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

Fun but not Pest [是生趣而不是問題]

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

As gardeners we value our plants most and may be quite alert on any ‘alien’ appeared on them, so does an arborist.  However, plants live with other creatures in the nature for millions of years, so for most of the time, we can just enjoy observing the amazing relationship between them rather than getting too panic.

Fruit trees are common in gardens.  Some of the species like Citrus spp. (金橘屬; e.g. mandarin orange, lemon, pomelo) and their relatives in Rutaceae (芸香科) are quite attractive to insects as much as to human.  Their leaves are particularly preferred by many species of swallowtails (i.e. butterflies from family Papilionidae 鳳蝶科), the species from which are usually quite big in size (with attractive wing tails in most cases) – e.g. lime butterfly / Papilio demoleus 達摩鳳蝶 (no wing tail).  The butterflies lay their eggs and entrust the life of their offspring to the trees.  The tree leaves then feed the caterpillars and the visited plants sometimes may look unhealthy without many intact leaves – this is when their presence is aware of.  Nervous ones may worry about the health of the affected plants and may look for way to treat the ‘pests’.

 

Mandarins (C. reticulata or C. erythrosa) are host plants of many swallowtails

Mandarin fed by lime butterfly caterpillar

In fact, not all species of butterflies or moths would outbreak like a pandemic (e.g. by moth Phauda flammans 燄色榕蛾 - refer to another article - or other sucking insects) that creates serious defoliation and may kill trees.  For swallowtails their number of eggs laid on a particular plant are usually limited.  As long as your trees are healthy with sufficient food storage, they can resprout quickly after the feast of the caterpillars, even though most of the leaves are eaten.

There is no clear boundary for the term ‘pest’.  Observing butterfly life-cycle or just flying butterflies does add interest to our gardens – for example the colourful lime butterfly likes to visit the flowers of Lantana.  Having butterflies in a garden also increase its vitality and ecological function, when urban development has destroyed much of their habitat.  Caterpillars do feed on plants in one way, but many of the adults also help plant pollination in another.

Picking caterpillar off the plant for observation and controlling the impacted extent of our plant

Having some caterpillars increase gardening fun

 If the number of caterpillars is big, manually picking some of them may help your plants.  Transferring part of them to an observation box with manual feeding would also limit the impacted parts of the host plants, keeping the plants neat while we can enjoy watching the growing process.

Lemon (C. lemon) is also recommended for attracting 'fun' to the garden

Adult of a lime butterfly

  

Applying pesticide is always least preferred as usually it would be an over-reaction to this normal wildlife relationship, not to say that we would harvest the fruits from the same trees!

A Sudden Defoliated Evergreen [突然秃頂的常綠樹]

It was extremely hot in most time of this August, especially when we found that the shade originally provided by the large banyan trees had gone suddenly.  This happened in various parts in Hong Kong according to some reports over the social media.  Witnesses felt worries about the fate of the trees.  According to the reports those affected trees were mostly Ficus benjamina (Weeping Fig; 垂葉榕) although F. microcarpa (Chinese Banyan; 細葉榕) were occasionally spotted to have similar problems.

Ficus tree row defoliated

The two tree species are very common in Hong Kong and are also housed by many of our parks and gardens.  With closer inspection, some gardeners may be able to identify caterpillars on trees or on the ground mixed with the dropped leaves.  The defoliation of these trees appears to be quite sudden – it took only one or just more than a week from full tree canopy to completely bare crowns.  In some severe cases all trees of the whole area had defoliated, leaving the branches only.  Some people were even afraid of the tree stability and the associated public safety.

Defoliated Ficus benjamina

Premature fallen leaves with chewing marks

In fact, the severity of this phenomenon has lasted (or increased to some extent) for at least several years but it appeared that it was (or was one of) the worst this year.  For evergreen trees like Weeping Fig they do renew their leaves throughout the year normally (in contrast to deciduous species which drop off all/most leaves within a short period of time) but in a gradual manner.  Such excessive defoliation is therefore extraordinary.

When you find any of the above Ficus trees (or even a few more other species) having this problem, the trees are probably affected by an outbreak of a pest namely, Phauda flammans 燄色榕蛾/朱紅毛斑蛾.  It is the leaf-feeding larvae of this moth which can chew off all leaves of a mature tree.  The pest is native to South China to South Asia and they have three generations a year.  The caterpillars may first appear in late June but the bigger and longer outbreak usually appears in Aug till October.  The larvae are relatively small with yellow and reddish strips, usually appeared sticky on the surface.  Before pupating, the larvae climb down the trunk and pupate between roots or underground.

Larva of moth creeping down the trunk

Adult of Phauda flammans
(source www.hkwildlife.net)

With severe infestation (leaves all consumed), the affected trees surely will suffer for a period of time.  Their resources and food storage are wasted due to the pest.  For healthy trees without much stress, the impact of merely leaf consumption is limited and the trees can recover themselves.  This would only affect the growth rate of the tree, but not tree stability.  However, if the impact happens repeatedly within a short period of time, stored foods of the trees may also be depleted and the health compromised.

Unless the situation is really bad, it is not recommended to spray pesticide (onto the large surface of tree crown) trying to eradicate the larvae.  Such attempt may also harm or kill beneficial organisms such as parasitic wasp, or even birds that feed on the larvae.  One suggested way to control the scale of next outbreak is to remove the larvae that creep down the trunk or that try pupating in the surface soil.  Providing a better growing condition for the trees such as adding mulch will also help tree recovery.  Over-fertilization (esp. nitrogen) would favor appetite of pests in general and should be avoided.

Weeping Fig Tree recovering from the infestation (photos taken with 10 days apart)

一日去兩個墳場咁大吉利是(香港重光75週年 翌日)

[寫於香港重光75週年 翌日]

最近看了一套港台記錄片(註一)，適逢昨天是二戰結束75週年，就往片中有提及的幾個地方憑弔（更貼切的是親身了解一下）。我不太了解現在的教科書如何介紹，但很明顯，近年香港官方或香港民間對這段近代發生而又影響深遠的歷史似乎漸漸淡化。

這幾年香港政治和社會動盪，要對自己的地方有歸屬感和團結社會，正確而不扭曲地去認清和緊記歷史至為重要。二戰一役以至之後的重建經歷是香港和中國歷史的重要部份，牢記歷史不是要緊握仇恨，但要銘記前人的犠牲和珍惜和平。

第二次世界大戰打了多年，到最後才完結的戰役是在遠東（太平洋戰役），香港自1941年聖誕節當日淪陷至1945年8月15日重光，受日本軍法統治經歷了三年八個月。由於寡不敵眾以及地方細小，在日本精銳部隊四面圍攻及瘋狂空襲下，正式的英日對戰至英軍投降僅僅是18天那麼短而已。網上/書籍資料實在有很多，前述的紀錄片亦有精簡的描述可自行參考，因此這裡只簡單紀錄一下這兩天紀念的所見和感受。

1) 赤柱軍人墳場（包括第一及二次世界大戰）：埋葬的死者以二戰中的軍人最多，共有近700人，其實當中有些也並非軍人。

除少數沒有文字或文字失落外，墓碑上大部份的碑銘都很清楚。不少人是在1941年黑色聖誕前後（即英軍不敵而投降當日）至1942年初就喪生了。很多墓碑連陣亡的地點亦有顯示，不少身亡的地點正正是赤柱（香港保衞戰最後的戰線）和墳場邊的聖士提反書院（大概是紀錄片中所述的軍醫院傷兵大屠殺事件）。以下照片就顯示這一組是"Met Their Death at St. Stephens on 25 December 1941"。

其他死不去的軍人就多數成了戰俘。很多捱不過去繼後死了，以下這一組是還差一點就見到重光：死亡日期是1945年1月，還差那幾個月，非常不幸：

墳場內除了超過500個英國人、印度人等，還安葬了近160華人或香港人，以下這一組便是，姓名的拼音就像是我們身邊的朋友一樣。昨天（16/8星期日）我們在那裡憑弔了約半小時，失望的是我只見到另一對香港人前來墳場，這些亡者不是為香港捐軀的同胞嗎？香港人是否太過善忘了?

除了安葬了的，還有更多的中國人名字立於墳場南面的四塊大石碑上，他們包括了兩次世界大戰中陣亡的中國軍人（初步看以二戰死者為最多），他們的死亡地點大都不在香港。

2) 西灣國殤紀念墳場：相比於赤柱墳場，這裡紀念的是清一色在二戰其間陣亡的軍人（也有少數平民），人數也更多（共約1600名）。墳場內分區比赤柱更加清楚整齊，分別長眠了1,013名英國人、283名加拿大人、104名印度人、53名香港人、33名澳洲人、1名紐西蘭、1名緬甸人、72名荷蘭人（數字根據維基）。

名字、年份非常清晰。但也有為數不少的無名碑，碑上都寫著：

"A SOLDIER OF THE 1939-1945 WAR - KNOWN UNTO GOD" ，意思是：一名在二戰中的士兵：他的名字天主會知道

實在倍感悽涼。

除以上者，還有2000名以上身死異處的軍人，他們的遺體當然也不在這裡。幸好還留下了一個名字，都刻在正門建築的牆上，而前面橫放的大石碑可算是他們共用的墓了。

在香港保衞戰中除了英軍、義勇軍和本地民兵，盟軍不同地方都有派軍人支援，當中最多的為約2000的加拿大軍人，還有其他地方的。墳場低台就是加拿大和荷蘭更軍人的墓。碑上有楓葉的便是加藉軍人。

憑弔當天恰巧見到荷蘭國的花圈。我著實希望香港人也不要忘記曾保衞香港的他們，況且按紀錄這裡有至少50名是香港本地人。我們的花圈呢？

要多了解陣亡的外國士兵資料，尚有小冊子可免費取閱。期望不久將來有人可以追查到並公開華人陣亡者的資料。


這次憑弔除了兩個墳場，也到過其他幾個地方：

3) 赤柱炮台 - 據講這炮台是港英守兵作海上防衞的重要防線亦是守到最後的地方，曾令日軍有點束手無策。現今炮台位置在解放軍軍營內是無法參觀的。解放軍繼續駐守這要塞以防衛香港當然無可口非。能夠參觀的就只有這巴士站或登山遠眺了。

4) 匯豐銀行總行：淪陷後日軍的軍政府總部，現在總行地下大堂原來也有相關的展板。希望香港人也珍惜那對經歷過二戰的獅子，無論甚麼原因也不要破壞它們。

5) 和平紀念碑：香港重光後中（當時是國民政府）英雙方就在此舉行重光典禮儀式（1945年）。

近年幾乎不見任何官方紀念活動，不知是否因為甚麼政治因素或去殖民化政策。無論怎樣，在繁忙的中環每天怱怱路過的人們，有幾多會駐足了解和懷念？

雖然二戰是75年前發生的事，但這是香港近代史的重要組成而且是至為關鍵的部份。一個地方如果沒有了歷史（或淡化/曲解/破壞了歷史）那裡只是一個沒有根、沒有靈魂的軀殼，不論把現今的成就說得多漂亮，或把未來包裝得多動人，那個地方都沒有任何吸引人之處。要重振香港和留著人才，我們已不能單靠ＧＤＰ、利益、主題公園或購物，這些都是可以在另一處複製的。

唯有歷史，是一個地方無可取替的魅力所在，也是團結人心的方劑。

註一: 歷史二三事 History on Tuesdays - 馬克公公打仗的日子 My Grandparents' War - Mark Rylance
(https://www.rthk.hk/tv/dtt31/programme/historyontuesdays/episode/690312)

Training our trees [樹木的生長調控]

(The original article was written for Hong Kong Gardening Society) 

The third wave of local coronavirus surge has broken the dreams of normal life for many of us.  It is the time to stay home again to pick up garden works which had been left behind.  Amongst various gardening tasks, taking care of trees is usually put at a lower priority because many people may think that they are long life plants and our response to their growth could be slower.  The same thought is also shared amongst many properties management.

 

Many of us would like to plant trees as they provide shade, blocking and screening unwanted subjects, increase privacy and create extensity in space – that many other plants cannot do easily.  Unless you do have a big land (not common in Hong Kong) for tree growth or you do not have any species which is an “ordinary tree” in nature, overlooking the timely step of pruning may create some kinds of problem later.

 

Trees planted in restricted areas (e.g. podium planters, strips of planting beds nearby building or pavement, pots or somewhere near brick/stone wall) strive to survive and their roots always struggle to look for water source.  If they fail in the trial trees will remain small and look unhealthy gradually – some species like Bauhinia spp. (洋蹄甲屬) respond more obviously by dropping of leaves, looking yellow with leaf size significantly smaller.  In such situation pruning is less demanded than a boost of nutrient and watering.

A Bauhinia with sparse and yellowing leaves on a podium

 

For many other cases, space below the ground is less limiting when compared with how it appears above the soil.  Tree roots may find their way through gaps between structures or rocks, drains or even through leaky pipes toward a world of rich water supply.  The trees then look normal and healthy.  One may be surprised of the growth rate and size with the setting of the environment, which gradually becomes less and less compatible to those healthy trees.  The trunks may become too big and pushing upon structures, facilities or just the curb; the crowns may become too big and shady for example.

 

If you do pruning at this stage usually it would be a bit late and cannot fix the situation in a long run.  If the pruning as a ‘treatment’ is too massive, it may create severe damage to the tree or affect the tree health.

Therefore, it is important to imagine the final desirable size of a tree at the very beginning – when the tree is still young.  Training a young tree through pruning can meet certain purposes such as: (1) directing the growth of the tree; (2) creating a form that can achieve / maintain the expected function(s); (3) controlling the size; and (4) reducing conflicts.  The rules of thumb for such pruning is to minimize the amount of live tissues to be removed at one time and to minimize the size of cut wounds.  In general, all cuts shall be made clean at branch unions (with branches less than a finger-thick or still green to be an exception).  To control crown size pruning cut is usually made on the thicker branch ahead of the union.  If you need a handsaw or a chainsaw in the training exercise this properly suggests that you have waited too long.

 

Many potential structural defects can be minimized or removed (before the loading on them becomes much) during the process at the same time.  These include point of splitting risk (included bark), wounded/cracked branches, competing stems, heavy and drooping branches for instance (part of a training process called structural pruning).  Refer to the figures for illustration and real examples.  Please note that the above guideline is just for general reference and is introductory only.  The high diversity of tree species that we can have implies different responses of our trees to pruning.  It should be noted that excessive pruning or improper pruning would trigger massive sprouts, deform the crown and jeopardize the expected normal function of our trees too.  If a tree is already big or mature the strategy of pruning is also different from small one.

 

With proper training the useful life expectancy of our trees can be prolonged while we can enjoy the benefits from them longer too.  Consult appropriate professional for the most suitable pruning works when there is uncertainty.  In fact, there are alternatives to just controlling tree growth through pruning.  Fitting your garden with a species of compatible properties and suitable size at maturity is always a recommended option.

The below are examples of structural pruning and crown size control.