The following is reproduced from my piece in Plant Heritage Journal autumn 2015:
Monarda (bee balm, bergamot) are those striking late summer clumps of ‘fireworks’ in the herbaceous border, herb garden, prairie garden and nowadays, dwarf forms in containers too. But much as we love the show, they can be rather temperamental too. They mainly hate it to be too damp, yet need some moisture and dislike drying out. Apart from those cultivars such as M. ‘Marshall’s Delight’ or M. ‘Jacob Cline’ that have powdery mildew resistance, their leaves will also often succumb to its symptoms, with unsightly foliage from usually later in the summer.
Assuming that site and aspect were right at the time planting; good drainage and mainly full sun, then you’re half way there. Nurturing your Monarda plants in the autumn is all about increasing the chances of success even further; the chance of both lowering the percentage of overwinter plant loss and that of maintaining vigorous and healthy surviving plants into next season. A reminder before the maintenance starts is to collect seed for next year’s plants from any Monarda grown as an annual, such as M. punctata or M. citriodora. Whilst propagation of perennial Monarda is possible by various methods, you may prefer growing from seed, so consider general seed collection too.
Firstly, pruning and dead plant material collection – once the plants are dying back prune down to a few centimetres from the ground, not for the sake of tidiness, but critically to improve plant vigour and remove diseased material. If fungal leaf spot has been noticed, by clearing dead leaves from the soil around the plants any chance of the overwintering stage of the fungus being left in situ will be lessened. Now you can see the soil again, the next step is to mulch around the plants with say wood chip, leaf mould, or compost. This is to create the right conditions now for next year’s healthy plants. They will then retain moisture in hot weather, which will reduce stress and the likelihood of disease. Whilst many perennials are divided in autumn, it’s not a good idea for Monarda, simply because it may add more stress and therefore increase the chances of winter plant loss, so if required delay until spring.
Most Monarda are hardy, but even so It is normal to experience at least some level of either die back or even just reduction of plant quality in the winter. Whilst this autumn care helps, this is also your chance to pot up at least some plant material from the runners to act as an insurance policy in case of the worst!