Forsythia is a fast-growing shrub that produces bright yellow flowers in early spring. It takes a weeping, natural form that may look wild or unkempt in formal settings, where it can become weedy or invasive. Homeowners may unwittingly worsen the situation by pruning forsythia into an unnatural shape. Plant forsythia as a specimen shrub, giving it plenty of room to grow, or remove shrubs that are improperly placed. Forsythia is a hardy plant, and repeated herbicide applications may be necessary to eradicate it.
Spray forsythia with an herbicide containing triclopyr. Spray on a clear, windless day in spring, after foliage and flowers have emerged. Follow package directions carefully, and avoid spraying the herbicide on desired plants. Wait two to six weeks for the foliage and branches to wilt.
Cut the branches 6 inches from the ground, using pruning shears. Dig a trench with a shovel around the shrub, digging at least 12 inches into the soil. Wedge the shovel under the shrub and rock it back and forth to dislodge the roots. Grab the stubs emerging from the ground and pull to wrench the shrub free. Turn the dislodged root ball over so it is exposed. Cut any roots that remain affixed in the ground with the pruning shears. Remove the root ball and discard it along with the branches.
Treat any suckers that emerge from the soil by spraying them with triclopyr. Wait until the next spring to plant anything new to minimise the chances of herbicide injury.
If your forsythia has been damaged by overzealous pruning, restore it by pruning the entire shrub to the ground. The forsythia will grow and take its natural shape once again. Thereafter, prune forsythia only to remove old, dead growth.