I don't have the answer to your question, Susan, but in my garden I notice more of a correlation between temperature and growth. When it's cold and cloudy during the day the roses seem to hunker down and do little in terms of growth. After two or three days of warmth and sun there is very noticeable growth of leaves and development of buds and flowers. That's not to say that length of days doesn't also play a major part but it's more difficult for me to quantify. Interesting question; will be waiting for more knowledgeable people to come on and give an explanation.

Ingrid

I do not have the technical info you asked about, but I think that most plants are very "length of daylight" sensitive, at least here in No CA. I notice every January around here that the acacia trees start blooming, whether we are having cold weather, sunshine, or rain, it doesn't matter. In Feb ditto the fruit trees, Japanese quince, camellias, etc. My bulbs start coming up in Jan also, no matter what the weather is doing. (Sometimes this is annoying, because the daffodils get smashed by rain, but they come up then anyway.)

My roses start leafing out every Jan/Feb as well. I have noticed that warm weather does make the growth larger sooner, and affects when they actually start blooming a bit, but when new growth starts seems to be length of day dependent. Some folks think the soil temperature is critical, and of course it may be where the soil actually freezes, buy here in our mild climate it appears to me that the length of daylight is the determining factor.

Jackie

Roses are day-neutral, that is, the length of light vs dark does not trigger either growth or flowering. Many plants are either long-day plants or short-day plants. Some such as onions come in both kinds, which is why norther gardeners have to grow different onions than southern gardeners and those of us in the middle are scratching our heads trying to figure out which ones are best.

Rosefolly

Thank you, Rosefolly.
That is eggs-ackly what I wanted to know !!!!!!

Susan

So, Maurizio, you are saying that roses that originated close to the equator (where day length does not vary much) are not impacted by the change in day length, whereas roses which originated farther from the equator (where day length varies by many hours from solstice to solstice) ARE impacted by the length of the day and not just temperature and water?

Interesting
Susan

We've been tracking bloom times and cycles in the cemetery garden in Sacramento for several years. Definitely, Asian roses are influenced by temp. We've had bloom vary by three weeks, depending on the weather. We also have more of a dormant period in the summer when it's very hot. Last summer, which was mild in Sacramento terms, found much more August bloom than usual.

I'll have to look at the data and think about the European OGRs and how they are affected. I think that, indeed, they tend to bloom at about the same time regardless of weather. They bloom in late April to early May here, not June as they do in Europe. Often, it's hot here by then, and the blooms don't last very long. Also, we often don't get enough winter chill for Gallicas to bloom at all well. I believe that these roses are like apples and other fruits that need a minimum amount of chill hours.
Anita

Susan, it's a little bit complicated than that, but roughly speaking, yes.

Roses like Asian species, originated in mild, subtropical climates, and they hybrids, aren't affected in their growth by day shortening - simply, it's not in their gene/DNA pool.
On the contrary, North Europe and North America species "knows" that where days shorten, it's time to go dormant, and that's written in their DNA.

Here, Roses such as Rosa chinensis spontanea, 'Fortuneana', LadyBanks, 'Cooperi', for instance, start growing rampantly in September after our long, dry, hot summer, as soon as it rains again and temperatures cool down a little, keep on going right into the middle of winter. They have a very short "stop" time in the cooler days (temperatures going down to 4-5 �C minimum in January) and then immediately restart to grow as mid-day temperatures raise up. If it's mild in January, you may have some unexpected surprises: I had, for instance, some open blooms on 'Belle Portugaise' for Christmas!
They follow the same cycles as "mediterranean" plants: we have in our gardens plenty of plants following the same cycles, as southafricans (Zanthedeschia, Dimorphotheca...) or mediterranean plants (Euphorbia, Acanthus, Cistus, ...artichokes!).
It's interesting to point out that these plants, if cultivated in areas with settled climate conditions (such as England, some areas in North California, etc) bloom for a very long time, and often re-bloom. I think of Cistus in England as well of LadyBanks, as Jeri reported sometime. It's like they're "stopped" in a never-ending Spring. We experienced something similar a few years ago, with an unexpected cooler and rainy climate in late May: my Rosa banksiae 'Lutea', who started blooming as usual in mid March, re-bloomed in May and was still in bloom by early June!!

My Gallicas, Damasks, Centifolias... drop their leaves by early November, when it's anything but cold, with min.temperatures of 10-12 �C and 18-23 �C max. And they don't start to grow again before late March, no matter how mild it's the winter or how it could be hot and sunny February.

When I hold pruning courses here in Italy, I always point out to my students :) that this different behaviour has a big influence bot in time of pruning and in the way you prune.
So, speaking for climates in zones 8-10, you can prune daylinght-sensitive roses as soon as they drop their leaves -for instance, in early December-, and you can be sure they won't move before they have sufficient daylight again.
Contrariwise, it's best to prune temperature-sensitive roses such as Chinas and Teas during their summer dormancy.

Maurizio

My information came from an article on photoperiodism. Scanning quickly around the internet I see several references to roses being day-neutral. However, Maurizio brings up an interesting point. Outside of the old rose community, most people don't even know that there are roses other than modern hybrid teas, floribundas, and perhaps minis or climbers if they are really into roses. Very likely this judgment was made strictly on observing the responses of hybrid teas and floribundas. Onion varieties differ in their photoperiodism, so why not roses of different ancestry? I don't know that anyone has actually done serious study on this issue. Maurizio's observations lead me to believe that it might be worth while. Any graduate students in horticulture out there looking for a dissertation topic?

Rosefolly

All of which is why THIS is where I come to get answers when I have questions. Sometimes the answers come in bits and pieces as we get to put pieces of the various puzzles together; and -- as now -- sometimes the answers lead to more questions.

Thanks everyone for contributing to my fascination for these beauties.

Susan

This IS a very interesting thread. I don't know much about this day-length stuff but have wondered about it. Winter has been a little different here in Maryland this year. While it has never been really cold, it seems like it's been in the 30s or below almost every day. We usually have quite a few warmish days here and there, days in the high 40s or more that are good for doing some work outside.

I am not too worried (yet) about my more tender roses though, because I have read (on this forum, of course, where I get most of my info) that the tender types are better off kept colder and dormant. Olga has said most winter die back is due to temperature fluctuations that cause the tender types to bud out and then freeze back.

We are having a high of 24 degrees today, and my witch hazel picked this day to begin to bloom. I think it bloomed this exact time last year. I plan to keep track of bloom times this year - especially for the roses.

Maureen

All of which is why THIS is where I come to get answers when I have questions. Sometimes the answers come in bits and pieces as we get to put pieces of the various puzzles together; and -- as now -- sometimes the answers lead to more questions.

Thanks everyone for contributing to my fascination for these beauties.

Susan

This has been fascinating, and it was interesting to me that Maurizio's explanation coincided with what I'd observed in my own garden. My roses are almost totally Asian and they respond beautifully to warmth and moisture. I vividly remember Mmr. Joseph Schwartz in the brutally hot summer of the year before last, when she was basically a bundle of sticks and I was ready to discard her, since I had watered her every day along with all my roses. I then got the idea of running a trickling hose on the soil around her for three or four nights in a row, all night long and in the early morning and after the sun went down. The result was dramatic! She exploded with new leaves and beautiful flowers, and even now in February is doing the same thing.

Ingrid

The thing about response to day length is that it has nothing to do with the weather at all. Plants that are day length sensitive are responding to the number of hours of light vs the number of hours of darkness, independent of temperature or rainfall.

My observations confirm what others have said here. The European old roses drop their leaves in the fall regardless of how warm the weather is. As for my Teas and Chinas, normally they begin flowering around the beginning of May, while the once-blooming European old roses get going around the middle of May and continue through mid-June. However, last year we had an unusually cold spring, and the warm-climate roses held off blooming for two or three weeks and began flowering at the same time as the European old roses. It just wasn't warm enough for them to bloom at their usual time.
Melissa

As Rosefolly points out, increasing day length and temperature usually coincide, so it isn't quite clear whether the variation in the roses' performances are in response to the warming/cooling of the soil, or the lengthening/shortening of the hours of daylight.

It does make some sense to me that roses that originate in day neutral environments might not have the inherent ability to change in relation to the shortening of the days when they encounter them and that they are simply differently equipped to deal with the changes. (But I should point out that what makes sense to me is often not what makes sense to anyone or anything else on the planet.:-)))))))

Thanks for all the input.
Susan

Definitely old European OGRs like gallicas, damask, mosses, etc don't get fooled by warm weather spell and wait until spring is in full force to start their growth. They are also wise to go dormat in fall even if it is warm. This makes them very good roses in climates like we have here. Weather swings can be really dramatic in winter here and many roses get canker and die (or almost die) because they react on each warm spell. Because European OGRs are not fooled by warm weather you can prune them in winter here. No danger pruning will stimulate their growth. I always prune my gallicas, etc, in December. If I would do this to my Teas, or HTs I would loose them, no doubt.
Olga