We have planted a Cortaderia Rosa pampas grass in our garden. But it does not seem to be growing very quickly. What could the possible reasons be for this? We have had the grass in our garden now for at least a year and it is only about 18-24 inches tall.
Check the culture - well drained soil, enough moisture, full sun, etc. - to make sure the plant has what it needs. Then consider the cultivar; 'rosa' is a dwarf variety, and may take several years to reach its full potential. https://www.gardeningknowhow.com/ornamental/foliage/pampas-grass/growing-pampas-grass.htm