Eucalyptus regnans (Mountain Ash) Growing Guide

What is Eucalyptus regnans?

Eucalyptus regnans is the tallest Eucalyptus species, and the tallest flowering plant on earth. The tallest living specimen is 100.5m tall and named Centurion, but one fallen tree discovered in the Watts River region is thought to have measured 132.6m.

It creates dense forests in Southern Australia and is easily recognised by its smooth grey bark that runs up the entire length of the tree. This is because, unlike many Eucalypts, it has no lignotuber – the basal section of the trunk that protects many species from fire damage. 

Where eucalyptus trees have lignotubers, they develop stringy or rough bark around the base, with a different appearance further up the tree. The lack of lignotuber makes mountain ash a much neater, cleaner eucalyptus than many other species.

The evergreen leaves of mountain ash are glossy on new growth but fade to a deep, dull green from older more mature branches.

It’s also worth noting that the flowering period for Eucalyptus regnans is shorter than most, usually occurring for just two months from March to May, followed by clusters of seed capsules that remain on the tree for several months. 

For alternative eucalyptus trees to grow in smaller spaces, read our guide to different types of Eucalyptus species.

Natural Habitat of Mountain Ash

Eucalyptus regnans is native across a small region of Victoria and Tasmania, almost exclusively in cool mountainous forests. Relatively high rainfall supports their rapid growth, and the natural leaf litter of the forest floor maintains drainage and soil structure, which is essential for these trees.

In most of its natural habitat, mountain ash is found growing in forests occupied by a single species, but in the Otway, there are examples of it growing in mixed forests with other species including Victorian blue gum and messmate.

Common Uses for Eucalyptus regnans

There are two common uses for Eucalyptus regnans, both just as useful as the other. And both, thankfully, are climate-friendly, potentially offering solutions to global timber production.

The first is simple; mountain ash is one of the toughest fast-growing timbers there is. It is cost-effective, requires no irrigation after planting, and can be used as coppiced firewood and building timber for all sorts of purposes.

The second, which has only truly been understood since 2009 thanks to a study by Professor Brendan Mackey, is its potential usefulness as a carbon lock. The mountain ash forests of Tasmania and Victoria lock more carbon than any other forest in the world (1,867 tonnes per hectare).

This is partly due to their age, and the lack of human disturbance, but there are potential longer-term benefits this discovery could inspire.

Edible Uses & Identifying Eucalyptus regnans

There are no edible uses for Eucalyptus regnans, but it’s still worth getting to know its identification traits to avoid confusion with other species.

As well as the smooth trunk, mentioned earlier, because of its lack of lignotuber, it rarely has any lower stems or structure once mature, preferring to grow as a single trunk, with scattered canopy over the top third of its trunk. 

The leaves are distinctly egg-shaped, but slightly narrower on mature growth, and clearly identifiable thanks to their glossy texture. This sheen is thanks to open pores on the upper and lower part of the leaf, which secrete oils.

Finally, while most eucalyptus trees have seed capsules and flowers in clusters of three or seven, Eucalyptus regnans develop their oval seed capsules in clusters of nine or fifteen.

How to Grow Eucalyptus regnans

Eucalyptus regnans are rarely cultivated in domestic gardens, due entirely to its size. But, because it grows up to a metre a year, it does make a very practical coppiced hedge.

The only caution with this is that it will require annual pruning regimes to be strict and religious to maintain its size and prevent it from becoming the mountainous tall tree it naturally wants to be.

The planting guide below is designed to suit planting regardless of how you intend to grow it, and for commercial plantations, spacing will be more critical, taking into account the overall height, and the required harvest height. 

Ideal Position for Growing Eucalyptus regnans

Eucalyptus regnans need full sun, and reasonably fertile soil, but can grow on poor and well-drained soils provided it receives good irrigation while it establishes. The most important thing to consider is its distance from other features. 

In forests, the competition of other trees exaggerates the growth habit and height of the tree, but in open landscapes, they are unlikely to reach the same heights. Regardless, mountain ash should not be planted near buildings and will outcompete other nearby trees.

If planting a coppiced hedge, the rules are a little different, as its roots will restrict themselves to the size of the hedge, and the height is restricted by you, the gardener.

In this case, you can plant it pretty much anywhere you like, provided it has sufficient sunlight and adequate drainage.

Soil & Drainage

Mountain ash has two other common names that seem to contradict themselves; swamp gum and Tasmanian oak. But it prefers the drainage of elevated conditions to swamps.

Tasmanian oak refers more to its timber than its habitat, but the rainfall in Tasmania is a good indicator of the sort of irrigation these trees need. Aim to plant into a reasonably fertile, well-drained soil, and water well for the first three years.

Once the roots have been established, you can stop watering entirely.

Light & Temperature

The reason mountain ash grows so tall is because it wants sunlight. In forests, where there is constant competition, the only choice saplings have is to shoot up, and quickly.

Planting Eucalyptus regnans in a well-lit sunny spot will help to regulate its growth, but it can cope with dappled shade.

Shelter

No shelter is required from wind or coastal conditions.

Planting Eucalyptus regnans

Provided you get the soil conditions right (see above), Eucalyptus regnans isn’t particularly fussy about how it is planted. Stick to general tree planting rules and it will be fine.

Dig a hole twice the size of the root ball, and backfill it with soil. Once it’s in place, firm it in well with your heel, and water thoroughly to remove air pockets in the surrounding soil.

Finish with a tree stake to support the tree against rocking while the roots develop and anchor it naturally in place.

How to Propagate Eucalyptus regnans

Eucalyptus regnans is a very typical eucalyptus, in that it will grow well from seed but rarely takes from cuttings. Both methods of propagation require specific treatment to work, but growing mountain ash from seed is the most reliable choice. 

Having not grafted mountain ash before I can’t give a first-hand guide, but if you want to give it a try, follow our instructions for grafting Eucalyptus punctata. 

Propagating Mountain Ash from Seeds

Eucalyptus regnans is a Tasmanian and Victorian native, and like most trees native to that part of the country, cold stratification is an important part of the process.

The seeds themselves are quite easy to handle and easy to identify too thanks to their pyramidal form. To stratify them, place them in an airtight plastic bag and store them in the fridge for 3-4 weeks. This mimics winter conditions and triggers growth when they are raised to higher temperatures.

Once the mountain ash seeds are stratified, fill a seed tray with a well-drained compost mix and sprinkle the seeds directly onto the surface. Do not cover the seeds, just water them lightly to make sure they have good contact with the soil.

Then move them somewhere warm to germinate (21°C or higher). They do not need direct sunlight to germinate, so can be left somewhere warm until they show signs of growth. 

Keep the seeds moist and wait 4-6 weeks for germination.

Eucalyptus regnans Propagation from Cuttings

Eucalyptus regnans can be propagated from cuttings but with difficulty. There is no special technique to how you should take the cuttings, but provide a well-drained potting mix (75% compost/coir, 25% perlite is ideal) and dip them in rooting powder. 

After that, keep them somewhere warm, humid and out of direct light. The ideal conditions are 25-32°C, and humidity can be maintained by placing a plastic bag over the top of the cutting.

 Success rates will vary and can’t be guaranteed, but there should be signs of growth within a month.

Caring for Eucalyptus regnans

Eucalyptus regnans is the world’s tallest flowering plant, so you might think it was one of the toughest, but there are actually one or two things to consider when planting it that will greatly help with future care.  

Most importantly, because it has no lignotubers, it is very susceptible to damage by wildfires, and will not recover from them. Its seeds will replace the parent tree in time following a fire, but if you are planting woodland in dry parts of Australia, this is not the tree to go for.

Instead, plant it by itself as a landscape feature, or, in small settings, use it for coppice, and make sure you keep on top of pruning, remembering always that its typical growth rate (it can be faster) is 1m per year.

Pruning Eucalyptus regnans

Mature Eucalyptus regnans should not be pruned unless it is to remove damaged or weak branches that pose a risk to safety beneath the canopy. If this is the case, employ a professional tree surgeon to carry out these cuts.

For coppiced mountain ash, cut back once a year, immediately after flowering. Not only will this reduce potential self-seeding, but it gives the longest growth season for the returning growth. 

We recommend coppicing mountain ash completely, rather than in staggered years, but depending on the height you want the hedge to be, or if you are coppicing as a firewood, you can skip a year or two, or prune back alternate stems.

Common Eucalyptus regnans Pests and Diseases

Eucalyptus regnans are hardy, handsome trees that rarely succumb to the results of pest damage or the effects of disease, but there are a handful of common problems that do impact the health of even the most durable mature mountain ash trees. 

Below, we have guidance on some of the most problematic, and information on how and if to deal with them.

Caterpillars, psyllids, gall wasps and other common eucalyptus pests are very much tolerable on mountain ash. The species is resilient against their damage and copes well with defoliation, recovering easily the following season.

However, sawfly and leaf beetles can do irreparable damage to young trees. Knowing what to look for can help stop them before they become an infestation.

Sawfly

Sawfly larvae are hard to manage because they emit an incredibly irritating substance when they are disturbed. But if left untended, they feed so voraciously on young eucalyptus plants that they can completely defoliate saplings and prevent them from establishing.

The only way to deal with them safely is to wear gloves and scrape them into a bucket.  

Leaf beetle

Eucalyptus leaf beetles do the same sort of damage as sawfly, but are easier to deal with. There are many natural predators, including birds, so adding bird feeders in your garden will help to attract them and put them to work against beetle infestations.

Otherwise, they can be scraped off, or shaken off the plant when you see them and disposed of however you see fit.

Canker, crown rot, and other common fungal problems caused by excess moisture in the soil, and damaged crowns don’t generally affect mountain ash, but, like the insects above, there are some very common garden fungal problems that can do irreparable damage to young eucalyptus trees.

Powdery mildew

Powdery mildew is problematic on Eucalyptus regnans, and quite common for young trees or coppiced mountain ash. While they do like moisture it is essential that it is paired with good drainage so that they are not sat in damp conditions, particularly through winter.

Powdery mildew can be treated with organic fungicides, but avoid using them on trees that are frequented by birds and other wildlife as they feed on the foliage and could ingest the chemical.

Myrtle rust

Myrtle rust is a common issue with young or coppiced Eucalyptus regnans. It is easy to spot thanks to the rusty lesions dotted over foliage in sections of the tree. If spotted on mature trees, it will usually self-resolve within a season.

On smaller trees and coppiced mountain ash, it should be cut out and burned to prevent further spread.

Cinnamon fungus

Another issue caused by waterlogged, poorly drained soil, is cinnamon fungus, a fungus with a visible fruiting body, but that is most commonly hidden in the soil, If tree roots are exposed to the fungus, coupled with damp, stagnant conditions for prolonged periods the fungus can work its way up through the tree, causing serious damage and reducing the vigour and shine of the leaves above.

Eucalyptus regnans Frequently Asked Questions

What is the range of Eucalyptus regnans?

Eucalyptus regnans has a small range, from North-Eastern Tasmania in the South to the Gippsland Forests in Eastern Victoria, and the Otway ranges in Southern Victoria. It is generally limited to mountainous ranges well above sea level.

Is smelling too much eucalyptus bad for you?

Over-inhaling eucalyptus can cause severe nasal irritation, and the redness may cause itching. Anybody with chronic respiratory conditions should avoid regular exposure to eucalyptus as it can exacerbate conditions like asthma.

Are Eucalyptus regnans toxic to dogs?

Eucalyptus regnans are toxic to dogs even in small quantities. Eucalyptus essential oils are especially bad for your pet, and you should seek help from your vet immediately if inhaled or swallowed.

What Eucalyptus stays small?

There are plenty of wonderful dwarf eucalyptus trees to grow at home without having to coppice them. Eucalyptus gunnii is one of the most popular dwarf eucalyptus trees that will stay small, only ever reaching 2.5-3m tall, and can even be grown in containers.

How can you tell how old a eucalyptus tree is?

Depending on the species of Eucalyptus, you can date its age by the circumference of its trunk, Eucalyptus regnans are often dated this way, and specimens of particular interest are also measured vertically.

The more common and better known method is to count the rings inside the trunk (though this is obviously not possible for living trees).

What happens if my pet sniffs eucalyptus?

Eucalyptus is toxic to pets (cats, dogs, and rabbits) when ingested in small quantities, but inhaling it is not toxic. However, it will irritate their airways and can cause prolonged sneezing and itching.

Pets with respiratory infections or chronic lung conditions should not be allowed to sniff eucalyptus essential oil.

Does Eucalyptus regnans go dormant?

Eucalyptus regnans do not go dormant over winter. Its growth does slow slightly over the cooler months, but keep in mind that these evergreen trees are mostly winter flowering, and Eucalyptus regnans in particular flowers from March to May, with seed production throughout winter.

Do Eucalyptus regnans have deep roots?

Eucalyptus regnans do have tap roots, but they are nowhere near as deep as you might imagine. Their main roots, and all of their feeding roots are the lateral roots that grow out in the shallow ground for at least the same distance as the tree’s height. 

Are Eucalyptus regnans safe for birds?

Eucalyptus regnans are not only safe for birds, but it is a great treat to give them. If you have parrots or budgies at home, they adore the sensation of eating eucalyptus leaves and can be fed regularly, with some sources claiming it is not only non-toxic but actively good for them.

Is Eucalyptus regnans invasive?

Strictly speaking, Eucalyptus regnans is not an invasive species and is not noted as one. It can be planted anywhere in Australia. However, it is a tree with a rapid growth habit, and can, and will, overtake nearby trees, shading them out and stealing their nutrients.

If allowed to self-seed or cross pollinate there is always a risk of disturbing local ecosystems when planting trees from other similar habitats.