I just re-read the descriptions on Crug..
Begonia grandis 'Sapporo':
A very hardy form of this species from Sapporo, the capital city of Hokkaido the northern island of Japan, where the winters are severe. Distinct in foliage in this form, with large palmate dark green leaves, dark red below. On erect stems that are red at the nodes, which in turn producing bulbils in the autumn. The pink flowers are borne in terminal sprays opening from reddish buds in late summer
Begonia grandis ssp. evansiana - BSWJ11188 (the numbers at the end are the collectors initials Bleddyn and Sue Wynn-Jones - owners of Crug and plant hunters, and a collection reference number which is given as a rule until a species is given a valid name.
From a collection we gathered from the forest floor, situated at the base of the snow capped Fuji-San at 650m, in the autumn of 2005. Where this hardy species, of a mostly tender genus, grew in deep pine needle litter. With large palmate leaves, dark red on their undersides, which had borne sprays of pink flowers opening from reddish buds.
I'm seeing two things in the description of Begonia grandis 'Sapporo'. One is the very hardy description from a place with severe winters, the other is the mention of bulbils at the leaf nodes which yours does Myriam, I know it does because mine does too.
It also got through the extremely hard winter we had, although it was in a cold greenhouse it survived where many other bulbs I had in the same greenhouse did not, and they had survived quite hard winters before.
There is no mention of Begonia grandis ssp. evansiana producing bulbils, this could be just an omittance but it could be that it doesn't make bulbils. That can be the case within the same genus, as in Arisaema ciliatum and A. ciliatum ssp liubaense, the latter is stoloniferous making small corms at the ends of threads sent out from the main corm.
It's possible that yours is actually Begonia grandis 'Sapporo' Myriam! Plant hunters share amongst each other, and I know there are some serious growers in Europe.
We might be able to find out for sure by searching, maybe not. We could contact Crug Farm to ask what they know about the differences.
Looking at the descriptions on MOBOT,
Begonia grandis:
Plants will self-propagate by tiny bulblets which form in the leaf axils in autumn and drop to the ground. Bulblets may also be harvested from the leaf axils and planted as desired. Plants may also self-seed. New season growth is usually late to appear.
Begonia grandis, commonly called hardy begonia, is the only species of Begonia that is winter hardy to the St. Louis area. It is a monoecious, tuberous-rooted, clump-forming perennial that typically forms a bushy mound of foliage to 2’ tall on branching stems. Large, obliquely ovate leaves (to 4” long) with cordate bases are medium to olive green above and reddish green with red veining beneath. Male and female pale pink flowers (to 1” across) bloom in pendent clusters (dichotomous cymes) from July to early fall
http://www.missouribotanicalga...
Begonia grandis subsp. evansiana:
Best grown in moist, organically rich, well-drained soils in part shade to full shade. Soils should not be allowed to dry out. Winter mulch is advisable in the St. Louis area where plants are not reliably winter hardy. Deadhead flowers to extend bloom period. Self-propagates by tiny bulblets which form in the leaf axils autumn and drop to the ground.
Hardy begonia is a clump-forming, tuberous-rooted perennial that typically grows to 2' tall on red stems. Ovate olive-green leaves with red undersides and conspicuous red veining form an attractive foliage mound. Drooping clusters of pink to white flowers (to 1" across) appear in August-September, sometimes blooming to frost.
http://www.missouribotanicalga...
According to those descriptions, both produce bulbils.
One main difference is they seem to be saying that Begonia grandis is hardier than Begonia grandis subsp. evansiana, although they recommend mulching in areas where B. grandis might not be so hardy.
The other main difference is the leaves, B. grandis mentions "Large, obliquely ovate reddish green with red veining beneath" where ssp evansiana mentions "Ovate olive-green leaves with red undersides and conspicuous red veining".
I would say yours is closer to the description of B. grandis in leaf shape, they are large and obliquely ovate more than ovate. The underneath colour though matches ssp evansiana, or perhaps more fitting Begonia grandis 'Sapporo'.
If you look at the bottom of the page on MOBOT you will find two more botanical varieties, reading the descriptions of leaves they vary from these two.
Where did you get the plant from Myriam, and is it possible to find out more from the supplier?
