Wednesday, January 4, 2017

Mildew Resistance and Garden Suitability of Bee Balm

Mt. Cuba Center just released their evaluation of Monarda. So I thought I'd talk about it, Chicago Botanic Garden's 1998 trial, and my experience growing various forms. This will be a less image intense post than some of my more recent ones. Despite growing a number of Monarda through the years, I apparently haven't taken pics of many of them. I make up for the lack of pics with wordiness. Sorry peeps, this one is long.

Bee balm has been a popular plant for a long time. Centuries actually, it was grown in the late 1700s in Europe as a garden plant. Before that it was used medicinally by Native Americans and as tea by the American colonies. So it's no surprise that it's a staple of perennial gardens throughout the world. It's hardy, easy to grow, and easy to propagate.

Like many other Americans, Monarda had to travel to Europe to find itself. European breeders focused on Monarda didyma and Monarda fistulosa and bred them to create many selections and hybrids in an array of colors with large flowers. What they didn't focus on was disease resistance. If you've grown bee balm, you know what powdery mildew is and what it can do.

Monarda had to come back to America before selection for disease resistance would take place. It started at Morden Research Center in Manitoba, Canada with 'Marshall's Delight', Grand Marshall ('AChall'), Grand Parade ('ACrade'), Grand Mum ('MCmum'), and 'Coral Reef'. More recently other breeders (Walter's Gardens, Ball Horticulture, and others) have also focused on compact habit.

Monarda can be a spreader in the garden. Especially if you haven't grown it or a new variety exceeds your expectations of vigor. Monarda didyma and its hybrids are highly rhizomatous. They can take up a good chunk of garden real estate. This habit can also make it quite dense which exacerbates the mildew problems it can have. Plan on giving it space, dividing it every few years, and thinning stems out to promote good air circulation. Easy to grow, yes; low maintenance, no. Worth it.

Besides promoting good air circulation, there are several other cultural considerations for reducing mildew. While tolerant of a wide range of soils and conditions and highly drought tolerant, excess moisture or extreme drought will lead to mildew susceptibility. Also, clean up old stems and foliage as completely as possible. Mildew overwinters on leaf litter and in soil. If you have a serious mildew problem, you can use several fungicides to good effect. My preferences are organic fungicides like copper and Green Cure. Actinovate can also be used as a foliar spray for prevention of mildew.

Being that bee balm is a garden staple and I've been in the horticulture industry for 20 years, I've grown A LOT of it. Many I've only grown as a container crop for sale but I've grown quite a few in gardens as well. I haven't found a greater susceptibility to disease when growing in containers, and no variety is 100% mildew free. Both Mt. Cuba and Chicago Botanic rated plants based on disease resistance, habit, flower coverage, and bloom time; and for the most part their results match pretty well.

One glaring exception is 'Marshall's Delight'. Chicago found it "had the lowest level of infection with no more than 5% ever observed" while it ranked almost at the bottom of 40 varieties at Mt. Cuba "due to severe powdery mildew infections that caused significant damage and defoliation by midsummer." This could be attributed to regional differences. But my experience in gardens and containers has matched up very well to Mt. Cuba's findings. This is one of the least mildew resistant plants out there. I've tried it several times and it's always been awful.

Another exception is Monarda fistulosa 'Claire Grace'. Chicago rated it a 2 and Mt. Cuba gave it their highest rating of 4.5 for "its sturdy, upright habit and prolific floral display." They rated its mildew resistance as good. I have not tried this one yet, but will pick it up to compare it to straight Monarda fistulosa which also has excellent habit but terrible mildew resistance. On the topic of M. fistulosa, forma albescens (often sold as 'Alba') scored ok in Chicago's trial with 3 stars and a good resistance rating. There aren't a lot of white bee balms and most are very susceptible to mildew.

Red bee balms are always popular and often the worst spreaders and most susceptible to mildew. Both trials rated 'Gardenview Scarlet', 'Colrain Red', and 'Raspberry Wine' very highly. Mt. Cuba also scored 'Jacob Cline' a 4.0. I've had mildew to the same extent on 'Jacob Cline' and 'Gardenview Scarlet' and it wasn't too bad. Both had a looser very spreading habit for me. 'Colrain Red' I've only had as a container crop and only once for a short time as it sold out very quickly. 'Raspberry Wine' probably wins the award for most vigorous of all the ones I've grown. My plant quickly grew to nearly 5' tall and 10' wide. It was removed from the gardens for being too vigorous as I didn't have the space for it. Despite this vigor it was very dense, had almost no mildew, and is still a favorite of mine for color. I will be putting it in the new gardens and giving it ample space.

'Raspberry Wine' in year 2
'Raspberry Wine' in year 3. 5' tall x 6' wide. This plant was removed a year later.
'Raspberry Wine' in year 3. 5' tall. 

A close second place in the vigor category for me is 'Purple Rooster'; it's slightly less vigorous and dense than 'Raspberry Wine'. It also ranks very highly in the color category for me as it's the only true dark purple I've seen. This was selected from a batch of seedlings at The Flower Factory here in Wisconsin. It is also very mildew resistant. Mt. Cuba ranked it a 4.1.

'Prairie Gypsy' is a hybrid of M. bradburiana and scored a 3.8 at Mt. Cuba. They rated its mildew resistance as fair. I have grown this, actually in a quite shady spot, and not had any mildew issues on it. I will concur that it has a lax habit after bloom. Even after moving to the sun, the floppy habit persisted. I like this one regardless. Bloom time is intermediate between M. bradburiana and M. didyma, usually in bloom mid-June.

Best picture I could do of 'Prairie Gypsy'

'Peter's Purple' is a hybrid of M. fistulosa 'Claire Grace' and the Mexican species M. bartletti. It is usually listed as being zone 6 hardy and that's probably the safest cold zone for it. I did grow it for a couple of years though and it did ok until we had a very wet winter. So it may be doable in zone 5 with proper drainage and planting early in the season. Mt. Cuba rated it 3.7 based on fair mildew resistance and a spindly floppy habit. Mildew resistance for me was ok, mostly the lower leaves were affected. Spindly? Yes. It never filled out well and wasn't very dense. Floppy? No. The stems were quite sturdy. Much sturdier than the average bee balm. Flower color is dark lavender.

Grand Marshall ('AChall') was the highest scoring compact variety from Mt. Cuba. They say "its growth habit is still Monarda-like, growing in a uniform mass that spreads slowly outward. Many of the newest compact selections are dwarfed to the point of looking artificial and out of place in the landscape. The habit of Grand Marshall™ habit better lends itself to blending with its neighbors in an attractive and natural way." So far, I have to concur. Grand Marshall has been one of the best performing bee balms I've grown. Almost no mildew, fuschia-purple flowers, and compact and slowly spreading but still a good grower. This is the standard by which I judge all other compact Monarda, and so far none measure up.

Grand Marshall on the left looking very pink. 'Raspberry Wine' on the opposite end.

Grand Marshall after being moved to the new garden. This color is much more correct. 

So what about those other compact hybrids? Let's start with other Morden selections. Grand Parade ('ACrade') wasn't too bad, it's almost as good as Grand Marshall. Mildew resistance is excellent. Vigor is quite good, like Grand Marshall. Habit is not as good, tends to hollow out in the center quickly. Mt. Cuba found that to be the case as well.

 'Petite Delight' scored in the middle 3s (3.4 and 3.5) in both trials. It shows reduced vigor and is slow to establish. Mildew resistance is good. It suffers from low flower coverage and very short peak bloom.

'Petite Wonder' might just be one of the worst varieties ever introduced. It scored near the bottom of Mt. Cuba's trial with a meager 2.7. I think they pretty much cover it: "Monarda ‘Petite Wonder’ is one of the worst performing cultivars in our trial. The plants hardly grew over the three year period and both its floral display and powdery mildew resistance were mediocre. In fact, there was no other cultivar that displayed such a significant lack of vigor."

'Coral Reef' I have not grown in the garden. But it had a lack of vigor in container culture. Flower color is a strong medium pink and is quite nice. Mildew resistance is good. Flower coverage is below average. Mt. Cuba had issues with some sections dying out over winter. Based on the lack of vigor I experienced in container culture, I guessed that the 36-42" advertised mature height was unlikely. Mt. Cuba seems to have confirmed this as their plants only grew to 20".

Neither trial covered Grand Mum ('MCmum'). I only grew it once as a container crop. The mildew susceptibility I encountered in container culture convinced me that I never wanted to put it in anyone's garden.

Continuing in the tradition of other European introductions, the Lace series ('Pink Lace' and 'Cranberry Lace') have only fair mildew resistance. Mt. Cuba had trouble with 'Cranberry Lace' overwintering and attributed it to defoliation from mildew. I have also had trouble overwintering the lace series, but I didn't have significant defoliation problems. Flower color on both is excellent, but I guess not worth the trouble.

'Balmy Purple' received a low rating of 2.7. Mildew resistance was fair, habit was irregular, flowers were large, bloom time was short, and vigor was low. All the basic traits of a compact Monarda. In containers the mildew resistance seems good.

'Pardon My Pink' and 'Pardon My Purple' both got low marks, 3.1 and 2.6 respectively. Typical complaints about compact Monarda apply here. Though I didn't have any issues with mildew in pots or in the garden. The colors are really quite good and an improvement over some older varieties.

Species etc in the trials:
Monarda clinopodia and Monarda 'Bryan Thompson': These two plants are apparently remarkably similar. I don't fully agree with Mt. Cuba's logic that "‘Bryan Thompson’ is unlikely to be M. clinopodia because the species does not grow in Texas where ‘Bryan Thompson’ was discovered" without knowing more about the area and the collection. Plants escape cultivation, though M. clinopodia isn't exactly widely grown. so I guess it's just as likely to be an undescribed species. I'd really like to pick both up and compare them and maybe try to key them out and see what happens. In any case they both seem to be decent plants. They only scored low due to unkempt habit. Mildew resistance was excellent. White flowers. Worth breeding with.

Monarda austroappalachiana 'Snowbird' received low marks for all of the typical complaints associated with compact hybrids. Except mildew resistance, that was excellent. Another white flowered thing worth using in a breeding program.

Monarda bradburiana is another compact species with great mildew resistance but a floppy habit. I concur with Mt. Cuba: "If further research of this species focuses on sturdier habits it would easily be one of the best bee balms for garden use." Fortunately, this is happening. More about this shortly.

Monarda citriodora and M. citriodora 'Bergamo' both scored low for floppy habit. 'Bergamo' was the worse of the two in this regard. This is an annual species that can self seed in gardens. Flowers are stacked. A neat plant, but goes dormant early. Excellent mildew resistance.

Monarda punctata scored quite high, a 4.0. This is another species with a stacked inflorescence. The flowers themselves are yellowish with brown spots and not terribly attractive. The bracts surrounding the flower however range from white to pale yellow or light pink and are incredibly beautiful. This species is a tough one to grow and requires very well drained soils. Here in Wisconsin it is found on sand dunes along Lake Michigan. It's been said to be annual, biennial, and perennial. I've always leaned towards biennial as that was my experience but Mt. Cuba found it reliably perennial. Chicago had all plants die in the first winter. It can look pretty rough later in the season as it defoliates after flowering. Lots of potential for selection. Added bonus is that it's one of the most attractive to pollinators. Especially predatory wasps.

New things not in the trial:
Monarda 'It’s Majic' – "Most likely a hybrid of fistulosa and didyma. These extra vigorous  plants reach 5’ tall. Dark foliage and some purple stems add interest too. Bright  purple flowers are super eye catching and last an extra long time blooming in  summer."  From Intrinsic Perennial Gardens. We got a few of these for sale this past year. I liked the foliage, form, and flowers. They did get some mildew in pots though. Unsure how resistant it will be in the garden.

The Leading Lady series is a pair of plants from Walter's Gardens that are hybrids of M. bradburiana. Supposedly densely branched, that may solve the floppy habit problem. 'Leading Lady Lilac' is lilac-lavender and 'Leading Lady Plum' is more typical purple. Flower time is late June and habit is compact. We'll see how vigorous they are.

The Sugar Buzz series has been around for awhile, includes 8 varieties, and I'm surprised none were in the Mt. Cuba trial. Also from Walter's Gardens, this is another compact series. I have not grown them, but mildew resistance is apparently quite good. Not sure about vigor.

There are also further plants in the Balmy series and Pardon My series, though I expect them to be similar to those already trialed.

Monarda 'Balmy Lilac'

Monarda 'Balmy Pink'

Despite all of the problems associated with Monarda and mildew, it has continued to be a garden staple. That tells me that people are willing to overlook spreading habits, ugly foliage, and lack of vigor in the case of compact varieties. While we focus on the flowers of these plants, much can be said about the fragrance of the foliage too. It's a welcome addition to the garden. There is much improvement that can be done to this group of plants, and we've really only started to branch out beyond M. didyma and M. fistulosa. I've often thought about working with them, and may yet do so. Much of the focus is on compact habit, but I think tall varieties are the way to go. Especially restraining vigor of some of the big plants and getting a good white flower. And of course continued improvement of mildew resistance.