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Lepidote Crosses in 2003 using
Rhododendron tomentosum
as Seed Plant

Kristian Theqvist, 09-Aug-2006, rev. 019

tomentosum flower time on the marsh
Figure 1. Rhododendron tomentosum shrubs are flowering in June on the marsh near to my garden.


Rhododendron tomentosum (Marsh Tea) has now been included in the subsection Ledum of the subgenus Rhododendron of the genus Rhododendron. There are only few known crosses done with other lepidotes. It would be interesting to know whether it would be possible to get new cultivar forms of this very hardy shrub. I'm pretty sure that most of the crosses that I'm trying are doomed to fail, but without trying I'll sure get nothing.
(The comments in parenthesis in the following list were my feelings on the probability of succeeding before I made the pollination in June 2003.)

R. tomentosum x
  groenlandicum (should match)
  hypoleucum (should match)
  hippophaeoides (maybe?)
  minus (Milde) (probably not)
  minus, pink (Göteborg) (probably not)
  kiusianum (totally hopeless)
  ledebourii (maybe?)
  dauricum (maybe?)
  impeditum (maybe?)
  minus (Hachmann) (probably not)
  'Robert Seleger' (maybe?)
  'P.J.M. Elite' (maybe?)
  'Mother Greer' (maybe?)
  'Ramapo' (maybe?)
  'Blumiria' (maybe?)
  cinnabarinum var. cinnabarinum (hopeless)
  'Dörte Reich' (minus x cinnabarinum Concatenans) (picture © Glendoick Gardens) (hopeless?)

A total of 17 crossings with tomentosum of which I hope that at least a couple or three would be successful.


Status 20-Jul-2003 on tomentosum crosses
The ovaries of the pollinated flowers are swelling and elongating on surprisingly many of the crosses that I made. Of course it is possible that the seeds are not germinating or the seedlings do not live long if the genes of the father differ too much from the genes of the mother. Anyhow, I'm eagerly waiting on some exotic results.

Pollen (father) Swelling ovaries / Total Comments
Non-pollinated stigmas as control  0 / 11 This was to verify that the pollination method with emasculated flowers and covered stigmas was good.
groenlandicum 8 / 9 Expected good result as groenlandicum is closely related to tomentosum.
hypoleucum 1 / 9 I had an extremely small amount of pollen from hypoleucum. Otherwise the result would probably have been better.
dauricum  5 / 8 Pretty interesting, if gives seedlings.
ledebourii  5 / 8 Same result as for dauricum. Some regard ledebourii as a regional variation of dauricum.
cinnabarinum var. cinnabarinum  5 / 8 This was a big surprise and totally unexpected! Probably the seeds do not germinate, but still I'm impatiently waiting for extraordinary results. smile
minus (Milde)  4 / 7 Could this be true? Very interesting cross, if it gives seedlings.
hippophaeoides  2 / 7 This could also yield to quite interesting hybrids.
minus (Hachmann)  2 / 8 Very interesting, if gives seedlings.
impeditum  1 / 8 Not much pollen.
'P.J.M. Elite'  1 / 7 Pretty interesting, if gives seedlings.
'Ramapo'  1 / 8 6 out of 7 stigmas and ovaries dried out. Reason is unknown. May be I squeezed the style a little bit too much during pollination?
'Blumiria'  1 / 8 6 out of 7 stigmas and ovaries dried out. Reason is unknown. Squeezed style?
'Dörte Reich' (minus x cinnabarinum Concatenans) (picture © Glendoick Gardens 1 / 8 6 out of 7 stigmas and ovaries dried out. Reason is unknown. Squeezed style?
The results from the remaining seed capsule could be very interesting.
'Robert Seleger'  0 / 10 All 10 stigmas and ovaries dried out. Reason is unknown. Squeezed style?
'Mother Greer'  0 / 8 Probably not a compatible cross.
minus, pink (Göteborg)  0 / 8 Probably not a compatible cross.
kiusianum  0 / 7 Probably not a compatible cross. A very expected result.

I was very careful when I emasculated the covered, but still not opened flowers. If a stamen at removal came too close to a stigma, I discarded it. After pollination I covered the truss with a paper bag. I used also a non-pollinated control truss to verify the pollination method.

tomentosum x ledebourii
Figure 2. Swelling and elongating ovaries on the cross tomentosum x ledebourii three weeks after pollination.




Status 18-Nov-2003 on tomentosum crosses
I did get seeds from all of the ovaries that had swelled up. The number of tomentosum crosses that gave seeds was 13, a lot more than I had expected!
The most surprising cross tomentosum x cinnabarinum gave very nice looking seeds and the seeds clearly were different from the wild collected tomentosum seeds that were collected on the same day from the surrounding bushes.

tomentosum x cinnabarinum seeds vs. tomentosum
Figure 3. Seeds from the cross tomentosum x cinnabarinum as compared to wild tomentosum seeds.

Number of seeds

Figure 4. Number of seeds from the crosses.
Note 1. The actual number of pollinated stigmas was 7 to 10. This means that the title "Number of seeds from ten seed pods per cross" is a bit misleading. However, the error is small and the mathematically adjusted number of seeds per ten seed pods would not give an essentially different graph.



Pollen (father) Germination percentage 2 weeks after sowing Comments
groenlandicum 90 % Nice looking seedlings.
hypoleucum 55 % Nice looking seedlings from the very few seeds.
dauricum  22 % Small, weak looking seedlings.
ledebourii  30 % Small, weak looking seedlings as for dauricum!
cinnabarinum var. cinnabarinum  100 % Very vigorously growing, good looking seedlings. smile
Specially large, round cotyledon leaves.
minus (Milde)  72 % Nice looking seedlings.
hippophaeoides  77 % Nice looking seedlings.
minus (Hachmann)  100 % Nice looking seedlings.
impeditum  0 % No seedlings from the very few seeds.
'P.J.M. Elite'  25 % Nice looking seedlings.
'Ramapo'  38 % Small, weak looking seedlings from the very few seeds.
'Blumiria'  60 % Some nice looking, but mostly small, weak seedlings.
'Dörte Reich' (minus x cinnabarinum Concatenans) (picture © Glendoick Gardens 67 % Nice looking seedlings from the very few seeds.
tomentosum CW  100 % As comparison wild collected tomentosum gave good germination, but the seedlings had considerably smaller and narrower cotyledon leaves than most of the hybrid crossings.




Status 17-Dec-2003 on tomentosum crosses
The tomentosum seedlings have now the first true leaves. I took a picture from each cross and tried to place them roughly in same scale for comparison (one unit is mm). Wild tomentosum has typically very narrow leaves, but most of the pollen given lepidotes have wider leaves. This seems also to have had an effect on the seedlings, but ... It is still too early to make any conclusion whether the crossings have been successful or not. May be everything turns out to be pure tomentosum in spite of the controlled pollination?

first true leaves of the tomentosum crosses
Figure 5. First true leaves of the tomentosum crossings.


Status 04-Jan-2004 on tomentosum crosses
Now it seems pretty clear that my crosses on tomentosum have resulted in true hybrids. The leaves of the seedlings differ distinctly from the leaves of wild tomentosum. Figure 5 shows seed plants of the cross tomentosum x cinnabarinum. A seed plant of tomentosum CW is shown for comparison in the insert of the figure.

comparing x cinnabarinum seedplants to tomentosum CW
Figure 6. Seed plants tomentosum x cinnabarinum compared to a tomentosum CW seed plant. (The scale unit is 1 millimeter.)



Status 23-Jan-2004 on tomentosum crosses
The seed plants are growing normally and only a few weak ones have died. The difference to wild tomentosum plants is now even more clear. Most of the seed plants have much wider leaves than tomentosum. However, the variation in the shape and size of the leaves is pretty large. A summary on the results as of 19-Jan-2004 is given in Hybridity of lepidotes in Rhododendron subsection Ledum based on some hybridization results.

comparing x cinnabarinum seedplants to tomentosum CW
Figure 7. The hybrid tomentosum x cinnabarinum seed plants are growing nicely and are now 12 weeks old. One of the plants has markedly darker leaves (upperleft picture).



Status 15-Feb-2004 on tomentosum crosses
A second growth cycle is now starting and the hybrids look pretty good. No more casulties so far. :)
Several of the hybrids have hairs on the new growth and some have reddish outlook.

15 weeks old tomentosum x cinnabarinum seedplants
Figure 8. The hybrid tomentosum x cinnabarinum seed plants show distinctive variation at an age of 15 weeks.


Status 19-Mar-2004 on tomentosum crosses
I took 12 pictures of the tomentosum x cinnabarinum and 6 pictures of the tomentosum CW seed plants to more easily see the difference of the hybrids to the mother. The picture gallery of the seed plants can be seen at Pictures of tomentosum seed plants.

19 weeks old tomentosum x cinnabarinum and tomentosum CW seedplants
Figure 9. On the right is one of the tomentosum x cinnabarinum seed plants and on the left tomentosum CW. More of these with large variation can be seen at Pictures of tomentosum seed plants.


Status 25-Apr-2004 on tomentosum crosses
I photographed all the seedlings in order to better see the variation between the plants. Some crosses show pretty large variation, some not so much.
However, all crosses clearly differ from the control group, wild collected tomentosum.
A selection of 124 pictures on the hybrid plants and 12 pictures on tomentosum CW seed plants can be seen at
Picture galleries of 25 week old tomentosum cross seed plants.
Below are four of the pictures shown as examples.
 
25 weeks old tomentosum x cinnabarinum seed plant 25 weeks old tomentosum x hippopaheoides seed plant
Figure 10. One of the tomentosum x cinnabarinum seed plants. Figure 11. One of the tomentosum x hippophaeoides seed plants.

25 weeks old tomentosum x dauricum seed plant

25 weeks old tomentosum x minus (Hachmann) seed plant

Figure 12. One of the tomentosum x dauricum seed plants. Figure 13. One of the tomentosum x minus (Hachmann) seed plants.


Status 17-Dec-2004 on tomentosum crosses
The seed plants from my 2003 crosses are now one year old and growing well after having a long several month break in growth during our cool summer. The plants probably felt that winter started when I brougth them out in June to temperatures close to 0°C (32°F) or even below freezing. Now the plants are again in my room at +20°C (68°F).

Most of the seed plants have inherited more features from mother R. tomentosum than from the pollen given lepidote. However, in all of the crosses there are plants that clearly differ from the R. tomentosum.


tomentosum x dauricum
Figure 14. This is one of the most beautiful seed plants, a cross R. tomentosum x dauricum. The leaves are long and narrow as on mother  tomentosum, but totally glabrous, without hairs as on the pollen given R. dauricum.

tomentosum x ledebourii
Figure 15. The glabrous leaves of R. tomentosum x ledebourii  are inherited from R. ledebourii (variation of dauricum). The growth is very dense.

 tomentosum x hippophaeoides and x minus

Figure 16. On the left is a glabrous, narrow leaved R. tomentosum x hippophaeoides and on the right a hairy, wide leaved R. tomentosum x minus. On top of the picture is partly seen another  R. tomentosum x hippophaeoides plant, this one with more rounded leaves.


 tomentosum x hippophaeoides
Figure 17. R. tomentosum x hippophaeoides seed plant has wider and shorter leaves than tomentosum.

 tomentosum x cinnabarinum
Figure 18. R. tomentosum x cinnabarinum seed plant has shorter and wider leaves than tomentosum. The stalks are exceptionally dark red and very hairy.

 tomentosum x cinnabarinum and x Dörte Reich and x ledebourii
Figure 19. Three different crosses and as a result three very different looking plants.
On the left is R. tomentosum x cinnabarinum with nice hairy leaves, somewhat wider than on tomentosum.
In the middle is R. tomentosum x 'Dörte Reich' with strange-looking twisted and very hairy leaves. New growing leaves are curled around each other.
On the right is R. tomentosum x ledebourii with almost glabrous, straight leaves.
(All plants are different from the previously shown ones.)

 

Status 25-Apr-2004 on tomentosum crosses
I gave the seed plants a two month artificial winter at +5°C to +10°C. The growth stopped and several plants got some "autumn" colors on the leaves.
In March I got the plants back in at +25°C. I pinched off the top of each plant and good new growth has started.

I took pictures of some of the now 77 week old seed plants and made a picture gallery that can be seen at Pictures of 77 week old R. tomentosum cross seed plants.
I placed a white paper with 1 cm (0.4 inch) grid to the backgound to make it easier to compare the size of the plants.
A typical R. tomentosum CW plant is shown in the first picture of the gallery as reference.

One of the R. tomentosum x cinnabarinum seed plants has two different types of new growth. On some branches the new leaves are smooth shaped and normal looking, but on some branches the leaves are twisted and somewhat malformed looking. I noticed this growth habit already last spring, but that time it was not so obvious. Some kind of mutation when the chromosome pairs do not match? See picture below.

 tomentosum x cinnabarinum
Figure 20. R. tomentosum x cinnabarinum seed plant with two different types of new growth: smooth shaped, normal looking leaves on some branches and twisted, malformed leaves on some. Non-matching chromosome pairs?

 

Status 09-Aug-2006 on tomentosum crosses
The seed plants were last winter first time out and without any winter protection. The lowest temperature was not that low, only down to -20C (-4F), but without help from any snow cover. I have now counted the losses and registered how each cross survived the winter. The results are shown in figure 21 and a comparison between x groenlandicum vs. x cinnabarinum in figure 22.

Winter survival of tomentosum crosses

Figure 21. Winter survival of tomentosum crosses during winter 05/06.

Some comments on the winter survival results are given in next table.

tomentosum CW Another nine plants from same batch were planted in other location. No damage, all OK.
tomentosum x groenlandicum As expected, very good survival rate and minimal winter damage.
tomentosum x ledebourii The batch included some weak looking plants which did not survive. Otherwise good survival rate.
tomentosum x 'Dörte Reich' Surprisingly good survival. One of the plants is vigorous with larger and wider leaves than tomentosum CW.
tomentosum x dauricum The batch included some weak looking plants which did not survive. Otherwise good survival rate.
tomentosum x cinnabarinum One third dead, one third got damaged leaves and one third survived without damage.
tomentosum x 'P.J.M' Some small weak-ones could not be found anymore. Surviving plants without damage.
tomentosum x minus Milde Surprisingly low survival rate. R. minus is not fully hardy in S. Finland.
tomentosum x minus Hachmann Surprisingly low survival rate. R. minus from Hachmann is not fully hardy in S. Finland.
tomentosum x hypoleucum The cross resulted unexpectedly in several weaklings that did not survive the winter.
tomentosum x hippophaeoides R. hippophaeoides is marginally hardy in S. Finland and gets easily damaged and even killed.

Winter survival of x groenlandicum vs. x cinnabarinum

Figure 22. Winter survival of tomentosum x groenlandicum vs.  tomentosum x cinnabarinum plants during winter 05/06.
x groenlandicum plants survived very well as expected.
One third of x cinnabarinum plants died, one third got damaged leaves and one third survived. This was also somewhat foreseen.


As a conclusion it seems that pollen given lepidotes have affected the hardiness of the hybrid plants but fortunately the mother R. tomentosum has given some good genes.


 tomentosum x 'Dörte Reich'
Figure 23. R. tomentosum x 'Dörte Reich' with wide growing leaves on the right as compared to largest R. tomentosum CW leaf that I found from the marsh.


I'll report on the growth of the seedlings on this page whenever something worthwhile reporting happens.

Pictures of 19 week old R. tomentosum cross seed plants
Pictures of 25 week old R. tomentosum cross seed plants
Pictures of 34 week old R. tomentosum cross seed plants
Pictures of 77 week old R. tomentosum cross seed plants

Pollinations in 2004 using R. tomentosum as Seed Plant

Pollinations in 2005 using R. tomentosum as Seed Plant
Pollinations in 2006 using R. tomentosum as Seed Plant
Leaf pictures of R. tomentosum cross seed plants
Taxonomy of species in Rhododendron subsection Ledum

Hybridity of lepidotes in Rhododendron subsection Ledum based on some hybridization results

Breeding Hardy Elepidote Rhododendrons in Finland