For the record, I love eucalyptus. But in the suburbs gum trees do have some drawbacks.
As I discuss in my other article Why Block It When You Can Borrow It one drawback is an aesthetic problem if these trees are blocking views and denying ‘borrowed landscapes’, but of course it could be argued that the communal benefits associated with increased tree canopy cover in Canberra generally should override an individual resident’s rights to views and solar access. In that article I am also at pains to sing the praises of gum-trees, particularly the old existing beauties throughout our ‘Bush Capital’ which I would personally chain-myself to if anyone dared cut them down in the absence of good reason (ie. genuine safety concerns where they are clearly diseased or at the end of their natural life). But in addition to the borrowed landscapes issue, solar access and such safety concerns (noting my emphasis of the word ‘genuine’ by the way) gums do have one other disadvantage worth mentioning.
They are incredibly difficult to garden under!
Although not impossible, once the right plants are selected and the soil is sufficiently amended, any site that has large eucalypts will always be a significant challenge for the following three main reasons.
Firstly, as evergreen trees, gums cast constant dense shade all year round. This factor in and of itself is not an insurmountable problem, and indeed could actually be viewed as a benefit given that more shade is something we are trying to achieve in our ever warming climate anyway. But permanent shade will obviously considerably restrict the range of plants we can choose to plant. Even many that will tolerate shade still won’t thrive like they otherwise would in a sunnier location (eg. a good number of native shrubs and groundcovers will survive in shade but will have a tendency to grow slowly and leggy in such conditions). Of course there are many exotic plants that will thrive in full shade, particularly some that may be described as a bit ‘weedy’ as I’ll elaborate on later.
The second reason large gums cause throbbing headaches for gardeners is the sheer size of their roots and the inevitable competition they create for water and nutrients. Every drop of water and handful of fertilizer provided to the understory plants will be voraciously devoured by these extensive eucalypt roots which is why the gum trees always seem to grow rapidly whilst the small suffering plants underneath just sit there doing nothing for years!
And the third factor is the detrimental direct impact gums have on understory plants both above and below the ground. Above the ground they are constantly dropping leaves and small branches which can physically damage plants below and create ongoing maintenance whilst, below the ground, some eucalypts are even allelopathic (ie. release chemicals in the soil, both from their roots and fallen leaves, inhibiting the growth of other plants), although there is some debate as to the extent to which they do that so i’ll leave you to research that further yourself.
Of the above mentioned three main reasons gums pose difficulties to growing understory plants, in my own experience I would say that the issue of shade and water/nutrients competition is the primary impediment.
Suffice to say that understory plant selection will need to be circumspect and you won’t want to attempt to plant any fussy sun-loving plants that require plenty of water or rich nutrient dense soil. Of course, most Australian native shrubs and groundcovers naturally prefer sun but there are many that have adapted to grow under eucalypts as that is often the situation they find themselves in out in the bush after all. For example, Correas, Croweas, Boronias, Hardenbergias (and other plants in the pea family which fix their own nitrogen). It is also worth pointing out that much of the water and nutrient competition is occurring further out from the eucalypt’s trunk closer to the drip-line where the feeder roots are located so positioning understory plants closer to the tree itself is a good idea for that reason.
Moreover, some bullet-proof shade tolerant exotic plants can be a good option for the ‘crummy-gummy’ site – even ones that have developed a bad reputation for being weedy garden-thugs (eg. Agapanthas and Acanthus) which I usually wouldn’t recommend as they can spread and take over the garden if planted in the wrong spot. But sometimes a bad ‘weed’ can become a good ‘weed’ when you’re desperate to get things growing under large gum trees – however, just be sure you’ll be happy to live with these for the long-term because once you have them good-luck trying to get rid of them. Obviously, less ‘weedy’ exotics such as the moderately shade tolerant Star Jasmine (grown as a groundcover) for instance can also be used but the whole point of going with something a bit ‘weedy’ is that it will tend to cope better with the water/nutrient competition so you may find that even a Star Jasmine will grow a bit sparse and leggy under a eucalypt, similar to those natives mentioned above, whereas that bomb-proof Aggy and beasty Oyster Plant won’t bat an eyelid! You may however need to down a few shots of vodka beforehand though to get up that ‘dutch-courage’ to actually go ahead and plant them. If the crummy gummy site is isolated from the rest of your garden the vodka may not be quite so necessary though because you’ll be safe in the knowledge that the ensuing jungle of Agapanthus and Acanthus will at least be limited to that area.
For the purpose of this article I have only included a few plant suggestions (both native and exotic) as the range, despite being restricted by the conditions, is broader than stated here but you’ll need to get me out for a consultation or garden-design to receive a more extensive detailed plant-list. Although I think you at least get the general drift as to what types of plants will struggle vis-a-vis those that will survive/thrive.
And as always there is also the option to not plant at all. Obviously as a horticulturist I’ll tend to try and overcome the hurdles to get plants into tricky spots but sometimes nice mulch and garden features (water fountains, bird baths, garden sculpture/ornaments etc) and of course visually striking ornamental pots will be all an area requires to still be well designed. Plants in pots and raised garden beds will also be lifted up above the crummy-gummy ground which overcomes the entire problem altogether so that is another approach to opening up the plant selection range under those troublesome eucalypts.
