Hypericums are a wide range of garden shrubs, but generally fall into two groups as far as pruning is concerned.
The shrubby varieties of hypericum such as Hypericum 'Hidcote' should be pruned in spring - March or April depending upon area and weather conditions.
Pruning later in the season may rob you of flowers for that year - although pruning even as late as end of May will still give you plenty of flower - but later in the summer.
This type of Hypericum flowers on the ends of new shoots, so the more new shoots you can get the plant to make, the more flowers you will get.
The best way to prune Hypericum shrub varieties, is by cutting all growth back to within a few inches of ground level. It will soon grow back to near its original size within one season. This will ensure that you get a well shaped shrub, with plenty of new healthy shoots that will be a mass of flower later in the year - normally starting July. Alternatively, simply trim off last season's flowered growths.
Dead heading the flowers should not be carried out with the varieties that have attractive fruits such as H Elstead, Hypericum androsaemum.
See also - Hard Pruning
Left - Hypericum calycinum - Low growing : Right - Hypericum Elstead - A shrubby Type
Hypericum calycinum - St Johns Wort - is also a shrub - but low growing and often seen in roadside plantings - that can be pruned back hard. There is no finesse needed in pruning this variety. If you have a large swathe of them as groundcover, simply run a rotary mower over then - set at the highest setting. Or you can use a heavy duty strimmer. Hedge clippers - shears - will also do the job. the aim is to get rid of all the existing growth by pruning down to ground level or just above.
Pruning Hypericum calycinum should be carried out in late winter - or early spring - and is a great start to the gardening year! It can either be pruned this way each year, or every two years.
By David Hughes - firstname.lastname@example.org