Monarda didyma, Scarlet bee balm

Bee balm (monarda) is a spectacular plant when grown under the right conditions. Given plenty of sun and well-draining soil, it will flower from mid to late summer. Still, the plant’s annoying predisposition to powdery mildew often makes it an eyesore in the garden. So, recently researchers at Delaware’s Mt. Cuba Center set out to determine which varieties offer the best resistance.

A POLLINATOR’S DREAM

A member of the mint family, monarda is known for its aromatic foliage. Both spicy and sweet, it smells like a bitter orange. But the plant’s real allure lies in its shaggy, nectar-rich flowers that are magnets for pollinators. When given ample room, monarda will spread rapidly, forming dense tufts of blooms that are a highlight of the summer garden.

WHAT IS POWDERY MILDEW?

Unlike many plant diseases that are difficult to diagnose, powdery mildew is easily recognizable. It usually shows up first as small white spots on the leaves and quickly spreads to cover most of the foliage. Eventually the fungus engulfs the entire plant, infecting the stems, buds and growing tips as well. Plants infected with powdery mildew often look like they have been dusted with flour.

Unfortunately, monarda is especially susceptible to this disease. The good news is that, while it may make them unsightly, powdery mildew rarely permanently damages plants.  As a result, most gardeners opt to cut stems down early and hope things improve the next year.

Powdery mildew on rose leaves

GETTING THINGS UNDER CONTROL

While there is currently no cure for powdery mildew, 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. Meanwhile, 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 propensity for mildew. No one 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 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’

TAKEAWAYS

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.

For more information on Mt. Cuba Center and its many plant trials, click here for mtcubacenter.org.