How to Prune Cape thatching reed?
No pruning required, tidy as necessary

Cape thatching reed · Elegia tectorum
Grow in moderately fertile, moist but well-drained preferably acid soil in full sun but tolerates alkaline soils. Grow under glass where temperatures fall below -5°C, in loam-based (JI No2) potting compost in full light. When in growth maintain low humidity and water sparingly in winter
No pruning required, tidy as necessary