A Bauhinia that flowers in two colours [開二色花的洋紫荊]

 The original article was written for the Hong Kong Gardening Society

The members of Bauhinia spp. (羊蹄甲屬) are so common in Hong Kong that many of our gardens have at least one or two.  They are quite hardy and adapt to trimming too (or even hard pruning, although this is not generally preferred in arboriculture practice).

Amongst all of them, Bauhinia x blakeana (Hong Kong Orchid Tree 洋紫荊) is perhaps the most commonly planted Bauhinia. This is not simply because it is our city flower; with its bright purplish colour and relatively long flowering period makes it a promising ornamental tree.  B. blakeana’s blossom lasts from spring to late autumn, while its pinkish counterparts B. purpurea (Purple Camel’s Foot 紅花羊蹄甲) and B. variegata (Camel’s Foot 宮粉羊蹄甲) flower in late autumn and early spring respectively.

Colour of Purple Camel's Foot varies from almost white to pink

Hong Kong Orchid Tree flower in bright purple

Sometimes, a careful inspection of a bunch of Bauhinia spp. may result in a fascinating discovery.  Two types of flowers – some belonging to the Hong Kong Orchid Tree and others belongings to Purple Camel’s Foot - may appear on the same tree!  The bright purple flowers of the former usually appear on most of the crown, with some bundles of pinkish flowers of the latter lying low close to the trunk base.  But occasionally, one may also be able to find a tree with both types of flowers forming the whole canopy in a way more or less half-and-half, side-by-side.  The flower of Camel’s Foot (B. variegata) rarely joins this matrix, however.

A Bauhinia tree having both types of flowers

This observation reminds us that Hong Kong Orchid Tree is a cultivated species, propagated by grafting – the scion (the plant on top) being B. blakeana while the rootstock is mostly, if not always B. purpurea.  If the grafting was not well done, the transport of sugar down the point of grafting might not be so effective and this triggers the resprouting of suckers at the root stock – which is a B. purpurea.  If sufficient time is allowed without human interference, the resprouts may be able to develop into major stems and contribute to the canopy.

grafting point on Bauhinia x blakeana trunk

Pay attention to the grafting point of a Hong Kong Orchid Tree, which tells the quality of grafting.  If the part above the grafting line has obviously swollen, this reveals some internal blockage of sugar transport. An improper connection of the scion and root stock indicates the chance of massive suckers, and also casts doubts on the stability of the grafting point (or called graft union), which may break sharply.

The Mystery of Maple Colour [楓葉之謎]

[The original article war written for the Hong Kong Gardening Society]

Some gardeners like to keep one or a few maple trees for their attractive seasonal colour change.  But unlike template region, it is not always easy to achieve this goal in Hong Kong, particularly in the central urban area where warm air prevails for most of time.

 

Seasonal foliage colour changes depend on a basket of factors such as species properties and weather.  While there are quite a lot of tree species which change colour before fall, not all turn red as maple trees do before leaves drop.  Some species like Ginkgo biloba (Maiden Leaves 銀杏) are well-known and popular for their golden colour in late autumn.  Ginkgo biloba is not common in the subtropical Hong Kong, but we have Celtis sinensis and Melia azedarach, both of which dress yellow before mid-winter.

 

Gingo biloba which turns yellow in autumn (taken at Japan)

The yellow leaves come from the pigment xanthophyll.  Sometimes carotenoids also play their role too.  These two pigments present in leaves for most of the time but are always masked by another green pigment namely, chlorophyll when the leaves are still actively producing food through photosynthesis.  Chlorophyll is essential for fixing carbon in the air into carbohydrate with the aid of sunlight, but it is also broken down gradually in sunlight.  In growing season healthy trees replenish chlorophyll through continued supply of minerals and water from other parts of the plant.

 

At some point after the growing season, this supply is gradually blocked by the formation of an abscission zone at where a leaf is attached to the branch.  The level of chlorophyll then drops and the yellowish and orangish hue then becomes visible after fading of the green shade.  Also due to this blockage, in some species – maple trees for example - the leaves gradually turn red / purple due to the accumulation of anthocyanins from the transformation of the trapped sugar in leaves.

 

progression of colour change in Liquidambar formosana

While variation in daylength triggers the formation of abscission zone, weather condition plays an important role to the duration and completion of the whole process.  After the abscission zone has formed, abundant light and low temperatures will destroy chlorophyll more rapidly.  Cool air particularly at night promotes the formation of red/purple pigments.  This explains why people found that maple trees in Hong Kong (at Tai Tong) showed their brightly red only in some years, when there had been a sharp and prolonged cooling.

 

There are other conditions which are detrimental to the colour show.  If the trees have suffered a prolonged drought, the abscission zone will form earlier and the leaves will drop before colour change.  Occasional heavy wind or rain during late autumn will also cause leaf drop before the colour change.

 

True maple trees are Acer spp. (槭樹), from which maple syrup is extracted (from some species).  Locally Acer spp. are not as common as Liquidambar formosana (Sweet Gum 楓香), which is usually mistakenly referred as maple by many people due to their similarity in appearance and they turn bright red.  In fact these two genera belong to different taxonomic groups.  They can be distinguished simply by the leaf arrangement – with Acer spp. being opposite and Liquidambar spp. being alternate.  Their fruits are completely different too, with Acer spp. baring winged seeds in pairs, while Liquidambar spp. have their fruits arranged in spherical shape.

 

Liquidambar formosana

Though being a minority in local flora, occasionally Acer buergerianum (Trident Maple三角槭) is seen in countryside or even planted in some parks.  It is referred as an exotic species to Hong Kong although its natural distribution covers Guangdong to the southmost.  Other than this large tree species, there are also other smaller amenity species of Acer spp. planted in local gardens.  Some of them look reddish even all year round - which is another mystery perhaps.

Acer buergerianum also turns orangish red before fall