Herbage production on many shallow Mediterranean montmorillonitic soils is severely restricted by phosphorus deficiency. A single addition of phosphorus fertilizer consistently increased herbaceous plant production in a community dominated by the dwarf shrub Sarcopoterium spinosum by a factor of 2.8 to 3.8 over 7 years. Even where phosphorus has been added and average annual rainfall is greater than 800 mm, inter-annual variation in biomass production remains relatively high (C.V. equal to 0.33). This variation (year effect) is represented in this study by an effective growing day index, G, based on an estimate of the number of days during which the available moisture in the rooting zone can balance the current evaporation demand. This approach was tested on data from a field experiment conducted for 7 consecutive years on a Sarcopoterium spinosum dwarf shrub community in the western Galilee of Israel. The index G was found to be highly correlated with biomass production of the herbage patches growing between the shrubs.