Grasses such as bamboos can produce upright stems more than 30 m tall, yet the processes that constrain plant height in this important group have never been investigated. Air embolisms form commonly in the water transport system of grasses and we hypothesised that root pressure-dependent refilling these embolisms should limit the maximum height of grass species to the magnitude of their root pressure. Confirming this hypothesis, we show that in 59 species of bamboo grown in two common gardens, the maximum heights of culms of 67 clones are closely predicted by the maximum measured root pressure overnight. Furthermore, we demonstrate that water transport in these bamboo species is dependent on root pressure to repair hydraulic dysfunction sustained during normal diurnal gas exchange. Our results established the critical importance of root pressure in the tallest grass species and provide a new basis for understanding the limits for plant growth.