Forsythia is an ideal container plant because it can be kept to size by careful pruning. There are dwarf varieties of forsythia, but some of these do not flower so profusely. To get the best from forsythia in a container you need to have a container big enough to give the plant a good amount of soil and heavy enough to remain upright in high winds with a fairly tall plant growing in it.
Forsythia can be grown from seed, but harvesting the seed is not so easy. The easiest way to start forsythia in a container is to use cuttings. The soil in the container should be moist and well drained. A little garden compost can be worked into the soil.
The cuttings should come from the tips of newly growing stems. A cutting about five inches long can be simply placed into the soil. Pack the soil firmly around the cutting and it should root successfully. A rooting hormone can be used to speed up the process.
Your container should be in a full sun position for most of the day. Because the forsythia will be bushy you need to keep an area around the container free to allow the shrub to spread. Forsythia is not a disciplined shrub when it comes to the direction of stem growth.
For some people this random development is one of the charms of forsythia while others prefer a more formal shape. In containers pruning to a formal shape is probably going to make the plant look more attractive and balanced.
Forsythia is an ideal container plant because it will recover from very savage pruning. After the flowers have finished old wood should be cleared out of the plant and about a third of the older and thicker stems. If you prune while the remains of the flowers are still on the plant you will be able to recognise the newest stems. These will have no flowers on them and should not be cut. Establish the rough shape that you want by selectively pruning individual stems.
Pay particular attention to those old stems that were not so prolific with their flowers. Don’t try to make the plant too tightly controlled because this will affect the quantity and quality of flowers. When the plant appears to be too big for the container you can take the stems back down to the soil level. New green shoots will soon appear and the plant will be looking quite robust in a couple of months. A benefit of such heavy pruning is that the plant tends to become a little more compact.
Container planting forsythia only adds the need to water as a regular chore with the plant. A mature forsythia will tolerate many conditions and will even survive being allowed to dry out. Keep the soil in the container moist for best results. An occasional feed with a diluted liquid fertilizer will help to maintain the plant but feed more often if you cut the plant back regularly.