How long can Welwitschia mirabilis leaves grow?

The permanent pair of straplike leaves of Welwitschia mirabilis grow continuously throughout the plant's life. While they are typically kept at a length of about 3 metres (nearly 10 feet) due to erosion from hot sand at the tips, they can grow significantly longer under certain conditions.12

One giant specimen was reported to have a leaf with a total length of 6.2 metres (20 feet), of which 3.7 metres (12 feet) consisted of living tissue. These leaves are perpetuated by a basal meristem at a rate of 8 to 15 cm (3 to 6 inches) per year. Over time, the leaves often become split and frayed, which can make them appear as though the plant has many leaves rather than its original pair.1