I have a reasonably healthy St John'w wort bush that had a decent flower production this season in Northern California zone 9. It did not have a serious prune in many years, and has now grown to 6+' hight, 6' diameter, is woody, and dense inside. With the bush near the house and fire season around the corner I would like to significantly reduce its size and the number of stems. I was wondering how much and how far can I cut down at this time of the year (mid-summer).
I would not cut it at all at this time of year. Wait until spring. This article will help:
https://www.gardeningknowhow.com/edible/herbs/st-johns-wort/pruning-st-johns-wort.htm
We have four St. John's Wort shrubs...and no matter how we prune them, we end up with long, stringy branches that do not divide and have one flower on each branch. I've looked all over the web for an explanation and can't find help. Please HELP!
They do have a flower cluster at the end of each stem. These articles should help:
https://www.gardeningknowhow.com/edible/herbs/st-johns-wort/st-johns-wort-plant-care.htm
https://www.gardeningknowhow.com/edible/herbs/st-johns-wort/pruning-st-johns-wort.htm
Your St. John's Wort is, fully, hardy in your area. You can leave it out for the winter so that it can go dormant.
https://www.gardeningknowhow.com/edible/herbs/st-johns-wort/st-johns-wort-plant-care.htm
As long as your Spruce is a true Spruce, then it will have no problem surviving your winters, outdoors.