to G. fornicatum in having fornicate, mostly 3–6 rays, of exoperidia and a cup-shaped mycelial layer.

[6][7] Geastrum welwitschii also has similar morphological characters to G. minimum in having small fruit bodies, whitish spore sacs, and fibrillose peristomes.

[9] According to mycologists Hemmes and Desjardin, the most common earthstar in the coastal Casuarina forests of Hawaii is a species "closely allied" with G. welwitschii, which they name Geastrum aff.

[nb 1] It differs from the main species in its much coarser pyramidal warts on the exoperidial surface, a sessile and sac-shaped endoperidial body, and smaller spores.