Dig a hole twice as wide and deep as the forsythia plant's root system in an unshaded or only partially shaded area of the garden. Forsythias prefer direct full sunlight but can also thrive in partial light, if needed. Make sure the hole is dish-shaped and slightly roughen the sides by scoring.
Mix the soil from the planting hole with peat moss, processed manure or compost to create a nutrient-rich planting soil. This helps the forsythia get the best possible start after planting.
Keep the plant's roots moist until just before planting and water them slightly right before placing the plant in the hole. Hold the plant straight as you backfill the planting soil into the hole, making sure to not leave any air pockets next to the roots. Roots can dry out and die if in contact with the air, even under ground. Pat the soil down slightly but not too tightly as the plants like loose soil.
Water the forsythia thoroughly immediately after planting. This helps the soil cling to the roots and aids the settling-in process. Keep the plant watered regularly during dry weather, but do not allow the ground to become waterlogged as this can lead to root disease.
Feed the forsythia once a year with a good quality rose or all-purpose fertilizer. The best time to do this is in early spring or summer.