Thuidium delicatulum, like Thuidium assimile, looks like a rather untidy and diminutive form of Thuidium tamariscinum. The branches are generally not as complanate as Thuidium tamariscinum, and the plant is usually smaller, but it can form good sized patches under optimum conditions. It is not so strictly calcicolous as T.assimile, and tends to prefer moister habitats, even those that are quite acid. It is typically a plant of damp grassland where the turf is short, and the edge of flushes.
In practice, a plant that doesn’t look quite right for T.tamariscinum should be checked microscopically. If the apical cell of the ultimate branch leaves is short with a single papilla then the plant is T.tamariscinum, which can look very much like the others if growing in exposed and drier conditions. The stem leaf apex of T.delicatulum is often a single long apical cell without the row of single cells below. Not all the stem leaves will have this long apical cell, but the apex does not show the long row of uniseriate cells that give T.assimile its attenuate apex. There is always the possibility of T.recognitum in basic upland habitats so it is worth keying out formally.Read the Field Guide account