I am planting two beds of English and old shrub roses next Spring. Up until yesterday, my plan was to take David Austin's advice and plant them in sets of three, about 18 inches apart. Yesterday, I read an article that this approach is mistake for english roses in the US as they grow bigger here than in England and that there is greater risk of disease planting roses that close together. I live in Pittsburgh, PA, Zone 6. Any experiences or recommendations?