Delphinium. Needs sun or part sun. Apply a thin layer of compost each spring, followed by a 2-inch layer of mulch to retain moisture and control weeds. Water plants during the summer if rainfall is less than 1 inch per week. Soil should never dry out. Stake tall varieties to prevent hollow flower stalks from snapping in the wind, and deadhead after flowering to encourage rebloom. After the first killing frost, cut stems back to an inch or two above soil line.