By Bellouise
United Kingdom 
I have some low growing mahonia shrubs which became diseased last year. After looking on the internet, I removed all affected leaves and the soil around them. This year they seem to have recovered but have now spread dramatically throughout the soil and are smothering all other plants near by. I would like to stop the spread and remove them completely. How can I achieve this.
28 May, 2015
Answers
Dig out the roots - if you can't get them out because they're too big, cut the whole thing back to the base, expose the woody parts of the roots, make holes or slash them and apply SBK, a brushwood killer. M. aquifolium does tend to have wide spreading growth, and growth doesn't always all necessarily come straight off the main root though.
Alternatively, keep the plant/s and just cut them back really hard - they'll grow again, but can be kept in bounds.
28 May, 2015
What Mahonia do you have? Mahonia japonica comes to mind as it is the most commonly available but I can't think of any diseases that effect it, so wonder if it is Mahonia aquifolium aka Oregon Grape? I know that gets a number of viruses and fungi attacks and it has a spreading habit unlike japonica.
28 May, 2015