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Eucalyptus deglupta Blume is a huge evergreen tree from the seasonally-wet humid tropics. Indigenous to Indonesia, Papua New Guinea and the southern Philippines, it grows to more than 65m (213ft) and upwards of 2m (6.5ft) in diameter. Away from its natural environment it rarely exceeds half this size.
It is the world’s fastest-growing forestry tree, cultivated mainly for pulpwood on plantations throughout the tropics. It also provides a source of hardwood timber for use in cabinet-making, construction and joinery. In the Philippines the even-textured wood is known as “Bagras” or Southern Mahogany and in Papua New Guinea, “Kamarere” is a major export timber. E. deglupta is one of four eucalypt species not endemic to Australia, and is the only eucalypt to occur naturally in the northern hemisphere.
As an ornamental specimen, E. deglupta is popularly admired for its multi-coloured “painted” appearance, from whence the common name, “Rainbow Eucalyptus” originated. Fine layers of bark exuviate in strips of varying shape and size, revealing a smooth, white to pale green surface, which with age turns to vibrant green, grey, pink, red, orange, blue, and purple. The rainbow effect is most impressive on habitat specimens, and tends to diminish the further the tree is found growing from the tropics.
Cultivation outside the tropics
In less favourable regions, a degree of cold tolerance can be expected, perhaps even enduring an overnight frost. However, E. deglupta is unlikely to survive prolonged exposure to sub-zero temperatures. In Mediterranean climates with plentiful irrigation and sunshine, new growth will emerge almost year-round. Some foliage burn is inevitable on young saplings where low temperatures of 4°-5° Celsius (39°-41°F) are expected. The juvenile leaves are soft and tender, and damage easily during the first year when planted out.
By the second summer Eucalyptus deglupta develops adult foliage. Once delicate, velvety leaves now emerge a lighter shade of green, turning a lot thicker, with a glossy appearance. During this period, the incredible growth rate becomes apparent. The tree in the adjacent photo is perhaps the first Eucalyptus deglupta in mainland Spain. At 1.5m (5ft) tall, planted out in May 2010, it had already doubled in height by the end of October the same year. All that is required to maintain this rate of growth is full sun and plenty of water!
About the seeds
The seeds are tiny, measuring approximately 2mm x 1mm*. There are an estimated two million seeds per kilo, or 2000 seeds/g. From my own experience, I believe this figure is based upon seed volume alone. Harvested seed of E. deglupta is inseparable from the chaff, and since the amount of chaff varies from one batch to another, this would in turn influence the total seed volume.
I sell Eucalyptus deglupta seed in 0.25, 0.5g and 1.0g quantities, allowing a 30%-40% margin for chaff. Taking into account a field germination rate of 60%, I equate this to roughly 300-400 viable seeds per 0.5g packet. This sounds more than enough for the average grower, but E. deglupta is fickle in the early stages. Additionally, when sowing an entire 0.5g packet of seed, expect around 10-20% of the tiny seedlings to damp-off after the first few weeks. Make sure you observe the highest standards of greenhouse hygiene when sowing and cultivating E. deglupta seedlings.
Of the seedlings that survive, many will remain undersized. At least half will make the grade and require pricking out and potting up ahead of the rest. If germination yields the best results, and you’re only interested in the strongest plants, then expect around 100-150 seedlings from a 0.5g packet of seed. If all you require is a few plants, read on. Throughout the early stages of growth there will be setbacks, and these will be discussed in detail.
Seeds germinate readily in 4-8 days. To maintain viability, make sure you keep the seeds dry, and refrigerated at around 3°-5° Celsius (37°-41°F). This will extend their lifespan by a considerable number of years.
Every gardener has their own technique for sowing seeds. Even so, I am often asked about the best method for germinating E. deglupta. As I mentioned earlier, this is a very fickle plant to raise from seed, and the first few weeks will seem painfully slow by comparison. Living in the Mediterranean, I find the best time of year to start is in April.
I prefer to use a lightweight sterile medium such as vermiculite, sometimes mixed 50/50 with coir (coco-fibre). Similarly, you can use perlite mixed with regular compost. Both are fast-draining. More importantly, these loose mediums will not form a “crust” once watered. This is critical, as the emergent seedlings can easily become trapped under the surface.
I typically germinate 0.5g of E. deglupta seed in white, semi-opaque, plastic trays measuring approx. 30 cm x 18 cm x 6 cm (12″ x 7″ x 2″) deep. I use a second tray as a lid to maintain humidity. Air flow is beneficial, so a loose-fitting cover which lets light through is ideal. Bright light, natural or otherwise, is sufficient during the early stages of germination. Full sun is only necessary once the seedlings are at least 5 cm (2″) tall and ready for pricking out.
The importance of cleanliness cannot be stressed enough. Make sure your germination containers are cleaned and sterilized. Use only fresh, quality compost if you prefer using soil, and use only clean water in you mix.
Fill a tray to about 3-5cm (1″-2″) with lightly moist germination mix. Level it smooth but do not compress the mix. Then sprinkle the seeds evenly over the surface a pinch at a time. Use a hand-held spray bottle to wet the seeds down. I add soluble, organic seaweed fertilizer to the water when using a sterile mix, as the tiny seeds hold very little nutrition. Now sprinkle some more of the medium over the seeds, just enough to cover them. Finally, water well, but do not soak. Cover the tray for up to 12 hours a day, and maintain a good level of light, with temperatures between 20°-30° C (68°-86°F).
Keep the mix moist, and do not let it dry out. If the temperature is consistently above 30°C (86°F) it is better to have the mix wet, even to the point of saturation. At this temperature, the emerging seedlings will tolerate a wet germination medium. Finally, do not despair if after your seedlings have sprouted, they appear to stay the same size for several weeks!
Sowing your E. deglupta in early April means they will be ready for pricking out in less time than if you sow them in February or March. Any time of the year will do, as they’ll happily survive lows of 4°-5° Celsius (39°-41°F) in a dormant state. If you like to see steady progress however, it is better to wait for the onset of Spring.
6 weeks later..
Around this time you can prick out the biggest seedlings and pot them up. Pricking out is not difficult, just hold the stem with thumb and forefinger and pull gently upwards. It helps to use a fine point to dig below the roots as you lift the seedling. For successful transplanting, try to transplant after sundown when the air is cool. I use plug trays, and a soil mix that is just dry enough to stir up finely. Ensure the seedling is upright, and that there is no air around the roots, by watering in well.
In practical terms, this is the earliest stage at which you can pot up E. deglupta seedlings, and it calls for a steady hand! If you are sowing fewer seeds using a bigger tray, then you can wait a few more weeks, until the seedlings are at least 5 cm (2″) tall.
Your newly transplanted seedlings, after watering in well, need be incubated in a covered box immediately. Keep them in the shade at around 20-25 Celsius (68-77 F) for at least 3 days. Many will wilt as a result of transplant shock during the first couple of days, but 80-90% of them should bounce back. After this time you can keep them uncovered, preferably in a greenhouse. If you live in an area of low humidity, it is best to keep your seedlings covered until you can visibly see new growth. Make sure they receive plenty of all-round light, as they are very responsive to the angle of sun.
By mid-summer your seedlings should be enjoying the outdoors. Now almost 7 cm (3″) tall, they will have doubled in size from when they were pricked out. Make sure they are sheltered from strong winds, as they blow over easily. They are somewhat top-heavy at this stage, and until the stems turn woody, they will want to lie flat even in light gusts of wind, and invariably they will grow crooked rather than straighten up.
You may decide that this is a more manageable size at which to prick out your E. deglupta seedlings, in which case you should still follow the same procedure for incubating against transplant shock. The only difference being, that you should not pot them up into plug trays like these shown here.
The root growth on E. deglupta is phenomenal, the equivalent of a weed. Furthermore, and this is true for all eucalypts, they really do not perform well in pots. From this stage onwards, the growth rate is determined purely by how much root space each plant is given. In 5 or 6 more weeks, these little seedlings can be popped out of their trays and potted up into any size pot, preferably no smaller than 2 litres (0.5 US gallons).
If you have plenty of spare seedlings, try experimenting with different-sized pots, and you’ll be amazed at the difference in growth rate! The ideal size for planting out is any time from 17 weeks, up to a year old. After that time, any manageable-sized container will soon start to restrict growth. Aim to have your Rainbow Eucalyptus planted out before the second growing season commences.
If you don’t have a garden, did you know that E. deglupta make excellent bonsai specimens? Even when planted out, if space is an issue you can prune a young E. deglupta sapling into a shrub, to create a ‘large bonsai’. Although it is the rainbow-coloured trunk that attracts most attention, it is the newly emerging foliage that is prettiest.
Pests and diseases
E. deglupta seedlings are easily plagued by aphids, which are little more than a nuisance. However, there is a leaf blight that affects this eucalypt, of which I still know little about. It can appear on the tiniest of seedlings as small red dots and quickly spread, eventually killing an entire batch of plants. What is puzzling, is that it seems to live on some seedlings without affecting their health.
The best advice is to dispose of affected seedlings by burning, ensure good ventilation, and separate your seedlings into small groups. The blight will only kill seedlings up to about 6 months, after which time it seems to pose no major threat. Remnants of the blight are just visible in an earlier photo here.