We recently had guest speaker Amelia Williams talk to members of Twickenham bonsai club about tropical bonsai. She has moved entirely to keeping tropicals & sub-tropicals, to the extent that her back garden is full of her ex-non-tropical bonsai trees which are now planted as a foliage bed! This post references some of Amelia’s talk, if you would like to read more she has written several articles for Swindon Bonsai Club.

So what is a tropical tree? At a basic level it’s a tree which lives in the tropics. The tropics are the region of the Earth between the Tropics of Cancer and Capricorn^ref, shown on the map below. The Tropic of Cancer is a line at 23.4° N where the sun is overhead during the northern summer solstice, and the Tropic of Capricorn is the line at 23.4° S when the sun is overhead during the southern summer solstice; the sun is never fully overhead at locations outside of these lines.^ref In the middle of this region is the Equator where there is always 12 hours of daylight.

The first thing you will notice from the map is that the tropics and the subtropics cover a large proportion of land mass on Earth (36%^ref), and aside from being where 40% of the world’s population and over 50% of its children live, it’s also home to 80% of the planet’s terrestrial species and over 95% of its corals and mangroves.^ref

It’s estimated that over 40,000 tree species live in the tropics, compared to only 124 in temperate Europe.^ref These species have distinct ranges – that is, species in the Americas are different to those in Africa which are different to those in the Indo-Pacific. So tropical tree species offer bonsai enthusiasts a huge opportunity to diversify our collections and explore trees we may never have known about before. Add in the sub-tropics where trees can handle a wider temperature variation, and the selection becomes absolutely enormous.

Tropical trees live fast and die young, growing twice as fast as trees from temperate and boreal regions and living on average 40% shorter lives (around 200 years). Research shows a relationship between high temperatures, fast growth rates and shorter lifespan for trees which is most evident in the tropics. It’s particularly the case where annual mean temperatures exceed 25.4°C and trees die much earlier than in cooler places. Below is a map showing the longevity of 3,343 tree populations across 438 species worldwide – the darker dots are locations of the oldest trees, none of which fall into the tropical region.^ref

How did the researchers work out the age of all these trees? They used tree rings – the science known as dendrochronology. Despite what you might have heard, tropical trees do produce annual growth rings, not based on seasonal growth like in temperate areas, but instead on limiting environmental factors such as water shortage during the dry season or root anoxia in flooded forests during the wet season.^ref

The good news is that tropical trees grow quickly, which is awesome for bonsai and helps us get nicely shaped trees faster, just as long as we provide the tree with the environment it needs. So what is that environment?

A defining attribute of the tropical environment is its weather. The temperature in the tropics doesn’t vary much, ranging between 25^oC and 28^oC all year round^ref. It never gets cold in this region and it certainly never gets frost. As you get closer to the Equator, the annual cycle of the Earth’s angle of rotation has a smaller and smaller effect, so these locations don’t change a lot in terms of their distance to the sun. Since the distance to the sun doesn’t vary that much, neither does the temperature.

The same applies to daylight. This never varies from 12 hours at the Equator, and even on the edges of the tropics the daylight period in winter is only 3 hours less than in summer (see the daylight chart for Alice Springs, on the Tropic of Capricorn). Compare this to London where the difference is 8 hours between summer and winter.

The amount of sunlight a tropical tree actually receives of course depends on its habitat and position within that habitat. In Costa Rica a study found that understory plants in a tropical forest received only 1-2% of the total light available, and that up to 77% of the light they did receive was from ‘sunflecks’ (spots of light which make their way to the forest floor through gaps in leaves).^{ref1, ref2} So whilst trees which occupy the forest canopy or live in a wet-dry tropical desert environment may require intense light for 12 hours a day, there are plenty of tropical species which can thrive in shady positions as well. As with any plant care, it’s all about understanding where your tree naturally thrives and trying to emulate that environment.

As well as constant temperatures, tropical plants receive a lot of rainfall. Two thirds of annual global rainfall occurs in the tropics and sub-tropics^ref with different patterns in different zones. While the Earth’s rotation has less of an effect on temperature in this region, it has a greater effect on weather systems, which occur more spontaneously in the tropics than elsewhere.^ref

The equatorial zone has high monthly precipitation (60mm or more) and annual precipitation of 2m or more. In this zone are many of the tropical rainforests, where there are often dry, humid mornings and rain in the afternoon. Outside of this there are ‘Dry and Wet’ regions with lower rainfall and distinct dry and wet seasons which depend on position relative to the Equator. Finally some areas of the sub-tropics are categorised as monsoon zones, where there is higher temperature variation (for example going down to 13^oC in Chittagong in January), but also periods of dry and periods of significant rainfall (known as the monsoon).^ref Your tropical tree will be expecting a lot of water at some point in the season!

What all of this means is that to keep tropical bonsai in non-tropical areas we need to create a suitable environment, with four main attributes: (1) a stable, high temperature, no drafts or strong temperature variations and definitely no frost, (2) a consistent level of light between 10.5-13.5 hours (with intensity depending on the species) (3) high humidity and (4) a good watering regime. For anyone who doesn’t live in a tropical location, this means keeping them indoors, in a house or heated conservatory, near a window with some sunlight and away from drying drafts or wind.

In their natural home many tropical species will be used to temperatures above 18^oC and up to 28^oC, but Amelia Williams recommends no less than 12^oC and for Ficus it should be above 15.5^oC. This is the lower limit for temperature for your tree, ideally it should be higher, so room temperature of around 20^oC with maybe some heat from the sun during the day should work well.

If your tree is in a conservatory or even in a window, it might get quite hot during the summer. Medium heat stress in trees is thought to be transitory and doesn’t result in long-term damage (although it does increase net energy use), but “long or exceedance of heat tolerance thresholds leads to irreversible damage to the photosynthetic biochemistry and leaf tissues”.^ref One study found in Phaseolus vulgaris that very hot conditions (over 40^oC) photosynthesis declined rapidly and the cost of respiration exceeded the energy from photosynthesis at around 43^oC. Damage to the leaf and death of cells and chloroplasts was visible from 48^oC.^ref This means that a tree which is extremely heat stressed may need to regrow leaves to recover. However even in this situation they need to have enough water. Trees under heat stress which have enough water keep their stomata closed and minimise water loss, but when they don’t they actually open up their stomata. This causes more water loss through transpiration and the possible death of the tree.^ref So it’s super important at high temperatures to keep your trees well watered.

Sunlight can be a difficult commodity to provide your trees all year round, but a protected window position (and occasional rotation of the pot) will give it the best light, which can be augmented by artificial light if you want to more accurately emulate a tropical photoperiod. Don’t worry if you have no natural light though, it is possible to use entirely artificial light, your trees just won’t be as vigorous. A study on houseplants in Uzbekistan found that those with no natural sunlight required artificial light of a minimum of 2000 lux per day and ideally 5000 lux.^ref

Taking your tree outside in the summer is an option, but be careful taking one from behind the protection of UV-filtered glass straight into a hot sun. Model plant Arabidopsis was found to take 8 days to synthesise and accumulate maximal levels of sinapate esters in its leaves, which act as a UV sunscreen.^ref Instead give the tree some shade or protection for a period while it acclimatises (this actually applies to any plant you keep inside for part of the year).

Humidity can be provided by regular spraying (or misting if you have the facility), but Amelia Williams uses humidity trays. These are trays or dishes filled with a layer of pumice which the bonsai pot sits upon. Water is added to the tray and evaporates up to the foliage, the roots also detect the humidity below (and can sneak out of the holes in the pot in search of this water).

As a watering regime, in the UK once a week is enough in winter, twice a week in spring/autumn and daily in summer, but as with all bonsai watering, this needs to be considered based on each tree’s needs. Water well in hot or dry weather to minimise heat stress.

Other tips from Amelia when keeping tropical bonsai – as with all bonsai use soils with good aeration and differing particle grades (see Bonsai growing medium), repot when actively growing, and avoid ‘cold’ soil substrates or substrates which don’t warm up easily or quickly (such as grit).

Choosing which tropical or sub-tropical bonsai you are going to start with is probably the hardest part, there are just so many options!

On the angiosperm (flowering tree) side of the fence, there are many, many options. Acacia, Diospyros, Eucalyptus, Ficus, Adansonia (Baobab), Bougainvillea – the list goes on – but if you are in the UK or a place with similar weather, Amelia Williams’ articles on Swindon Bonsai’s website give you lots of great suggestions – in this one she has a list. A quick tour of the websites of bonsai sellers located in the tropics yields a lot of the same species we see in Europe – Japanese pines and junipers. But there is inspiration out there – here are some specific African bonsai styles, tropical bonsai from Indonesia by Gede Merta, a bonsai farm in China and one in Florida (strictly Florida is sub-tropical so this owner takes his trees inside during cold weather). You don’t even need to stick to trees, as there are also tropical non-trees which look amazing as bonsai – such as these ‘Rambo form’ Adeniums from Thailand (note that Adeniums have toxic sap which was used to create poisoned arrow heads^ref).

If you’re a conifer fan there are fewer options, since angiosperms (flowering plants) have dominated in the tropics since the Cretaceous period. One exception is the Podocarpaceae family which thrives in nutrient-poor soils in these environments, including bogs.^ref Tropical species are found in the Podocarpus genus including Podocarpus macrophyllus or Buddhist Pine. They are also found in the Dacrycarpus genus for example Dacrycarpus dacridioides is apparently a ‘popular bonsai subject’ according to this website, although it’s not mentioned anywhere else I can find online. Other tropical gymnosperms include some members of Araucariaceae such as Agathis or Kauri trees, Araucaria cunninghamii the Hoop Pine and the sub-tropical Araucaria heterophylla Norfolk Island Pine. Two pine species can also be found in the tropics, Pinus merkusii from South-East Asia and Pinus hondurensis from Mexico.

The main constraint you will have in adding tropical or sub-tropical trees to your bonsai collection is probably going to be accessing plants or seeds, so let your imagination fly and see what works for you (I admit to being side-tracked while writing this post by this UK Adenium seller and now await my first packet of seeds!)