Monarda didyma, Scarlet bee balm

Monarda, commonly known as bee balm, is a spectacular plant when grown under the right conditions. Given plenty of sunlight and well-draining soil, it can flower from mid-July to late summer. Still, the plant’s annoying propensity to develop powdery mildew often make it an eyesore in the garden. That’s why researchers at Delaware’s Mt Cuba Center recently set out to determine which monardas offer the best protection against disease in the Atlantic region.

A pollinator’s dream

A member of the mint family, monarda has been a staple of the perennial garden for centuries. Perched high atop tall, square stems, its shaggy, nectar-rich flowers are a magnet for pollinators. The colorful blooms range from scarlet to purple and the foliage is tantalizingly fragrant, reminiscent of the bergamot orange.

When given ample room, monarda loves to propagate by rhizomes, spreading rapidly underground to produce new shoots. In no time, it forms dense drifts of brilliant color crowned by scores of butterflies, bees and hummingbirds.

Male ruby-throated hummingbird hovering by monarda flower

What is powdery mildew?

Powdery mildew is a fungal disease that affects a wide variety of plants. It usually starts off as small white spots on the leaves and stems and quickly spreads to cover the upper surface of the leaves causing them to yellow and dry out. With time, the fungus multiplies to cover the entire plant, distorting the buds and growing tips as well.

It’s easy to spot powdery mildew – it appears as a greyish white powder on the upper surface of the leaf. While they don’t usually die, plants affected by powdery mildew are an unsightly mess, prompting most gardeners to cut them back to the soil and hope for a better season next year.

Powdery mildew on rose leaves

Getting things under control

While there are suggested methods of control including pruning, no overhead watering and fungicides, the best strategy is to choose plants that are resistant or at least tolerant to powdery mildew. This is where the researchers at Mt. Cuba Center come in.

I recently attended a lecture by one of their staff members who described the results of a three-year trial that  included 40 monarda species and cultivars. Each week, researchers evaluated each cultivar for quality of foliage, abundance of bloom and resistance to disease. The top performing varieties, listed below, would make great additions to any garden.

Monarda fistulosa ‘Claire Grace’, one of the most floriferous, ranked high for its sturdy, upright stems and better disease resistance. The dark purple flowers lasted for a solid three weeks in July.

Monarda fistulosa ‘Claire Grace’/Mt. Cuba Center

Monarda ‘Dark Ponticum’s’ bluish green leaves (a relatively rare color for monardas) were found to be susceptible to powdery mildew, but in an unusual turn of events, it didn’t effect the health of the foliage. Masses of violet-purple blooms covered the plant for three straight weeks in July after which the unusual-colored foliage stayed lush and green until early September.

Monarda ‘Dark Ponticum’/Mt. Cuba Center

Monarda Violet Queen, a favorite among bees, has lavender-pink flowers that are slightly lighter than ‘Dark Ponticum’s. Short, silvery hairs lend its leaves a dull green appearance. It displayed excellent powdery mildew resistance.

Monarda ‘Violet Queen’/Mt. Cuba Center

Monarda ‘Achall’ Grand Marshall’s spectacular purple-red flowers and compact habit make it a great addition to the middle border. Topping out at just 28” tall, it tended to have little sections that dried out, but exhibited good resistance to powdery mildew.

Monarda ‘Achall’ ‘Grand Marshall’/Mt Cuba Center

Monarda ‘Purple Rooster’

According to Mt. Cuba researchers, this was the best cultivar for no 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 others.

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. 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.