Forsythia are an excellent choice for your container garden – they are hardy in cold and heat, do not require much water, and require little care. Yet they will reward you with bright yellow flowers on bare branches in the spring, followed by beautiful green foliage in summer and fall, and bare branches of interesting shapes in the winter. They may a great backdrop when left to grow freely, and can be easily trained onto a trellis for use as a garden-room screen, a wind break, or as shade for more delicate plants.
Forsythia can grow as large as ten feet tall and wide, depending on cultivar and how much they are pruned. Some dwarf cultivars are available, growing to as little as 3 feet tall and 6 feet wide, but the flowers are less attractive than the full-sized forsythia.
These beautiful shrubs prefer full sun and moderate water. They require little fertilizer, but will reward a little compost or manure tea with lush growth. They are susceptible to few pests and diseases. Plant all cultivars in a basic, pH balanced, well-draining potting soil.
As a backdrop shrub, any cultivars of Forsythia intermedia are a good choice. These vary in size from 7 to 10 feet tall with a 4 to 7 foot spread. All have long, arching branches, reminiscent of a fountain.
For Autumn interest, Forsythia viridissma koreana (greenstem forsythia) is an excellent choice. The deep green leaves turn a purplish bronze as the weather turns cold. The shrub is more upright and stiff than some Forsythia, and has olive green stems. Most cultivars grow 8 to 10 feet tall, with large, bright yellow flowers in the spring.
To train on a trellis, there is no forsythia more beautiful than Forsythia suspensa, or Weeping Forsythia. Train and support the main branches on a trellis, fence, or other support, and allow the branchlets to cascade in gently drooping arcs. The result will be beautiful in all seasons, and especially breathtaking when the gracefully arcing stems are covered with bright yellow blooms in the spring.
All Forsythia should be pruned and thinned after bloom. Remove the oldest branches, and clear away any dead or weakened wood. Also remove any misshapen branches, generally thinning the shrub by cutting back to the ground a third of all the branches by the time you are through. After pruning, spread a light layer of compost over the soil and gently mix into the top few inches.
Forsythia can be lightly pruned to shape them throughout the growing season. Watch for aphids as the new green leaves emerge after bloom and treat accordingly. Water as the soil 1” to 2” deep becomes dry, do not over water but never allow the soil to become completely dry. From spring through summer, your shrub will appreciate an occasional watering with compost tea or manure tea.
As your forsythia grows you may need to transplant it to a larger container. This is best done while the shrub is dormant, after loosing its leaves in the fall or before bloom buds form in the spring.