Hardy Begonia collection at OpdeHaar
This page contains information on the hardy begonia collection at OpdeHaar
- Begonia grandis hardiness
- Spectacular red leaves
- Seedlings and Bulbils
- Sports and cultivars of B. grandis
- Begonia grandis “Evansiana”, B.grandis ssp holostyla, B.grandis ssp sinensis
- At OpdeHaar
- Blister or reflective pigmentation in begonia’s
- Colour variation in begonia’s
- Anthrocyanin pigmentation
- Begonia Taliensis, B. Petadifida
- B. Torsa
- Chinese begonia’s and hydrids
- Hydridization of begonia’s
Begonia grandis ssp. Evansiana is fairly widespread throughout China and Japan. Its common name of “Hardy begonia” sums up the major claim for its inclusion in the hardy exotic hall of fame. During most winters it survives very well in our gardens in The Netherlands without any special protection. However, I think I would cover it in dried leaves if temperatures dipped below – 10oC or so.
This hardy begonia thrives in quite deep shade. Although moisture is preferred it can cope with dry conditions and, once established, a fair level of sun. It is therefore an excellent woodland plant. When the first frosts arrive, it dies off above ground but the tuber rootstock lives on. Eventually new growth emerges the following spring. Reports on the Internet hint that this emergence is “late”. In my experience, I find that it is the extent of exposure to sun which is a major factor here. In heavy shade, it can be towards the end of April before growth is seen. But, even after a normal winter, there are signs of life in a sunnier position in early April. It could possibly be related to the extent to which the soil has been warmed.
As the photos illustrate, Begonia grandis is instantly recognisable as a begonia. Many gardeners may just walk past it thinking that it is a pot plant which has been pushed in for the summer to fill in a space. Nothing could be more wrong. Stop and look more closely! If it can be arranged, try and position it so that the spectacular red undersides of the leaves are visible. A side view of the plant or looking up at it on a slope are two ways of achieving this.
As growth gets underway, a crop of little seedlings will appear. These can be potted up and will grow away very easily. The seedlings develop from bulbils which form in the leaf axils before the leaves die in the autumn. Bulbils are nascent tubers. They germinate very easily and provide a straightforward means of vegetatively bulking up all the different Begonia grandis cultivars.
By this time, you are probably wondering where “the catch” is in this story. I can honestly say that I haven’t managed to find it yet! This is a wonderful shade plant and it isn’t at all difficult either to propagate or grow. Furthermore, it isn’t irritatingly invasive although it provides offspring enough (from bulbils) for bulking up a group. It is a mystery to me why hardy begonia’s are not available in all the garden centres.
Normal flower colour is pink but there is also a white or “Alba” form.
For readers interested in gaining more botanical insights into Begonia grandis, the Pacific Bulb Society is a good reference source.
Over the years, plant hunters and the occurrence of sports in nurseries have given rise to a variety of interesting Begonia grandis cultivars. An article by Dan Heims of Terra Nova Nurseries provides a comprehensive summary of the situation of what is available to the gardener.
Heim’s article is based on a talk given to the US Begonia Society. It complements a somewhat earlier blog by John Boggan which provides additional insights and completes an instructive review of hardy begonia. These articles will bring the gardener up to speed on what is going on in this sector of the plant world.
The Flora of China recognises 3 subspecies in Begonia grandis. “Evansiana” is Begonia grandis ssp. grandis – a plant with rather sturdy stems and a solid abaxial leaf pigmentation of broadly oval leaves. The other two variants (Begonia grandis ssp holostyla and Begonia grandis ssp sinensis) both have longer and weaker/thinner stems. Begonia grandis ssp holostyla has triangular-ovate leaves. From a gardener’s perspective it seems to have been created especially for the “Nanjiang Silver” variant. In Begonia grandis ssp sinensis the red pigmentation is generally concentrated along leaf veins and sometimes stems. The majority of the Chinese B.grandis introductions fall into this subspecies. It goes without saying that this is only an extremely superficial look at the taxonomy. However, it alerts one to the nomenclature which is appearing in nursery catalogues and on plant labels.
We have been growing several hardy Begonia grandis cultivars for several years. When we came across new variants we try to purchase them out of curiosity. Left to their own devices they bulk up largely thanks to bulbils. This vegetative propagation preserves the integrity of the variant. Whereas with seed propagation there is a risk of cross-pollination when different cultivars are in close proximity to each other. This isn’t a problem in the garden. However, it can in theory introduce a bit of confusion in the nursery trade. Cross-pollination is maybe the explanation why, when looking around nursery beds, occasionally one or two plants seem to have numerous subtle differences from the surrounding stock. That said, from a gardening perspective, if the difference is an improvement, then it is worthwhile.
Many plant species have varied or variegated leaves and this is usually caused by variations in the concentration of chlorophyll. However, that is NOT the case in Begonia grandis species. Recent work in Taiwan has shown that the leaf structure will trap air. Curiously enough, that gives an impression of white/silver variation.
While variegated leaves offer the gardener fun options, nature always has a reason for something. In this case, air in the top of a plant acts as a protection mechanism against sunburn due to excess sun. It’s called “Blister of Reflective Variegation”. Georgina Reid summarizes the reasons for the variation. “The unpigmented top layer of the leaf is separated from the pigmented bottom layer by an air pocket that reflects light and creates silver metallic patterns. These may be possible dots of blemishes or may run along blades of edges. Reflective variegation is part of a plant’s composition, so these crazy plants can be reproduced by seed, cuttings”.
Important here is that, the air confinement originated in the cultivar, the pattern of the variation is form-fixed. Herein lies a possible explanation for the differences and variation in the white/silver variegated Begonia grandis species.
Among the Begonia grandis varieties, there are several with this white/silver variation on the upperside of the leaf.
This hardy begonia variant was collected by Dave Demers in Sichuan. The botanist Riz Reyes introduced it in the USA as Begonia “Nanjiang Silver”. It displays a more extensive leaf variegation. Look at a leaf of “Nanjiang Silver” from different angles and notice how the variegation seems to change. A similar phenomenon occurs when you walk around a plant. There are reports that the variegation decreases as the season progresses. I am still bulking up my own stock from bulbils to comment on this. My tentative observation is that the contrast associated with the variegation possibly appears to fade somewhat. The photo illustrates this.
Some leaf sports of B.grandis combine decorative upper leaf surfaces with an earlier appearance in late April. Begonia “Sparkle and Shine”, another hardy begonia is an example of this. It was christened by Russell Gardens in the USA. Their description of “Sparkle and Shine” below says it all:
“Begonia grandis ssp.evansiana `Sparkle and Shine’, white, 15″, September-October. House cross (happened on the farm without any help, just “bulbils mixing it up” if ya know what I mean?). The color of an Ox’s blood fills the backs of these wide HEART-shaped leaves and a PROMINENT SILVER-overlay adorns the fronts! They SPARKLE and SHINE in the dappled sunlight. Shade. They colonize by bulbils so FALL and SPRING clean-ups will not allow this to happen.”
In contrast to “Nanjiang Silver”, early observations indicate that the leaf pattern retention (i.e. spots) is much higher on “Sparkle & Shine“. Differences in light intensity do not appear to be key to these differences/changes.
“Sparkle & Shine” spreads effectively from bulbil distribution in our experience. In 2020 I harvested and pricked out bulbils in seed trays. (It is noteworthy that a small percentage of the bulbils gave rise to plants lacking the leaf variegation.) The following photographs illustrate the patterns on the upper surfaces of the leaves. It is somewhat reminiscent of the popular house plant, Begonia maculata.
As you would expect from a renowned nursery, one line of Terra Nova’s thinking was how to incorporate the hardiness gene into the decorative begonia varieties associated with pot plants or annual bedding displays. Begonia “Silver Splendor” ( PP 21946 ) is an early example of this line of thought.
Inspection of “Silver Splendor” indicates that the silver colour is also most likely a result of blister variegation. In fact it is more intense than that of “Nanjiang Silver”. Indeed, the extent of leaf colouration remains both constant and extends to well over 90% of the adaxial leaf surface. This is consistent with the Taiwan findings that the efficiency of photosynthesis is not impaired by air entrapment. We are wondering why the leaf patterns on the two variegated Begonia grandis cultivars do not seem to be constant. This could provide an interesting extension to the Taiwan findings.
“Silver Splendor” is a very decorative foliage plant which emerges late in Spring or even as late as early summer. However, it persists until the autumn frosts. It can easily be combined with early spring flowering plants whose foliage is gone by the time the Begonia appears.
The late emergence is also a characteristic of B.taliensis which is the pollen parent of the Terra Nova hybrid. This is about a month later than I expect to see B.grandis or B.sinensis varieties awakening from winter dormancy.
For completeness it is worth noting that there are two more mechanisms which cause colour variegation in hardy Begonia leaves. The vivid colours so characteristic of Begonia rex houseplants and the deep red-purple on the underside or abaxial leaf surface of Begonia grandis are associated with anthocyanins. These are the same chemicals which are responsible for the vivid autumn colours in some deciduous forests. In the context of Begonia see brief details here.
Quite a few woodland plants found in shaded areas have a heavily anthocyanin pigmented abaxial leaf surface. It is often stated that this reflects light back through the leaf. Thus giving the chlorophyll a second chance to utilize it in photosynthesis. American botanists recently set out to test this hypothesis and found it not to be the case. However, their conclusion amounted to not knowing what the function of the pigmentation is. Nevertheless their paper provides a nice summary of the various proposals of what the role of anthocyanin might be.
The red/purple anthocyanin pigmentation occurs in random areas on the under surface of Begonia taliensis leaves. This generally escapes notice. However, it is this abaxial pigmentation which causes the dark leaf mottling when plants are viewed from above. It is quite possible that the degree and extent of anthocyanin pigmentation will vary in different populations of B.taliensis. This is also the case with B. grandis variants. Indeed it would be interesting to see if selective propagation could maximise/enhance this decorative feature.
Some Begonia, notably B.pavonina, develop a blue iridescence in low-light conditions. This is indicative of the formation of highly uniform structures within the leaves which scatter light in a very regular manner. Bristol University originally discovered this and it has been further elaborated on by Chinese botanists.
The emphasis of this short review is on B.grandis and its cultivars. However other candidates for use or for trialling in the garden are starting to appear in specialist nurseries. I am growing B.taliensis and B.petadifida quite happily in the Central Netherlands. In the UK, specialist nurseries such as Pan-Global plants and Crug Farm list numerous Begonia species with various degrees of cold hardiness.
Fortunately there is a begonia hydrid which is hardy – Begonia “Torsa”. It is a cross with a unidentified species from Bhutan which contributed to the large leaves. It looks like B.grandis on steroids once tubers are mature. A short article by Paul Tsamtsis describes it and offers an insight into other B.grandis hybrids. He reports that New World begonia hybrids with B.grandis are weak and that crosses with B.rexes lack hardiness. However, the various cultivars of B.grandis are stable sports or mutations rather than hybrids in the sense of species cross-fertilizing.
Begonia “Torsa”, like its B.grandis parent, forms bulbils and is undoubtedly vigorous. It is claimed to exhibit a degree of winter hardiness. Indeed it comes through Dutch winters when protected by a layer of mulch. However, in a personal communication, Dan Heims told me that it wasn’t hardy in Ontario, Oregon where B.grandis is hardy.
I have only just acquired my first “Torsa” plants. It lacks the intense red pigmentation on the underside of the leaf although the leaf veins are red. One cannot help but wonder if back-crossing “Torsa” with either “Sparkle & Shine” or “Nanjiang Silver” would yield interesting progeny. Maybe even better leaf variegation (upper and/or lower) and even enhanced winter hardiness would be possible. Also maybe even an earlier start into growth in Spring rather than June/July could be an additional enhancement. This could make a nice project for a botany student?
It won’t come as a big surprise to learn that Chinese botanists have identified and are growing numerous Begonia species. Many of these will or are likely to display (a degree of) winter hardiness. A publication “Diversity and conservation of Chinese wild begonias” provides interesting insights into the wide variety of Begonia species. Of particular interest, are the comments from these expert Chinese botanists on the “Morphological diversity”. (See section 1.3 of the paper on Chinese begonia). They highlight the diversity of leaf type, colour and variegation and the paper contains excellent illustrative colour photographs. However, their comments on natural hybrids provide food for thought. Particularly when bearing in mind the variations in Begonia grandis and the deliberate crosses conducted by Terra Nova Nurseries.
In summary, they point out that different species in the same locality do form hybrids. Indeed they claim that B.helmslyana, B.longifolia and B.palmata are doing this particularly frequently. However, most of these hybrids are F1 in character. Therefore their continued existence depends on the proximity of the parent plants being preserved. In short, the hybrids are sterile and do not self-propagate; much like Begonia “Silver Splendor”. Dan Heims reports that although numerous interesting hybrids with B.grandis had been produced, at the time of his report, they lacked the hardiness gene.
A publication entitled “Occurrence and characteristics of natural hybridization in Begonia in China” by Daike Tian and coworkers is interesting. The article tabulates the hybrids they have identified and lists the parent species of the hybrids and other useful data.
It is noteworthy that B.grandis does not occur at all frequently as a parent species in the hybrids. This and other work start to create a feeling that maybe Begonia somehow have a facility to genetically adapt to a location provided there is consistency in the environment. I’m maybe totally wrong on this but it is a point for discussion.
Dan Hinkley introduced a sterile F1 hybrid from a collection in Central Taiwan in 1999 as Begonia X chungii DJHT99168. This has been shown to be a hybrid between Begonia palmata and B. longifolia. (See the article: “Begonia x chungii (Begoniaceae) – a new natural hybrid in Taiwan“). It is the ideal plant for gardeners who regard the self-propagation of B.grandis by bulbils as a bit of a nuisance. However it is reportedly very easy to propagate vegetatively. It is offered for sale by several nurseries in USA but it is harder to find in Europe and UK. This is a shame because it is very richly flowering in the autumn season. Furthermore it has proven to be hardy here in the central Netherlands over the last 3 years or so.