Pollinations in 2005 using Rhododendron tomentosum as Seed Plant
Kristian Theqvist, 15-Apr-2010, rev. 007

The table lists the lepidotes that I have used as pollen giving plant in 2005.
The number of seeds for each cross is shown graphically in picture Number of Seeds.

I made a total of 27 crossings of which most gave seed.

I covered the still not opened R. tomentosum flower buds on 5-Jun-2005. I emasculated and pollinated the flowers two week later when the covered flowers had opened. The stigmas were clearly wet and receptive and I checked that none had got any pollen while covered. In previous years I had emasculated and pollinated not yet opened flower buds. So, I had now made a change in my pollination method. This of course brought a risk that some of the covered but non-emasculated flowers would be accidentally open-pollinated. To get some assurance of the method I made also non-pollinated control trusses, 30 non-emasculated and 31 emasculated ones. The result indicated that there is a slight possibility to get seeds from emasculated, non-pollinated stigmas. However, the amount of seeds was small (approx. 0,4 % out of normal open-pollinated CW) and germination of the seeds was also very low (total < 0,1 % out of normal open-pollinated CW). Could it be apomixis, asexual reproduction, or accidental self-pollination? Normal CW-pollination gives a lot better germination. In any case, this sets doubt on some of the crosses with low number of seeds and low germination rate. The seed plants need to followed closely to distinguish the true hybrids.

Status 13-Oct-2005 on tomentosum 2005 crosses
Almost all crosses yielded seed as was anticipated from the swelling seed pods. The amount of seed varied very much from cross to cross. The seeds were mostly very viable looking and with variation in outlook (width, crookedness). However, on several crosses there were only few seeds and the seeds were small and weak looking.

I had pollinated ten stigmas for each cross. The number of seeds is shown in the graph of figure 1.

Figure 1. Number of seeds from the crosses.

The pollination with the Ledum diversipilosum gave clearly seed best, approx. 70 seeds/pod. Next best came pollination with too Lapponica lepidotes, fastigiatum and hippophaeoides hybrid 'Flämingpärle'.
An almost expected surprise was the high seed count with R. cinnabarinum var. xanthocodon. This was consistent with results that I had got with R. cinnabarinum var. cinnabarinum in 2003. For some reason R. tomentosum seems to cross pretty easily with Cinnabarina.

I had made a note on three pollinations that had very little pollen. I assume that R. lapponicum as pollinator would have given more seed if there had been enough pollen.

Status 23-Feb-2006 on tomentosum 2005 crosses
The seeds germinated pretty much as expected. Some crosses had smaller seeds than normal and those mainly did not germinate. The most exciting seed plants resulted from crosses with RR. cinnabarinum var. xanthocodon, desquamatum, hanceanum 'Canton Consul' and lindleyi. Genetically interesting are also crosses with RR. lapponicum and subarcticum. The amount of seed plants for each cross is documented in the top table.

Status 25-Nov-2006 on tomentosum 2005 crosses
I had the cross plants during summer out on the porch at my summer cottage. Unfortunately a couple of caterpillars had found the plants. Most of the plants had suffered and several died. Luckily I could save two thirds of the plants and I have them growing in my workroom at room temperature. They have now got a new growth cycle and I expect them to have a couple of more before next summer.

I took some picutres of the cross plants. The most astonishing plants have resulted form the cross with the hippophaeoides hybrid 'Flämingperle'. The germination had been good and most of the cross plants have nice leaves that clearly have features from the pollen given 'Flämingperle'. Could it be that 'Flämingperle' is also a tetraploid as R. tomentosum? Other lepidote hybrids that I've tried have not given such good germination and good characteristics as the cross with 'Flämingperle'.

Figure 1. The cross tomentosum x hippophaeoides hybrid 'Flämingperle' resulted in several seed plants that have nice leaves with features from the pollen given 'Flämingperle'. I got 'Flämingperle' from Hachmann while visiting Germany in May 2005.

More pictures of the cross plants are in picture gallery Pictures of 50 week old R. tomentosum cross seed plants.

Status 22-Dec-2006 on tomentosum 2005 crosses
More and more of the plants have started to show nice hybrid features. The most astonishing ones are the crosses with the hippophaeoides hybrid 'Flämingperle'. Another nice group of plants are the crosses with fastigiatum. Almost all tomentosum x fastigiatum plants have fastigiatum type of leaves. As follows are pictures that I took today of some of the 54 weeks old plants.

Figure 2. Most of the plants from the cross tomentosum x hippophaeoides hybrid 'Flämingperle' have nice leaves inherited from the pollen given 'Flämingperle'.

Figure 3. Two tomentosum x hippophaeoides hybrid 'Flämingperle' plants with different type of leaves. The left side plant has nice almost glabrous leaves typical to hippophaeoides while the right side plant looks more like tomentosum but with twisted leaves.

Figure 4. The cross tomentosum x hanceanum 'Canton Consul' resulted in this odd looking plant.

Figure 5. Most of the plants from cross tomentosum x fastigiatum have leaves with features from the pollen given fastigiatum. The leaves are shorter when compared to x 'Flämingperle' leaves but larger than on species fastigiatum.

Status 15-Apr-2007 on tomentosum 2005 crosses
The plants are now 70 weeks old and are well growing in my work room. I made a new picture gallery of some of the plants.
Pictures of 70 week old R. tomentosum cross seed plants 

Figure 6. Some of the plants have small miniatyre leaves and usually associated weak growth. However, one of the tomentosum x hippophaeoides hybrid 'Flämingperle' plants with small leaves has a very dense growth. The plant could potentially become a good low growing ground cover? Flower color is to be seen, white or light red?

Status 27-Oct-2007 on tomentosum 2005 crosses
Couple of the seed plants have got their first flower buds and it's getting exciting. Also several plants from my 2003 crossings finally have flower buds.
Two weeks ago, on 13th of October I noticed to my astonishment that one of the tomentosum x 'Flämingperle' seed plants had a flower!!!!

Figure 7. One of the tomentosum x hippophaeoides hybrid 'Flämingperle' plants had an autumn flower! Or is that a flower?

Is that actually strictly speaking a flower? By more close look-up I noticed that
a) It has six petals instead of five.
b) It has nine anthers instead of ten.
c) The stamens are malformed.
d) The curved style is pretty thick and the stigma does not look fertile.
e) The ovary looks normal.
f) Two of the otherwise white petals have some greenish on the tips.
g) I had not previously noticed any flower bud on that branch.
Based on some of these observations I do have strong suspicion that a normal shoot had somehow got damaged and changed its growth habit to flower type growth.
Can we say that this is a flower? It has all parts of a flower and probably it has features from the flowers that appear in the spring from flower buds.

Figure 8. Close-up of the odd-looking "flower" of the tomentosum x hippophaeoides hybrid 'Flämingperle' plant.

I bought a good microscope in order to be able to look at the glands and scales of the R. tomentosum hybrids.
The first microscopic comparison photos are from the tomentosum x 'Flämingperle' plant (pictured above) and from its parents.

Figure 9. Microscopic picture of glands and hairs from the leaf underside of R. tomentosum. The doughnut shaped glands stand on thin pilars extending from the leaf surface. In addition to the short white hairs there are long brown hairs on the midrib of underside of the leaf (not shown in the picture).

Figure 10. Microscopic picture of scales from the leaf underside of lepidote R. 'Flämingperle'. The scales look very typical of a lepidote Rhododendron.

Figure 11. Microscopic picture of glands and scales from the leaf underside of the cross R. tomentosum x 'Flämingperle'. Some notes: - The hairs typical to R. tomentosum are missing. - There are scattered glands inherited from R. tomentosum. - There are clear scales but not as nice formed and dense as on 'Flämingperle'.

The microscopic pictures give strong proof that it is a question of a hybrid between a Ledum and a lepidote.

Status 15-Apr-2010 on tomentosum 2005 crosses
One of the tomentosum x 'Flämingperle' seed plants has four flower trusses and one of the trusses has now opened! The plant is growing in a greenhouse at Arboretum Mustila. The seed were sown by Jaakko Saarinen and he took today pictures of the flowering plant.

Figure 12. R. tomentosum x 'Flämingperle' plant with four flower buds in a greenhouse at Arboretum Mustila on 27th of March, 2010.

Figure 13. R. tomentosum x 'Flämingperle'. The leaves are covered with white, woolly tomentum.

Figure 14. R. tomentosum x 'Flämingperle'. The underside of the leaves is covered densely with scales, without indumentum.

Figure 15. R. tomentosum x 'Flämingperle'. The flowers open pink and change to white. Picture taken by Jaakko Saarinen at Arboretum Mustila, 15th of April, 2010.

Figure 16. R. tomentosum x 'Flämingperle'. The flowers resemble more the hippophaeoides 'Flämingperle' hybrid than R. tomentosum. Picture taken by Jaakko Saarinen at Arboretum Mustila, 15th of April, 2010.

Figure 17. R. tomentosum x 'Flämingperle'. This is the best of my Ledum cross plants so far, not a perfect one yet but shows the potential in trying to pass the good hardiness of R. tomentosum to lepidote hybrids. Picture taken by Jaakko Saarinen at Arboretum Mustila, 15th of April, 2010.

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

Lepidote Crosses in 2003 using Rhododendron tomentosum as Seed Plant
Pollinations in 2004 using Rhododendron tomentosum as Seed Plant
Pollinations in 2006 using Rhododendron tomentosum as Seed Plant
Hybridity of lepidotes in Rhododendron subsection Ledum based on some hybridization results
Taxonomy of species in Rhododendron subsection Ledum
Breeding Hardy Elepidote Rhododendrons in Finland