Young trees display an upright pyramidal shape with yellow to light-green foliage on their swooping branches. Khaki-coloured adult foliage is scale-like, barely recognisable as leaves. Lower branches drop off as the tree gets older and the crown rounds out substantially. In high rainfall hill country where trees are more sparse, crowns become enormous and the trunks develop fluting towards the ground. Such trees frequently harbour botanical communities in the large lateral branches of their crowns including epiphytes, broadleaf, orchids, and kamahi. In the North Island Rata often entwines itself around Rimu, extending its root system to eventually encompass the entire trunk.
The bark of large Rimu breaks up and drops off in sizeable frangible chunks leaving characteristic patterns on the trunk.
Flowers develop at the tips of branchlets, and in female trees these branch tips turn up, creating a feathery aspect. The male tree usually exhibits the drooping form associated with young Rimu. Male catkins are upright, green and inconspicuous. Female flowers are wind-pollinated from October to January.
The small oval dark brown nut-fruits ripen from March to May the following year and are mounted on bright juicy red cushions which kereru, bellbirds and tui adore as an autumnal feast. Birds are the main method of seed-dispersal. Good seed years are often 2-3 years apart but in a mast year a large Rimu can produce as many as 200,000 seeds.
Rimu are found all over the country in lowland to montane forest, and while they thrive on rich, moist soils with shelter and shade, they also tolerate infertile poorly drained soils. D.cupressinum is a regal specimen tree well suited to parks, large open gardens, and also in native regeneration plantings in sheltered gullies.