Answer: Phormiums are adaptable to dry soils, but you may need to wean them off the frequent waterings they have come to expect. I'd continue to water them with the same frequency until the rainy season arrives. Next spring you can put them on a lighter ration of water. Since they are planted on a slope, I suspect a lot of the water you are applying is rolling down the slope before it has a chance to soak in. This spring, make a watering basin beneath each phormium by piling up a few inches of soil in an 18-24 inch circle all around the plant. Then once a week fill the basin, allow it to drain, then fill it again. This method of watering will concentrate the moisture over the root mass and allow it to slowly trickle down, wetting the entire root system. This will encourage the roots to grow deeply. The deeper the roots are, the more drought tolerant the plants will become. Best wishes with your plants!
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