Monarda, or bee balm, is a spectacular plant when grown under the right conditions. With plenty of sun and well-draining soil, it will flower from mid to late summer. That said, the plant has an annoying predisposition to powdery mildew that can make it an eyesore in the garden. So, recently researchers at Delaware’s Mt. Cuba Center set out to identify which varieties offer the best resistance.
A POLLINATOR’S DREAM
Monarda is a member of the mint family and is known for its aromatic leaves. At once spicy and sweet, they smell a lot like bitter orange. But the plant’s real allure lies in its shaggy, nectar-rich flowers that are magnets for pollinators. When given plenty of room, monarda will spread rapidly, forming dense clumps of blooms that are a highlight of the summer garden.
WHAT IS POWDERY MILDEW?
That said, by mid summer, these same flowers often look like they’ve been dusted with flour. The cause is a fungal disease called powdery mildew. Appearing first as white spots on individual leaves, the disease quickly spreads to cover the foliage. Eventually it engulfs the entire plant, infecting the stems, buds and the growing tips as well.
Monarda is particularly susceptible to powdery mildew.
Powdery mildew on rose leaves
GETTING THINGS UNDER CONTROL
Unfortunately there is currently no cure for powdery mildew. Instead, the recommended methods of control include pruning, no overhead watering and fungicides. That being said, the best strategy is always to choose plants that are resistant or at least tolerant to the disease. And fortunately, the experts at Mt. Cuba Center are providing a roadmap.
In the hopes of improving the home gardener experience, researchers at the botanical garden recently conducted a study to assess the disease resistance of 40 monarda species and their cultivars. The three-year trial evaluated each plant, each week, for quality of foliage, abundance of bloom and disposition for mildew. No one plant performed perfectly, but there were several deserving of honorable mention. Those top-performing varieties are listed below.
THE 6 TOP DISEASE-RESISTANT VARIETIES
Monarda fistulosa ‘Claire Grace’, one of the heaviest flower producers, ranked high for its sturdy, stems and good disease resistance. The dark purple blooms lasted for a solid three weeks in July.
Monarda fistulosa ‘Claire Grace’/Mt. Cuba Center
Monarda ‘Dark Ponticum’s’ bluish green leaves are a relatively rare color for monardas. Unfortunately, they were found to be susceptible to powdery mildew. But in an unusual turn of events, the disease was found to have no effect on the foliage. As a result, the plant stayed lush and green until early September.
Monarda ‘Dark Ponticum’/Mt. Cuba Center
Monarda ‘Violet Queen’ displayed excellent powdery mildew resistance. And its lavender-pink flowers were a favorite among bees. However, short, silvery hairs lend its leaves a dull green appearance.
Monarda ‘Violet Queen’/Mt. Cuba Center
Monarda ‘Grand Marshall’s’ spectacular purple-red flowers and compact habit make it a great addition to the middle border. It tended to have little sections that dried out, but overall exhibited good resistance.
Monarda ‘Achall’ ‘Grand Marshall’/Mt Cuba Center
Monarda ‘Purple Rooster’
According to the Mt. Cuba researchers, this cultivar exhibited the best resistance to powdery mildew. It also has the darkest purple flowers of all of the monardas. The stiff, rigid stems give it a different look from the more delicate varieties. The one downside is that ‘Purple Rooster’ doesn’t produce quite as big a floral display as the other species.
Monarda ‘Purple Rooster’/Mt. Cuba Center
Monarda ‘Gardenview Scarlet’ was the only red-flowered cultivar to rank among the top performers. Its large, 4″ wide flowers attracted hummingbirds in particular. It is about one foot shorter than the popular variety ‘Jacob Cline’, with which it shares characteristics. However, ‘Jacob Cline’ struggles with powdery mildew.
Monarda ‘Gardenview Scarlet’
While all of the above new cultivars exhibited good resistance to powdery mildew, the fungus still occasionally appeared on leaves and stems. Researchers also discovered that in many cases, the more the mildew the better the flower display. In sum, it’s probably best to bear the struggle in mind and site monardas in areas of the garden where the leaves can be masked by other species.