The mature stems (culms) are thick and square in shape with clusters of sterile branches at the nodes. The young culms look very attractive with dark green, soft and fluffy looking foliage.

Even though most restios adapt very well to a regime with little or no watering during summer, Restio quadratus has to have regular water to look at its best.

This is a plant for large gardens because the plants are not only tall, up to 2 m in height, but can also easily attain a diameter of 1.5 m at ground level. The stems are upright and slightly curved at the top, where most of the clusters of sterile branches are situated. The flowers are small and white but the female plants provide more of a show than most restios, as they flower profusely for a short, concentrated time in July or August. 

The mature plants of Restio quadratus look very similar to Calopsis paniculata, but can easily be distinguished by the square stems. These two species, although from two different genera, do not only look very similar with their spreading habit and nearly the same height, but also flower and fruit at more or less the same time.Apart from the stems, the other big difference is in the shape of the seed. C. paniculata seed is a very small nut in an indehiscent ovary, whereas the seed of R. quadratus is much larger, a small, barrel-shaped, dark brown nut, dropping out of the inflorescence as soon as it is ripe.