Plant forsythia in rich soil with good drainage at a location in the garden that receives full sun or partial shade. Add compost to sandy soil, and both compost and sand to heavy clay soil in order for a forsythia bush to adapt to poor conditions.
Water the forsythia bush frequently to keep the soil moist. Allow an open hose to run at the base of the bush for three to four minutes weekly through the summer and fall, and more often in hot, dry weather.
Prune the forsythia bush after it has bloomed. Cut away one third of the oldest branches at ground level. Use sharp, long handled garden pruners. Cut away shoots that sprout up in the ground surrounding the bush. Lightly prune again in mid-June to shorten the longest branches.
Apply a one-inch layer of compost around the base of the plant in early spring when flower buds appear. Spread a two-inch layer of mulch around the forsythia bush in late spring to help retain moisture in the soil throughout the growing season.