Shrub-type St. John's wort requires considered pruning
Question: I have three St John's wort plants. When I purchased them, they had many buds, and I thought I would have a full, beautiful yellow bush, but they bloom only sporadically. Do they need to be pruned?
Answer: Though you do not mention which type of St. John's wort (Hypericum species) you are growing, I'm assuming it's one of the shrub types vs. the lower growing perennial/groundcover varieties. These are very wonderful plants that deserve a place in our landscapes but proper pruning does indeed encourage more flowers and a better shaped plant.
St. John's wort blooms on new wood. The flowers that appear in mid to late summer develop on the new growth that occurs in the spring. For this reason, pruning should take place in early spring just before active stem growth begins. Many species of St. John's wort can be cut all the way back to the ground if necessary, but the shrub types should have about a third of the total height removed each season. Do this at the same time you prune your butterfly bushes back -- mid to late March.
When pruning your St. John's wort, use a clean, sharp pruning shear and selectively remove some of the branches. Also, remove any dead or crossed branches. Do not use an electric hedge shear to form them into meatballs -- this is not their natural form. Each place you make a cut will branch into two or more new stems, each of which should develop a flower cluster at its end. Doing this on a yearly basis will increase flowering and help the plant develop a good growth habit.
There are many types of St. John's wort; tiny rock-garden specimens, groundcover varieties, those that grow 12 to 18 inches, and lots of different shrubby varieties. All require full to partial sun and develop yellow, saucer-like flowers with lots of fuzzy anthers in the center. They are tolerant of poor soils, and their dense root systems are good at controlling erosion and out-competing weeds. The only negative to these wonderful plants is their tendency to have an occasional attack of spider mites, which are easily controlled with horticultural oil if necessary.
Another favorite trait of this plant is its interesting berry-like fruits, often found in the cut flower industry. Once the flowers have matured, tiny upside down strawberry-shaped yellow, orange, red or tan fruits appear clustered at the branch tips. They make wonderful additions to flower bouquets and add late season interest to the garden. And cutting a few of them off will not affect the following season's flowering.