There are two varieties of hypericum--a shrub and low-growing ground cover. Pruning should be used to keep the plant under control and to encourage flowers.
Prune shrubs early in the year. Early March to mid April is the timeline you should be looking at. Pruning later in the year will reduce the amount of flowering from the plants, but they will still be prolific. Since the hypericum only flowers at the end of newly grown stems the pruning should encourage new growth. Cut the shrub back to within a few inches of soil level.
The shrub will survive what might seem a savage attack and recover in time to produce a great display of flowers on the wealth of new shoots.
Pruning hypericums as ground cover is just as heavy as it is for the shrubs. The plants need to be cut down to as near soil level as possible. This can be done by a lawn mower or by careful use of a strimmer. The best time to prune this way is late winter or early in the spring when the plants are still dormant. Although this style of pruning can be carried out every year it is best to do it every other year so that you don’t exhaust the plant.
As the plant develops during the year there may be times when you want to control the way in which the plant is growing.
To get the best results from your pruning and to help prevent the growth of molds use very sharp tools. Sharp secateurs that have made a nice, near vertical cut through a stem will leave a smooth surface that water will flow off. The plant will be able to create a callus over the wound to prevent infection. Blunt secateurs can tear the stem and leave the shrub vulnerable to insect attack. The rough surface of the cut stem will trap mold spores and water and could be the start of a mold infection that could destroy the plant.
Reduce stems that are growing too fast or in the wrong direction at any time. Rather than shortening the stem it is preferable to remove it as close to the ground as possible. If it has branched off another stem it should be cut close to that stem.
If stems get damaged or die off for some reason they should be removed without delay. In the case of stems that have died off you must try to find out what killed them. It may be that you have already removed any source of infection but you need to know if it will attack a lot of the plants.
Used carefully, pruning can create plants with pleasing profiles that produce masses of flowers. One great benefit of hypericum pruning is that no matter how badly you do it, the plant will recover and hide your mistake very quickly.