A man-made vitreous fibre (MMVF) plant growth substrate is provided together with an irrigation strategy for providing water to the substrate. The substrate comprises a slab having a volume of 3 to 20 litres on which a single block containing one or more plants is disposed. In the context of this substrate, it is found that an irrigation strategy can be used which comprises a first period during which the desired water content in the slab is reduced to a minimum level before increasing and a second period during which the desired water content is maintained at a substantially constant level, wherein the difference between the minimum level and the constant level expressed as a percentage of the water content required to saturate the plant growth substrate is less than or equal to 25%. This provides excellent control over the balance between vegetative and generative growth in the plant.