Planting a garden on a hill can be a challenge because water drains to the bottom of the hill, making the top of the hill dryer than the bottom. Also, fertilizer can concentrate in the bottom of the slope or planting area as it mixes with rain or irrigation water and runs downhill. Then there is the problem of erosion if water runs over the bare soil. The secret to gardening on a hill is to protect the soil and make raised beds or terraces to prevent erosion and capture as much irrigation water and rain as possible.
Make raised beds that run across the hill, perpendicular to the slope. Beds can be as wide or tall as needed, but their function is to slow down and capture the water running down the slope. You can make raised beds by shoveling soil from what will be your pathways or by bringing in top soil.
Spread a one-inch layer of compost over the new beds and gently work in to the top 6 inches of soil with a hoe or shovel. The purpose of the compost is to add enough organic matter so the soil will hold water better than before. Too much compost will make the soil too light and highly erodible.
Plant appropriate garden plants in new beds as needed, keeping in mind that the top of the slope will be dryer than the bottom of the slope, even with irrigation. Taller plants will look better if planted at the top of the hill with smaller plants and ground covers planted toward the bottom.
Mulch with at least a two inch layer of mulch on all bare areas within the bed. Cover bare pathways with at least a one-inch layer of mulch to prevent erosion and rapid water runoff. You can leave pathways covered with well-established grasses or ground covers rather than adding a layer of mulch.