How can I stop the hypericum roots spreading around the lawn?
Thank you for your help. I hadn't realized just what a potential future problem when enjoying it's flowering period. It's a hidden time bomb waiting to explode. Have now treated all the roots that are rapidly appearing with "Roundup weedkiller gel" . (Glyphosate 7.2g/l).
Here are some suggestions: https://www.gardeningknowhow.com/edible/herbs/st-johns-wort/controlling-st-johns-wort.htm
I read that I could hard prune my hypericum which were growing very large and I did that in March. Now they aren’t showing any signs of green growth (May) and I’m afraid I killed them...
Thanks- it was early March so I do believe it was dormant. Should it be budding by now? There are slight bumps on the stems but no buds...
Was the plant dormant when you "stooled" it? If it had already leafed out when you pruned it, then yes, you may have killed it. Go ahead and cut the fattest branches down as far as you can to get more light into the center. Cut back one of the smaller branches. If the inside or pith is tan, it is gone. If there is a thin green ring just under the bark, it's alive.
Best time to plant St John's wort?
These are best transplanted in spring. Here is a link to some very useful articles on your plants: https://www.gardeningknowhow.com/edible/herbs/st-johns-wort/
It has tubers that form 1- 1 1/2’ deep in rock and clay. It's almost impossible to find and dig them all out. It's been in place for decades so the patch is large. A landscaper told me that it was popular in the 1970’s but that something killed off most of it. He doesn't know what did it.
Since there may be quite a few restrictions on what methods you are permitted to use in your area, I would opt for a treatment that is without chemical means. Solarizing soil can be a very good way to do so.