Grey plants are usually drought tolerant because they reflect the light and don’t get as hot as other plants so they don’t need as much water.
Grey Leaved Groundcovers:
Tanacetum densum ssp. amani
Artemisia versicolor ‘Sea Foam’
Silene uniflora ‘Druetts Variegated’. Many of the variegated plants like this one have a grey effect in the garden.
Potentilla canadensis
Artemisia ‘Powis Castle’
Raoulia australis
Grey Leaved Shrubs:
Grey Germander (Teucrium fruticans ‘Azureum’)
Eriophyllum lanatum ‘Siskiyou’
Salvia clevelandii ‘Allen Chickering’
Texas Ranger (Leucophyllum frutescens)
Leucophyllum langmaniae ‘Lynn’s Legacy’
Leucophyllum frutescens ‘Convent’
Lavender Cotton (Santolina chamaecyparisus)
Senecio ‘Silvery Velvet’
Festuca ‘Elijah Blue’
Leymus arenarius ‘Glaucus’
Pink Crystal Grass
Many Hosta Lilies have grey or blue-grey leaves.