Honey Fungus update 2019
Updated: Apr 28, 2020
So, the honey fungus is still here. I found this cluster last week in the corner where it all began:
I've been battling this problem since shortly after we moved to this house 16 years ago. I suspect it originated on a Norway Spruce that a previous owner had planted close to the house, probably after it had done its job over Christmas. (side note: I don't recommend planting Christmas trees directly in the garden, they're grown for a short term purpose so not always under the best conditions. Keep them in a pot where any problems won't spread to other plants). When we moved in the tree was large enough to be a threat to our foundations and was in poor health anyway so we had it felled. But a year or two later other plants nearby started to die, then the problem spread along an entire side of our garden. Sadly it took a while for the tell-tale rhizomes and fungi to put in an appearance, by which time it had destroyed a lot of our mature shrubs.
Over the years I have dug up and destroyed as much of the roots as I could, but it would be impossible to get to all of them without demolishing our house and probably some of our neighbours' buildings too.
I've tried limiting woody plants to those listed by the RHS as being resistant to honey fungus. The list has been recently updated and can be found here: https://www.rhs.org.uk/advice/pdfs/honey-fungus-host-list.pdf. This has worked reasonably well with new planting, probably because a healthy plant can often shrug off an attack anyway, but I've lost some older shrubs which are listed as resistant including a favourite Chaenomeles and a large Rosemary.
Last year I planted some more new shrubs but this time I used a physical barrier. I dug out large holes to as deep as I could get them - in my garden this was only 12-14 inches which is not ideal (18 inches would be better) - which I lined with pond liner. In view of the size of the holes I took the risk of refilling them with the same soil but I did check it extremely carefully and remove any roots or woody waste which could act as hosts for the fungus. Unsurprisingly, pond liner is waterproof so if you're lining the bottom of the hole as well as the sides it's vital to make lots of holes in that section for drainage. You can see how much work it took for one shrub below:
Anyway, once I removed the new outbreak shown at the top of this page I could see that it had formed on the "right" side of the pond liner, it the outside, so I'm hopeful that this time it won't spread to my new shrubs:
If you suspect a problem in your own garden, look for either the honey coloured fungi pictured at the top of this page or white "rhizomes" like the ones shown below under the bark or on the roots:
Good luck and I hope my experience will help you deal with this problem :-)