The 'Seasonal Stream' on the south edge of our property is supposed to run in the months of December through March. By 'run' we mean that in places it's eighteen inches wide and not so much as an inch deep and in other places it's completely below the surface of the ground. Whenever we get more than two inches of rain in an hour, though, it runs twenty feet wide, and three feed deep and roars like a freight train. The drought starts in February, but a snow storm or two will feed the seasonal stream with moisture well into spring.
By April it is dry a foot down, and it is expected to stay just about that dry until the next winter. Of course, because it is at the bottom of a big rock-lined basin and because precipitation from a quarter mile away drains through this 'stream' there is some lingering subterranean moisture here for a week or more after we get more than about half an inch of rain. My cunning plan was to take advantage of all these facts and plant roses in this stream bed. I figured that in the winter the roses wouldn't mind being in running water up to their knees. And during the summer they would enjoy some subterranean refreshment.
My trial run was a measured success. Four roses planted in spring of 2011 survived through winter and all last year. So this year I committed to planting the stream with roses. I made a small 'Old Rose' bed featuring Marie Louise, Compte de Chambord, Nuits de Young, Madame Plantier Duke of Edinburgh and Gros Choux de Hollande. And next to it I planted Graham Thomas (whom I like to believe would be honored to share a bed with some favorite old roses), Liebeszauber, Gingersnap, Mardi Gras, Oranges and Lemons, and Cary Grant. Then, realizing that these roses would be young this year, and therefore small, I planted between the roses a mess of dahlias.
I would declare the experiment a success. The golden dahlias in the modern rose bed are glorious right now. I'd never have planted dahlias there were I not trying to start a rose bed. Many of the roses are thriving too, though in some cases they are a little hard to see thanks to the dahlias.
But of course nature frequently does something different from what we expect. And this is where the interesting part starts... soon. This year we had maybe six inches of rain in July. Then we had some rain almost daily through August. The frequency of precipitation reduced to two or three times per week in the first two weeks of September. The last week or so has been pretty dry. That is, not a lot of clouds and not a lot of rain. The relative humidity, which was pegged at about 70% for two months has dropped below 45% during all daylight hours. But of course that level depends on where you measure it. Inches above a running stream it is higher.
In any case, the seasonal stream started running in early July and it's been running ever since. Some of the dahlias - the ones in the lower 'old rose' bed found their feet to be too wet and expired quickly. I feared for the roses. Every day I go out and look at the old roses in that bed and I expect them to have collapsed into a heap of mush; but they look bright, crisp, green, and happy. They have grown vigorously, although perhaps not quite so fast in running water as when the soil is simply moist. I know that the advice is not to plant roses in standing water, but perhaps freshly fallen running rainwater is richer oxygen and carbon dioxide than your typical standing summer pond. Not certain what accounts for my unexpected success here.
The stream has had a slightly different effect on some of the modern roses. Yesterday I noticed that Cardinal Hume which had been growing like gangbusters before the rains had lost about half its leaves to blackspot and was preparing to lose another quarter. Next to it, Gingersnap still had a lot of healthy foliage, bit it had a few leaves completely blackened by the fungus. Liebeszauber has some yellowing leaves, but the cause appears not to be blackspot, so I'm little worried. Perhaps it is smothering from lack of oxygen, the canary in the coal mine, so to speak. Speaking of canary, all three Sunsprite planted in this area are happy as a clam. Far away from the stream I have spotted just a few yellowing, black-spot infected leaves on Winsome, Toscana Vigorosa, and Paradise Found. These appear to be the aged leaves that roses tend to drop just because they are old. But they are lightly spotted with black spot: cause for raised alertness.
It's been an even better chance to discover which roses are immune to powdery mildew. The infestation began with Permanent Wave in an area where some dahlias were troubled by the disease last monsoon season. Many nearby roses were untouched: City of Leeds, Hobby, Kimono, Geisha, Ginger, Sexy Rexy, Crocus Rose, The King's Rubies, China Doll and Chevy Chase. Zepherine Drouhin got hit next, but recovered with some copper soap treatment. It has some deformed leaves, but new growth now seems to stay well-formed. Madame Alfred Carriere got the next wave. The fungus infested only new growth. Interestingly, Mme Alfred's leaves got distorted but they never got very powdery from the mildew. It was as if the rose was successfully fighting off the infection though being affected by it. Or perhaps my copper soap ministrations helped it, too. Baronne Prevost had a touch of the same thing, but is fairing well.
The very tippy tips of Ilse Krohn Superior were affected to the point of turning white with powder. That's the only place in the garden where I have observed the infection to get so advanced. Interestingly, established leaves seemed to shrug it off completely. Climbing Pinnoccho and Shocking Blue had slight leaf deformity, but no visible powder. Nearby Berolina, Rise 'n' Shine, Moonstone, Charles de Gaulle, Gemini, Rock and Roll, Blush Noisette, Emily Gray, JACclam, Looping, Grande Dame, Paradise Found, Rainbow Sorbet and Cherry Parfait, were unaffected. It looks like there might be some infection spreading to Double Delight. Handel, Antike 89, Penny Lane, and Desiree Parmentier are unaffected. I had thought Europeana was unaffected, but I am wondering whether its aged flowers are getting it. South Africa has remained free of infection as have Winter Sunset and Golden Celebration, but Folklore and Duftzauber 84 seem to have been lightly affected. The glossy leafed multiflora climber Psyche was free of it in all places where it lived in the sun; but where it both touched Lavender Lassie and spent much time in the shade, it was hit hard. Darlow's enigma and Ghislaine de Feligonde were unaffected. Only one sprig of new growth on Hermosa was affected, but it was devastated. That may have been the new wave of aphids, though.
I observe that shiny foliage seems to correlate pretty well with resistance. And that points of infection with powdery mildew are in all cases soft, new growth. I was interested to see that the first and only rose to show signs of both blackspot and powdery mildew was Merveille de Lyon. The new leaves at the top were deformed by mildew. The old leaves at the bottom are yellowed by blackspot. The middle-aged leaves in the middle are still relatively healthy, but the plant looks a wreck. Remind me not to plant Baronne Adolfe de Rothschild until I've learned how to control both infections.
My second round of spraying has been a biological control agent labelled "Biocide" that I bought a few years back from Gardens Alive. I'm hoping that the bacterium responsible for controlling the fungus is still viable. Spraying it seems to have materially exacerbated my fall allergies in a way that did not seem to happen with copper soap. Because the powdery mildew infection sort of advances in waves that correlate with weather, and because the effect of infestation seems a little subtle for some days while the fungus establishes on a leaf, it's a little difficult to tell whether success is aided by intervention or whether any success I might have is just a fortunate artifact of drier weather. Today it looked like plants that had been affected are no worse off than they were yesterday or the day before. That's good, because when the disease is winning, it can colonize a lot of new territory fast. And really susceptible new growth can become covered in powder in a day or two, it seems.
This is my first serious encounter with powdery mildew. At this point the only plant that I can say is 'ravaged' by the disease is Permanent Wave. I am contemplating waving goodbye permanently. The others that have been affected have been touched by it but have not really lost any leaves to it. This is also my first encounter with black spot on this property. Now I understand the limits of my good fortune with respect to raising roses here. If the weather becomes so damp that I do not need to water my roses, I will need to spray them. I wish I had understood that when I gardened in NJ. There the rule would have been the other way around: "If it is dry enough that I don't have to be spraying for blackspot, I need to be watering my roses."
The seasonal stream is still running in places. The air is getting drier and cooler every day. In four weeks frost will kill most of the rose leaves in my garden and the fungal infections along with any predatory bacteria will fall to the earth for winter. Horticultural oil and more biocide will arrive soon and be applied next spring. We shall see then how the rose beds in and not so far away from the seasonal stream will be affected by this year's particular events. Stay Tuned.