Neckera douglasii is mostly epiphytic, found growing on the trunk and branches of trees in coniferous or broad-leaf forests.
Neckera douglasii is a relatively large pleurocarpous moss with pale green to yellow green, flattened (complanate) shoots that may be regularly or irregularly pinnately branched. Plants can form dense mats.
The leaves of N. douglasii are broadly-lanceolate, ecostate and undulate. Their apices are narrowly-pointed, and toothed.
Neckera douglasii is dioicous. Pictured below is a close-up of a perigonium from a male plant.
The leaf cells are typically oblong to elongate in shape.
The sporophytes of this species are frequently abundant (). They are typically pale red-brown in color and can be seen growing from the the undersides of shoots.
The sporangium is generally oblong, smooth and shortly-exserted.
The seta is generally 4-5 mm long.
The spores of this species are papillose.
This species is most likely to be confused with Metaneckera menziesii, with which it shares the same general habit and appearance. With some practice, the two species can be readily distinguished in the field. Neckera douglasii tends to paler green in color (versus brownish green in Metaneckera menziesii), more strongly complanate and never has fragile branchlets, nor paraphyllia. The sporophytes in the former are also exserted, and the leaf apex is narrowly-pointed rather than broadly-acute as in Metaneckera menziesii. Neckera pennata may also be confused with Neckera douglasii, however, that species is more glossy than the latter and has immersed sporophytes. It is also much less common than either N. douglasii or Metaneckera menziesii.