Another aspect of hardiness that is often overlooked is optimum growth temperature.
The subject of temperature and growth has been studied from time to time, but the needs of commercial growers of cut flowers are not the same as those of gardeners. A commercial grower wants a plant that continues growing and blooming at low temperatures (approaching freezing) to reduce heating costs. A northern gardener, on the other hand, is better off with a plant that refuses to grow at low temperatures when frost is possible.
Then again, many growers expect their rose bushes to start early and finish late. These expectations conflict with hardiness. A plant that begins growing too early in the season risks damage from late frosts. And a plant that continues growing too
late in the year risks similar injury from
early frosts.
Rosen (1956) found that varieties that break dormancy late suffered less damage from late March freezes than the earlier types. And plants that grew slowly were less harmed than the faster types that started at the same time.
The late season is just as important in the search for total hardiness as Bugnet (1941) observed.
"With a much longer experience, on a larger scale, in stone-fruits breeding, I am led to believe that a plant, in order to withstand our climate, needs a very early ripening of its tissues. Winter-killing, apparently, is not caused by extreme cold but rather by a too early cold snap catching immature wood, like the 30° below we had in the first part of November last. Once, at dawn, on October 12, 1930, we had 16° below zero. The next day was rather warm. None of my hardy hybrids and no native tree or shrub suffered. I have often noticed that half-hardy plum or apple trees here, unhurt by December 1, passed unharmed through the rest of the winter no matter how intense the cold."
Risley (1958) pollinated 'Skinner's Rambler' by an assortment of other roses. He chilled the seeds, but found that the time to germination varied widely (83 days to 173), according to the pollen parent. Two of these caught my attention. The offspring of 'Persian Yellow' started sprouting in 91 days, whereas those from 'Diamond Jubilee' took 160 days to come up. Both of these varieties are descended from
Rosa foetida. However, 'Diamond Jubilee' is a seedling of 'Marechal Niel', a variety that loves heat and refuses to grow at low temperatures that other roses could enjoy.
The slowest seedlings to appear were from 'Queen of the Lakes', a Brownell "Sub-zero" rose.
I think that Risley got tangled in terminology. He assumed that the late appearance of some seedlings was due to a greater chilling requirement. But he was not watching to see when the seeds germinated, only when the seedlings emerged. The ability to grow (or not) at low temperature would skew the results. That is, one seedling might germinate early (short dormancy) but emerge late due to its slow growth at the stratification temperature. On the other hand, a different seedling might germinate late, then grow rapidly.
The point, here, is that growth rate can be assessed independently from dormancy-breaking. Plants that grow slowly or not at all at low temperatures will likely suffer less damage from late Spring frost and early Fall freezes.
Selection for growth temperature has been done successfully in the Calla, 'Green Goddess'.
http://www.actahort.org/members/showpdf ... rnr=766_19
Bugnet: The search for total hardiness (1941)
http://bulbnrose.x10.mx/Roses/breeding/Bugnet.html
Rosen: Resistance to Spring Freezes (1956)
http://bulbnrose.x10.mx/Roses/breeding/ ... pring.html
Allen & Asai: How Frost Damage Occurs (1943)
http://bulbnrose.x10.mx/Roses/breeding/Frost/Frost.html
Asai: Repeated freezing and thawing (1944)
http://bulbnrose.x10.mx/Roses/breeding/ ... g1944.html
Greeley: Temperature and Rose Bloom (1919)
http://bulbnrose.x10.mx/Roses/breeding/ ... y1919.html
Greeley: Night-Growth of Roses (1920)
http://bulbnrose.x10.mx/Roses/breeding/ ... rowth.html
Risley: Male controls sprouting (1958)
http://bulbnrose.x10.mx/Roses/breeding/Risley.html
http://bulbnrose.x10.mx/KKing/HeatGrowth.html
Another aspect of hardiness that is often overlooked is optimum growth temperature.
The subject of temperature and growth has been studied from time to time, but the needs of commercial growers of cut flowers are not the same as those of gardeners. A commercial grower wants a plant that continues growing and blooming at low temperatures (approaching freezing) to reduce heating costs. A northern gardener, on the other hand, is better off with a plant that refuses to grow at low temperatures when frost is possible.
Then again, many growers expect their rose bushes to start early and finish late. These expectations conflict with hardiness. A plant that begins growing too early in the season risks damage from late frosts. And a plant that continues growing too [i]late[/i] in the year risks similar injury from [i]early[/i] frosts.
Rosen (1956) found that varieties that break dormancy late suffered less damage from late March freezes than the earlier types. And plants that grew slowly were less harmed than the faster types that started at the same time.
The late season is just as important in the search for total hardiness as Bugnet (1941) observed.
[quote]"With a much longer experience, on a larger scale, in stone-fruits breeding, I am led to believe that a plant, in order to withstand our climate, needs a very early ripening of its tissues. Winter-killing, apparently, is not caused by extreme cold but rather by a too early cold snap catching immature wood, like the 30° below we had in the first part of November last. Once, at dawn, on October 12, 1930, we had 16° below zero. The next day was rather warm. None of my hardy hybrids and no native tree or shrub suffered. I have often noticed that half-hardy plum or apple trees here, unhurt by December 1, passed unharmed through the rest of the winter no matter how intense the cold."[/quote]
Risley (1958) pollinated 'Skinner's Rambler' by an assortment of other roses. He chilled the seeds, but found that the time to germination varied widely (83 days to 173), according to the pollen parent. Two of these caught my attention. The offspring of 'Persian Yellow' started sprouting in 91 days, whereas those from 'Diamond Jubilee' took 160 days to come up. Both of these varieties are descended from [i]Rosa foetida.[/i] However, 'Diamond Jubilee' is a seedling of 'Marechal Niel', a variety that loves heat and refuses to grow at low temperatures that other roses could enjoy.
The slowest seedlings to appear were from 'Queen of the Lakes', a Brownell "Sub-zero" rose.
I think that Risley got tangled in terminology. He assumed that the late appearance of some seedlings was due to a greater chilling requirement. But he was not watching to see when the seeds germinated, only when the seedlings emerged. The ability to grow (or not) at low temperature would skew the results. That is, one seedling might germinate early (short dormancy) but emerge late due to its slow growth at the stratification temperature. On the other hand, a different seedling might germinate late, then grow rapidly.
The point, here, is that growth rate can be assessed independently from dormancy-breaking. Plants that grow slowly or not at all at low temperatures will likely suffer less damage from late Spring frost and early Fall freezes.
Selection for growth temperature has been done successfully in the Calla, 'Green Goddess'.
http://www.actahort.org/members/showpdf?booknrarnr=766_19
Bugnet: The search for total hardiness (1941)
http://bulbnrose.x10.mx/Roses/breeding/Bugnet.html
Rosen: Resistance to Spring Freezes (1956)
http://bulbnrose.x10.mx/Roses/breeding/Rosen_Spring.html
Allen & Asai: How Frost Damage Occurs (1943)
http://bulbnrose.x10.mx/Roses/breeding/Frost/Frost.html
Asai: Repeated freezing and thawing (1944)
http://bulbnrose.x10.mx/Roses/breeding/AsaiFreezing1944.html
Greeley: Temperature and Rose Bloom (1919)
http://bulbnrose.x10.mx/Roses/breeding/Greeley1919/Greeley1919.html
Greeley: Night-Growth of Roses (1920)
http://bulbnrose.x10.mx/Roses/breeding/GreeleyNightGrowth/GreeleyNightGrowth.html
Risley: Male controls sprouting (1958)
http://bulbnrose.x10.mx/Roses/breeding/Risley.html
http://bulbnrose.x10.mx/KKing/HeatGrowth.html