 A stone mulch can significantly increase crop yields in arid areas . This is most notably the case in the Canary Islands : on the island of Lanzarote there is about 140 millimetres ( 5 @.@ 5 in ) of rain each year and there are no permanent rivers . Despite this , substantial crops can be grown by using a mulch of volcanic stones , a trick discovered after volcanic eruptions in 1730 . Some credit the stone mulch with promoting dew ; although the idea has inspired some thinkers , it seems unlikely that the effect is significant . Rather , plants are able to absorb dew directly from their leaves , and the main benefit of a stone mulch is to reduce water loss from the soil and to eliminate competition from weeds . 
