Good housekeeping of plants/sufficient air circulation can cut the risk of disease and fungal infections but some plants are naturally resilient. Perennials with this trait can be incorporated in the open space and have an impact on neighbouring varieties. While these are not free from problems, under the right conditions very easy to keep.
Phlox can be scented and provide you with summer flowers, the varieties mostly through to autumn. Pinks, purples and whites. Some mat forming and some upright. The perennial is good in most aspects although they need a certain degree of sun. P. ‘David’ is resistant to powdery mildew.
Monarda (Bergamot) a perennial in mid summer to late. Aromatic flowers with a clump forming habit. It is suitable in most aspects, the only condition is the plant is not keen being wet so well drained soil. In certain conditions it can be susceptible to powdery mildew but only if the air circulation is poor and this is easily avoidable. Varieties are more resistant than others. These include ‘Petite Delight’, ‘Colrain Red’.
Paeonia – garden and tree. The soil is important, they tend not to like acidic soil so balanced conditions. Peonies don’t like sitting in moisture so well drained too. Hybrids can often be more resilient and cutting down stems each season will help keep pests at bay. The most important point to remember about Peonys. They don’t like to be moved. P. ‘Scarlet O’Hara’ gives a show late spring to summer.
Sedums are perennials but also succulents due to their fleshy leaves. So these plants are often good in dry conditions and drought tolerant. They provide attractive foliage and flowers, they like full or partial sun at the very least. These plants are low maintenance but very effective. They attract beneficial insects too.
Geraniums are very good at the front of borders, the smaller varieties anyway. These generally are hardy and need minimal maintenance save cutting back spent growth. G. ‘Rozanne’ will provide flowers throughout summer. A condition is that the soil is well drained and they get sun. G. Macrorrhizum will give you aromatic foliage early to mid summer and are white rather than purple.
Verbena offers attractive lilac flowers in late summer. It encourages beneficial insects too. It needs some shelter but the stems are relatively tough. V. bonariensis is quick to establish and can be put almost any position because of it’s habit. It will not block out other plants. It is probably best to cut back when new growth appears in spring. Again, for this plant powdery mildew is usually what thwarts it and with good housekeeping this can be avoided. In warm, dry weather the spores are likely to be more active.