Here on the north shore of Long Island zone 6B we had a late Spring freeze which resulted in much damage to my roses in particular roses grafted to multiflora. As Spring approached frequent rainfall was a welcoming sight to this area which had been in a drought over the last few years. We have been upgraded from drought to abnormally dry. This summer has been very hot and extremely humid and unfortunately a perfect climate for black spot - warmth and moisture. I have several garden beds consisting of a combination of annuals, perennials, shrubs, evergreens and 100+ roses.
This Spring, I began my usual routine of spraying my roses including the soil beneath the roses with OxiDate depending on the forecast about every 10 days. It is not rainproof but has proven to be effective in the past. Morning deep watering was implemented. Bumper Crop was applied along with Mills Magic at intervals. The soaring high temps and 90% humidity continued day after day, week after week with no end insight. Often, the air was still, not a leaf moved. It felt so thick as though you could almost cut through it. Occasionally, we received a 15 min. hard downpour that resulted in no more than a runoff. the worst scenario was light drizzle rainfall that would last at 5 day intervals together with high temps. Suddenly without warning, black spot began showing up on various roses where it never appeared before in previous years. During this period of time, both growth slowed and few blooms appeared. While on the battleground, I discovered the perennials and shrubs were infected as well (Pass the aspirin please). With a new strategy in mind, I began cutting down hundreds of irises to 2 inches from the ground, staking, pruning, discarding and transplanting perennials and spraying ALL foliage. I would estimate at least 1/2 of the 100+ roses were infected. Any pruning required disinfecting the pruning tool with each cut followed by sealing them off with Elmer's glue. I don't know for sure if this sealing off procedure was effective but, it made me feel better that I was trying to limit contamination. After collecting and properly disposing infected leaves and observing partial and complete defoliation over a 2 week period of time, I began to see a turnaround with the emergence of new healthy growth. Finally, I began to feel as though I had control over this situation. What a relief! What a challenge!
Do you want to hear something really interesting?
As stated previously I have several garden beds, one of which stood out because it showed NO signs of black spot/fungus on any of the roses, annuals, perennials or shrubs. This bed contains 6 roses: Electron, 2 Mister Lincoln's, Over the Moon, Midnight Blue and The Yellow. A couple of years ago, I transplanted Peace from this same garden bed to another. It did not have a smithereen of black spot, however, it does now. In addition, the color of this Peace rose does not appear to be as vivid after being moved into it's new garden bed. I wondered why this particular garden bed with the 6 roses showed no signs of black spot. In my determination to get to the bottom of this I looked into the parentage of these 6 roses and compared them to others afflicted with b/s. Unfortunately, the parentage of some roses is unavailable, not disclosed or incomplete. I can tell you that Peace is in a lot of modern roses - 7,987 unique descendants. Some people believe r. foetida weakened disease resistance (b/s???). Many roses are descendants of this rose as well. This may be only one factor attributing to black spot. The unrestrained inbreeding and introduction of specific roses to achieve various colors by some hybridizer's has most likely led to less vigorous, disease prone offspring. It is regrettable, that many earlier vigorous roses were cast aside.
I knew I planted Mister Lincoln and (Ludwig's) The Yellow in that particular area because they are both shade tolerant. I also recalled the vivid colors in the Peace rose photo taken years ago with the Colorado Spruce in the background. Once again, an annual practice for me, I mapped out my entire garden and documented the number of hrs. of sun, shade, partial shade etc. in each summer garden bed between the hours of 7 a.m. and 5:30 p.m.
Result: the garden bed that resisted blackspot -
Receives full sun from 7 - 11 a.m.
Shade and Partial shade from 11 a.m. - 2:30 p.m.
Full sun from 2:30 - 5:30 p.m.
This is the only area in my garden that presents this scenario.
1) the early morning sun from 7 - 11 allows for dew or overnight rainfall to dissipate - to be absorbed.
2) the partial shade from 11 a.m. - 2:30 p.m. allows for transpiration - cool down and photosynthesis (remember the temperature dropped about 10 - 12 degrees during the total eclipse).
http://www.harlequinsgardens.c...
In other garden beds, the roses receive shade or partial shade in the morning and ALL other beds are exposed to full sun with high humidity during the hottest part of the day effecting transpiration and causing stress. In addition, the sun may effect the color of multicolored roses in particular roses such as Peace, Double Delight, Paradise etc. (phototropic). Some may fade, some may darken and still others may change color.
3) the full sun from 2:30 to 5:30 p.m. combined with the morning sun offers a total of 7 hrs of available sunlight.
Perhaps, by providing shade to roses during the warmest time of the daytime you will offer them necessary transpiration, lessen stress which may be a trigger, and the appearance of accurate color to your roses. Locating a duplicate site like mine in your own garden might not be feasible. Deciding to plant a rose near a tree in an effort to provide shade, keep in mind you will have to water more frequently. I have 6 huge town owned Maple trees on the western portion of my property. I find it necessary to almost flood my nearby garden beds in order to provide an adequate supply of water.
This Nov. I will mound up my roses with topsoil and mulch to provide winter protection. In the Spring, the mulch will be removed to eliminate any spores that may have harbored over the winter and ...
Being Again
The lesson learned: know your site...know your roses

MargieNY said:Some people believe r. foetida weakened disease resistance (b/s???). Many roses are descendants of this rose as well.

Correct, it is attributed to being a big factor in the BS weakness in modern roses, R.foetida is the main source of yellow in modern roses, also of note is lavenders and mauves came out of yellows. Given the yellows, lavenders and mauves being popular colours it's why R.foetida has been so prolific with descendants. R.foetida is very prone to blackspot when not grown in a climate it's suited for (it's a species that's designed to go dormant in summer) and this is often true of it's first gen offspring even when crossed with species that are resistant.

MargieNY said:This may be only one factor attributing to black spot.

Also correct as blackspot can be found in rose classes that were bred before R.foetida was added into the garden rose gene pool, it's just typically a lot less and not likely to take over the whole plant if untreated unlike a lot of moderns.

MargieNY said:The unrestrained inbreeding and introduction of specific roses to achieve various colors by some hybridizer's has most likely led to less vigorous, disease prone offspring.

Also kind of true if you look at the R.persica, quite a number of the descendants released have a lot of mildew issues (that seem to clear up when settled in but stress can see the plant become more covered in mildew), so it's kind of happening again to an extent....but if you look at the normal growing habits of that species it's not entirely unexpected that something didn't click well.

Margie, a very informative piece. There is one bed in my garden with relentless dawn-to- dusk light. And I have found a lot of roses lose vigor there. I know it has the highest pH of any bed in my garden and have begun adding sulfur. A very young Pink Lady apple tree thrives there and will soon bring some partial shade to bits of the bed.

I notice this year that the roses I sprayed very carefully last year and early this year with Actinovate are not the ones suffering from black spot. It's a whole new set of roses. So I'm thinking that there may be an ongoing competition between fungi and the bacteria in Actinovate on the surfaces of these roses. I am surprised that the action should last so long, given the high amount of UV light we have here.

This year I cannot characterize the BS outbreaks as belonging to any one location. There are four different spots involved with three different kinds of soil, six or seven cultivars and four different kinds of light pattern. I wonder whether there might be a microbiological explanation, maybe one that does involve interactions with peak temperature in the bed.

--- EDIT --->

After a few hours standing in the bright sun pulling blackspot leaves off roses that never before suffered from BS, it occurred to me that when I lived in TX I had more parallel experience to yours. Many of my roses went dormant during the hottest and dryest days of the summer there - maybe they did not lose their leaves but they did not set new ones. Something like that happens here in the month of June, but not quite to the same extent. But in locales with brutally hot and dry summers losing their leaves actually can be a pretty good way of preserving moisture. In places with blistering hot & dry summers and very long mild springs and falls, roses with lots of foliage might be served well by being a bit vulnerable to such a disease. R. foetida evolved in such a place. Of course, this might be to roses a little like the sickle cell anemia cell is to humans - conferring a definite advantage in places where a very specific condition prevails - and then only to the members of the population who have some offsetting gene, and conferring a heap of woe on those that get two copies of the gene. As noted above there is a lot of interbreeding, esp. among HT roses.

What blackspot seems to be doing in my garden to certain roses, Hermosa, Selfridges, and Sunsprite, is infecting and removing old leaves that have been on the roses since April, but leaving new growth alone. There is a sense that it is actually performing a kind of beneficia function, focusing energy on new growth the same way pruning does. So in this manifestation it might actually be an adaptive trait. On Colorific, though, it is attacking new growth leaves. And that is definitely maladaptive.

Protoavis - thank you for sharing your expertise and your confirmations - I appreciate your time.

Steve, I will try Actinovate. Thank you for the suggestion.
I am going to reevaluate my rose list and order duplicates of what roses did well this winter and summer.

This is a very interesting and educational topic. I wish I kept records so that I could contribute my experience here in a rather different climate. Black spot is definitely a problem in the spring and many of my roses partially or fully defoliate. By mid summer it is too hot for the fungus, but some are still leafless. Summer dormancy ? I don't know - no records, but this discussion may prompt me to put out the effort.

If you do, I'd love to hear the result!

This is a continuation from my last post https://garden.org/thread/view...
To review, my "perfect" garden bed location that resists blackspot receives:
SUN from 7 a.m. - 11 a.m. - helps to dry dew & reduce risk of blackspot.
PARTIAL SHADE from 11 a.m. - 2:30 p.m. - allows for transpiration (cool down).
SUN 2:30 p.m. on - offers a total of 6+ hrs. of sunlight.
I looked into the lineage to see the percentage of R. foetida contained in some of the roses. Here's a chart I made:




Next, I wanted to see if there were any roses in the lineage with shade tolerance. Two showed up.
1) Comtesse de Labarathe - an earthkind, cold tolerant, drought tolerant, vigorous, shade tolerant. With 15,623 descendants
2)Parsons Pink China - hybrid between chinensis & gigantea
- hardy, shade tolerant
- with 18,293 descendants
If you scroll halfway down to Parsons Pink China - read the square box on the right side.
http://bulbnrose.x10.mx/Roses/...

Here is a partial list of roses that have r. foetida, CDL & Parsons Pink China in their lineage background:
Double Delight
Electron
Fragrant cloud
Mister Lincoln
Over The Moon
Paradise
Midas Touch
Sunset Celebration
Peace
Velvet Fragrance
Shelia's Perfume
Midnight Blue
Rainbow Sorbet
I am assuming that providing 2 hrs. of shade during the hottest part of the day isn't going to negatively effect roses. In fact, it could be beneficial. Here's why I believe this may be true.
r. foetida wants to go dormant in the summer perhaps triggered by rising temps.
Teas want the sun and heat.
It appears the two are in conflict. And so a battle ensues.
Perhaps the mature leaves on the bush lose their nutritional value and become susceptible to fungus especially in high temps & humidity where transpiration is effected. What is interesting is, the newest growth does not contact black spot only the older mature leaves.
Conclusion: Possibly in changing the environment by providing shade during the hottest part of the day one can "trick" or prevent the rose to go dormant in the summer and eliminate it's susceptibility to black spot.

This is very interesting, Margie. You touch on a couple of issues. Let me see if I understand them. I've taken what I think you mean and developed it a little bit because I find the ideas to be a little non-intuitive and reasonably informative. I'd be happy to know if this is anything like what you meant...

Tension Between Leaf Loss and Minimum Growing Temperature
If I understand you correctly you are saying that roses such as R. foetida respond to temperatures above a certain critical temperature, LL, by losing their leaves. By contrast, roses that descend from the china roses simply will not grow unless the temperature is above some other critical temperature GR. Pretty much every hybrid tea rose has some foetida and some china heritage. This means that they will, on average, show some measure of each behavior. If we were to assume that these traits were passed along to hybrid tea roses unmodified and without some other compensating effects, the consequence is that the HT rose will not grow unless the temperature is consistently above GR. And it will drop its leaves or go dormant if the temperature is above LL. If other heritable factors from other roses affect things, then GR and LL can potentially be a somewhat different for each hybrid tea rose.

Without knowing what GR and LL are, we do know that if GR is very close to LL, the rose will grow satisfactorily in a very small range of conditions. Conversely, if GR and LL are very far apart, the rose has the potential to grow well over a larger range of temperatures.

GR, we must note, is not freezing. In the case of many hybrid tea roses it is not any temperature with thirty degrees F of it. My own guess is that it may be more accurately described by the soil temperature or average daily temperature than it is by daytime air temperature. For the sake of clarity let us specify soil temperature at about three inches depth as the thing to measure. I would speculate that for hybrid tea roses, GR is almost certainly greater than 70F/21C, based on some observations of roses in pots this season and some other factors. For some roses, I am tempted to believe it may be in excess of 80F.

I have no real idea at what temperature R. foetida would go dormant or lose its leaves. Maybe it's something like 115F/46C. If LL were much higher than this it would be such a very uncommon experience and would be of little help to the rose. If it were more than 20F lower than this, it would prove to be a handicap even where the rose is endemic.

So if these guesses were anything close to being correct, then in a place where the soil temperature is warmer than 70F and the air stays cooler than 115F, a hybrid tea rose with adequate water and not too much humidity is likely to do well enough. Coastal California, for example. Or most of Italy and Southern France. Or bits of Australia or South Africa. Places where the soil temperature never reaches GR (say 70F) during the summer will not be good places to grow many hybrid tea roses. In many places where this is true, frost is going to be a more seriously limiting issue.

Places where the soil temperature only reaches GR (70F) when the daytime air temperature is above LL (115F) would be places where it would be impossible to grow certain roses, even if they were frost hardy there. This combination of conditions comes rather close to describing a significant portion of the intermountain region of the western US. And so it would explain why gardeners in these regions might need to be a little picky about which hybrid tea roses they attempt.

Roses whose response to weather is more strongly influenced by multiflora, wichurana, gallica, and rugosa heritage will have GR set to a materially lower temperature. Because those species roses are quite reliably frost hardy, GR could be within ten or fifteen degrees of freezing without causing the rose to be harmed by the occasional but inevitable freeze. In dry, cool, or short summer areas it would serve the rose well in spring and fall by promoting growth when temperatures are marginal but soil moisture levels are more favorable.

We have absolurely no information on how breeding programs took account of these effects, if any ever did so. Nevertheless we do have reason to believe that roses bred by breeders like Poulsen, Radler, or Fryer might naturally tend to have a lower GR temperature than roses bred by Armstrong or Warriner because summer soil temperature in Denmark, Wisconsin, and Britain tends to be lower than it is in the rose growing parts of California. The natural selection process in Britain and Denmark would impose a greater disadvantage to roses that could not grow well in cool soil than they would in California's Central Valley. This would give us reason to believe that, on average, roses bred and selected in an area with a lower average ground temperature - however you define it - will naturally grow more vigorously in cool soils than those bred and selected elsewhere.

Why would a china rose have evolved to shut down completely when the weather is coolish? Evidently there is some cost to a rose to actually grow in cold weather; it is cheaper to grow in warm weather - when that is available in ample amounts every year. Dormancy in coolish weather also might arise from the fact that in Sichuan Province (likely a representative region for the origin of the china rose) summers are long and hot; winters short and dry. They have strong monsoon weather, making it much more favorable for growing during the summer. Furthermore, summer stretches out for many months. There is almost no advantage to the china rose in being prepared to grow vigorously when it is coolish, and some advantage in its growing vigorously when weather and ground temperatures are warm. This behavior, however, is not adaptive in places with coolish or dry summer weather or those with very cool nights which would keep soil temperatures low and retard growth.

The Use of PM Shade to Mitigate Blackspot Problems
We know that black spot only plagues roses when the air temperature is between about 65F and 85F and humidity exceeds a certain level for long enough. If we can create conditions that mitigate these factors, we might prevent or at least mitigate the effects of blackspot. Morning sun dries leaf foliage very effectively, so it is a very desireable factor in growing roses. But in summer roses can overheat, so a few hours of shade during the heat of the day can reduce the peak foliage temperature. and reduce transpiration losses of moisure which stress roses. Using this argument, we would give roses some shade.

I agree that roses under stress are more prone to black spot. There are several cultivars in my garden that only got black spot in their first season here. That said, I have never seen in my own garden or read anywhere that shade helps to prevent BS in roses. Shade might hurt. If the optimum temperature for blackspot cultivation is 74F, and if BS rarely grows in temperatures above, say 90F, then sunlight might actually help a rose heat up more quickly after a coolish night. Spending less time in the growth window for BS could reduce the rate of spread of the disease. This is more of an issue here in the mts. of AZ than it would be in places where summer nights never dip below 90F. If, on the other hand, leaf loss is a desired response to drought conditions, black spot on old leaves can promote a desired condition. One might argue, then, that shade could delay the need for leaf loss, but it might also augment the speed of leaf loss when needed. If this were so, then a bit of PM shade could help roses prone to BS stay alive both by delaying the need for leaf loss and by speeding the process when it is desireable.

Not an intuitive conclusion. A little speculative. Possibly helpful.

I have read that r. foetida has a shorter growing season - something like 3 - 3 1/2 mos.
From Wikipedia -
https://en.wikipedia.org/wiki/...
"native to the foothills of the Caucasus Mountains in Georgia". I tried to find out the temps in this particular area but was unsuccessful.
Perhaps by offering shade (cooler temps) during the hottest time of the day, it prevents r. foetida from going into summer dormancy - loose it's leaves. Perhaps, if it does go into summer dormancy, before the leaves drop they loss their nutritional value and may become susceptible to blackspot.

Last night, I found this article - maybe this will help explain it better. Scroll all the way down to the bottom to find this excerpt.
"CybeRose note: The lack of hardiness that Jerabek noted and the tendency to go dormant (or at least stop growing for a while) during summer heat are consequences of the "cold tolerance" inherited from Rosa foetida. That is, the species and many of its descendents do most of their growing at low temperatures — a fault also found in some Chinas and their derivatives. The short-lived foliage of R. foetida suggests that the species prefers to go dormant — leaf-shedding dormant — in summer, following its fairly brief growing season. This inherited tendency is directly related to the blackspot susceptibility of so many descendents that do not go fully dormant in summer. Short-lived foliage, repeatedly produced by continuous growth, lacks the "built in" resources needed to fight infection."
Here's the link: http://bulbnrose.x10.mx/Roses/...
Steve, thanks for trying to help me figure this out. You have a gift for writing.

http://bulbnrose.x10.mx/Roses/...

"Rosa foetida seems to be adapted to a short growing season in region where summers are fierce. It emerges early from dormancy, flowers, ripens its fruit (presumably) then sheds its leaves before the onset of scorching summer heat."

I would agree with greater than 70F/21C. My various roses (in pots mostly) only recently started growing new leaves (and flowers in some cases) but we've had temperates peak around 28c. This includes my rugosa hybrids (Dagmar Hastrup and Therese Bugnet, the seedlings too) but it may be the hybrid side there that's requiring higher temps as my R.glauca was putting out new shoots earlier (apparently survives down to zone 2) than everything.

I think the high temp would be anything over 38C or so (lower than 46c anyway) vaguely recall cellular damage starts happening around there with most plants, they recover because temps in most places don't remain that high for long.

I think it may be a pursuit of never achieving (BS eradication) as BS evolves along with everything else. Having said that I get very little BS despite being in a sub tropical/temperate zone (summer is usually the higher rainfall period but it happens most of the year) so hot and often humid but the roses are in half day sun and half day shade (not intentional, just the result of tall buildings along one side of the properties I move in to) which may play a part. The other big part would be a specifically used helpmefind to check roses to make sure they were fertile, fragrant and disease resistant...this is probably the main factor.

Either way the only foetida yellows I have would be sunsprite, typhoon (red yellow), and rise n shine....the first two are the worst looking rose plants I have during towards the end of summer and autumn....I just don't have better yellows (yet....have started breeding with R.ecea which has very interesting plant structure). So I can't really add too much on the feotida front.

Thanks for the info, Protoavis. 38C is 100F. This does seem a more plausible temperature. As I recall from more than twenty yeas ago... many roses that I grew in TX took summers off when daily temps peaked just below 100F.

I agree that BS is here to stay. And I think that checking at HMF is a really good step in finding out whether a rose has good resistance to the fungus. One of the reasons I like using Actinovate is that blackspot does continue to evolve. And if the bacterium in Actinovate were to evolve at roughly the same rate then the bacterium would continue to provide some measure of protection. I also like it because it seems less important to spray non-infected established leaves.

My Sunsprite roses are fine until we have a good monsoon in mid and late summer (and relative humidity soars from 15% to 95%) and then they become little blackspot machines. I've had problems with Golden Celebration and Julia Child; but my spraying program with them was successful this year. I was blindsided by finding BS on Hermosa and Blush Noisette this season, though. It may be most strongly promoted by west asian yellow roses, but it does not seem confined to them.

@Protoavis

I think the high temp would be anything over 38C or so (lower than 46c anyway) vaguely recall cellular damage starts happening around there with most plants, they recover because temps in most places don't remain that high for long.

I've pretty much stayed out of this thread because I only experience black spot in my garden in the spring, but I think I need to address the statement.

Protoavis, the summer temps in my garden average 38C / 100.4F all summer long. Some plants wouldn't be able to take that kind of heat, but a lot of plants do not have cellular damage and thrive. As for roses, I have to select roses with thick petal substance and dense foliage and provide sufficient water and they do just fine.

In other words the generalization above is probably a mis-statement.

Once temperatures are over 85F / 29.4C, bs spores are inactive.

Black spot was a problem with hybrid teas before r. foetida was introduced into the gene pool because they were a cross between a hybrid perpetual and a tea rose. The hybrid perpetuals, were a transitional class of roses that showed a high propensity to fungal diseases and required a careful spraying program (source: Brent Dickerson: http://www.paulbardenroses.com... )

I am headed down the mountain for shopping and can't add more this morning, but at least this is a start.

Steve ...

I agree that BS is here to stay.

You might find this article interesting:

https://today.agrilife.org/201...

Now, I am really off ....

Great article, Lyn.

Steve812 said:
My Sunsprite roses are fine until we have a good monsoon in mid and late summer (and relative humidity soars from 15% to 95%) and then they become little blackspot machines. I've had problems with Golden Celebration and Julia Child;

I agree with Julia Child. We bought it in autumn (potted) for my friends mother because of it's alleged scent (that first week before getting it to her it had a bloom, I did not smell anise/licorice but that may be because of numerous environmental factors, normal blooming season hasn't started yet so will see how it is in a few weeks) and being yellow. They were very covered in blackspot at the nursery.

RoseBlush1 said:@Protoavis

Protoavis, the summer temps in my garden average 38C / 100.4F all summer long. Some plants wouldn't be able to take that kind of heat, but a lot of plants do not have cellular damage and thrive. As for roses, I have to select roses with thick petal substance and dense foliage and provide sufficient water and they do just fine.

In other words the generalization above is probably a mis-statement.

Technically no, just not necessarily significant in all cases. The damage occurs, as you've said you've had to select for adaptions that resist/tolerate the visually obvious damage (like sunburn, wilt, etc). The damage at this lower end is generally reversible (usually over night when temps drop). If you were to take a slice of the plant and look at it's proteins there'll be degradation/anomalies relating to heat stress that won't be seen in the middle of spring. Even if not visually obvious it's likely to be a trigger for species that go dormant in summer.

Having said that, I did a google (to see temp/rainfall/etc things, I get curious and the Australia BOM website has spoiled me...I mean seriously look
http://www.bom.gov.au/climate/...
thousands of locations recording data even just small distances away from each other.)
and found
https://www.usclimatedata.com/...

which suggests you're more around the 96F most days of summer rather than 100F? I'm not sure how accurate that is though.

It's probably inaccurate ...

The problem with using that sight is that it averages data over several years and doesn't take into consideration the normal weather cycles of the Western United States and smooths the data out to the point where it is almost meaningless.

Also, I live in the mountains. Elevation plays a big role in both weather reporting and the climate in my garden. None of my roses have ever gone "summer dormant".

However, this takes us off topic.

The important part of my post was that HTs were susceptible to fungal disease even before Pernet introduced the genes of r. foedita to the rose gene pool.

Margie asked for some clarification on my use of GR and LL. And it occurred to me that I had left some things poorly defined. Maybe this will help...

GR is the minimum temperature at which a particular rose sets new growth. It might be an average daily temperature. Or it might be the soil temperature. I'm going with soil temperature at three inches into the ground. So if we suppose that for a specific (hypothetical) rose, say, Golden Warbler, GR is 70F then when the soil temperature rises above 70F Golden Warbler will set new growth (hence the name GR) with a relatively high degree of vigor. Below that temperature, it will kind of shut down.

Similarly, LL is the maximum temperature at which a rose will still demonstrate this same kind of behavior in setting new growth. I am thinking LL is an air temperature; the highest air temperature a rose is exposed to during a day. It might, though, be more like the maximum leaf temperature, which could be somewhat higher if the rose is in full sun at the hottest part of the day. So if we suppose that for a specific rose, say, Golden Warbler, GR is 96F, then when weather conditions are exposing it to that temperature, it will cease to produce new leaves. And if it is profoundly influenced by R. foetida, its old leaves will get black spot and fall off. And so it will experience very visable leaf loss (hence the name LL). And maybe this leaf loss will preserve moisture well enough that when the weather cools slightly and the monsoon rains arrive, the rose will have the resources to set new leaves with renewed vigor.

It seems to me that every rose has a unique set of these temperatures, GR and LL. Roses that we think of as cool season roses - rugosa hybrids, wichurana hybrids, and multiflora hybrids have GR set to a relatively low number. Maybe they begin to grow when the soil temperature reaches 50F or 60F. GR would be 50F or 60F. Roses we think of a warm season roses - tea roses and roses with a very strong affinity for R gigantea and R chinensis are likely to have materially higher GR temperatures, maybe 70F or even 80F. I noticed, for example that Incantation only bothered growing for the eight weeks in the year when nightime temperatures exceeded 80F. I would say that GR for Incantation is probably higher than 80F. As a practical matter, that means the rose cannot be grown on my property, indoors or outdoors. It is never warm enough to promote steady growth. I notice that Silver Jubilee, a hybrid tea rose with 6% R kordesii (wichurana and rugosa) is growing quite vigorously in our coolish fall weather where nightime temps hover around 50F and daytime temps are around 75F. Silver Jubilee has a GR of something less than 65F, I would say. Interestingly, Gemini and Firefighter are growing with a similar level of vigor right now. So I have to estimate their GR temperatures below 70F also. By contrast, Moonstone appears to be just sitting there. It grew during the heat of the summer. Its GR must be closer to 80F.

I noticed when I lived in Texas that most roses took the summer off. When the high temperature exceeded 96F virtually every day most of them stopped making any new foliage or flowers. This was a condition that persisted from about mid-May to mid-September. In Austin there were only occasional excursions below freezing, so it was like living in a place with three seasons: Spring, Summer, and Autumn. In Spring and Autumn the roses would bloom. In fact, the stretch of time in which no growth occurred in the summer was much longer than that stretch of time without growth in the winter. And I came to believe that in many cases it was better to prune a rose in late August before the fall rains started than at any other time of year.

I practiced this on the polyantha Marie Pavie this summer. In June its leaves had all turned brown due to drought and heat. I pruned it. Two weeks later it set new foliage; and it has never looked better. Maybe for Marie Pavie LL would be as low as 86F. But maybe the plant just got too dried out. As high temperatures hovered above 90F Sunsprite lost its leaves, with some aid from BS. So maybe for Sunsprite LL is close to 92F. For David Austin roses I would say that some cultivars might possess a definable LL, Abe Darby and Fallstaff for instance, lost most of their leaves when temperatures went into the 90F+ range. Others, such as the Crocus Rose, Teasing Georgia, and Susan Willaims-Ellis did not.

There were only eight weeks of weather here with high temperatures over 90F, and probably not two with temperatures over 95F. Only Sunsprite was fully defoliated. It does seem to me that a very large portion of the established roses began growing with renewed vigor - some with a vigor not seen at all earlier in the year - once summer transitioned toward fall and daytime temperatures no longer reached 90F. At the same time, nighttime temperatures were below 65F. Some of the new, unestablished roses have not behaved this way. And my prediction is that this presages their doom. A rose that does not grow vigorously when the conditions are most favorable here will die over the parts of the year when growth is difficult (something in excess of thirty weeks a year thanks to frost and drought.)

At this point I need to tell of an observation of the hybrid tea rose Beloved. I bought it in a pot from Jackson and Perkins, and it arrived in May. I potted it into a three gallon black plastic pot and set it in (just about) full sun. By mid August it was growing like topsy and blooming its head off. I settled on a place in the garden with at least as much sunlight, dug a hole and planted it. At this very moment it stopped growing. It did not drop any leaves. It simply stopped making new ones. I first attributed it to transplant stress. Maybe it was, but more than four weeks later there is still no sign of new growth. So now I am wondering if maybe the difference is that soil temperature there was materially cooler than the soil temperature in pots. I have had plastic pots containing plants get so hot that they physically melt and make the soil inside the container too hot to touch. They absorb a huge amount of sunlight. I know they encourage very harmful precocious growth in spring due to heating. So they must add five or ten degrees to soil temperature during the summer. Point is, now I have evidence to suggest that GR for Beloved might be lower than the soil temperature of my garden in late August. That does not bode well for its long-term prospects here.

At this point I only have enough observations to justify some tentative definitions of the categories. That is, I know that something closely related to the concepts of minimum soil temperature and maximum leaf temperature have some value in predicting when a rose will grow and when it will not. I have some strong reasons to believe that for some cultivars, soil temperatures at my location may preclude their horticultural viability. (I don't know, maybe all of this is very well known to people in the industry. But if it is so, it is not very well communicated to people who grow roses.) With some more work one could, I think, measure a meaningful range of values for GR. (BTW, Johnny's Seed catalogue does/once did, publish something analogous to this for every cultivar they sell/sold.) For certain roses with at least some R. foetida heritage one could define a leaf loss temperature, LL. Maybe for some other roses, too. If we knew these for every rose we would have a much better picture of which roses cannot hope to succeed in certain places. We would also have a clearer picture of when to prune each cultivar. As important as they might be I'm quite sure it is somewhat more complicated than simple USDA zone cold hardiness and simple late spring freeze protection.

Steve ...

Thank you for sharing your observations. I've had a long, hard day and cannot address all of them properly tonight, but thought I would address two of them.

The first:

When the high temperature exceeded 96F virtually every day most of them stopped making any new foliage or flowers. This was a condition that persisted from about mid-May to mid-September.

The temps in my garden are always in the high 90s to low 100s for the same period of time and my modern roses all continue to bloom in flushes. True, during the hottest part of the summer, the blooms are smaller and have fewer petals, but the roses are still blooming. So, I don't think it is high temps that caused your roses to stop making new foliage and blooms.

The second:

When you write about your roses losing their leaves when the temps rise, my first thought is that spider mites love hot dry climates and can defoliate a rose in a couple of days. Did you check your roses for a mite infestation ?

I am pooped. That's it for tonight ...