Have had my plant for about 3 years and have had only 1 bloom each year. How to get St. John's wort plant to bloom?
If the plant seems happy and healthy but just not flowering it may be to much nitrogen in your soil.
You can add phosphorous by using Bone Meal.
Here are a couple of links that will help.
How do l cut it back this spring from last year's growth?
St John's Wort blooms on new wood. The flowers that appear in mid to late summer develop on the new growth that appears in the spring.
Pruning should take place in early spring, before the new growth appears.
Shrub varieties of St John's Wort should be pruned back by about 1/3 of the total height of the plant.