Growing under gums

I love gums, I love living under gums - those majestic messmates and mountain grey gums.

But, for years I have struggled to get more than a few measly broccoli, watched the grass stagnate despite good rains and have waited more years than I should have for my first meagre harvest of apricots.
I have blamed the soil acidity and smothered my soil in wood-fire ash. I have blamed the hard clays, and dosed it is gypsums and mulch, and I have blamed the gum tree roots for stealing water - then wondered why even good rains and heavy watering fail to mitigate it.

Yes, my soil has its many issues when it comes to nutrients and structure, but none of these can explain the way things fail to thrive near gums.

The answer I have been missing all these years is allelopathy. It is the nature of certain plants to toxify the soil around them to keep competition subdued.

Allelopathy is an attribute of many plants, especially trees. It is well understood in walnuts, but in Eucalyptus it is less well known. When I tell people that I have trouble growing vegetables and fruit under gum trees at my home, most people assume the Gums are taking all the water. Of course, Gums like all trees take water, but their growth inhibitory effect is seen regardless of soil moisture.  Eucalyptus are not known to be huge users of water, however no doubt they are opportunists that will drink heavy when they can. Their roots tend to stay mostly below the grasses, so any drying effect they have is from the bottom up. Grasses and herbs will still get the first sip of rains. Eucaplyptus trees certainly would be on good terms with the soil fungi, and having the best position for their solar panels, they no doubt have the best bargaining power to buy water from the fungi in dry times.

But the dominance of Eucalyptus in the Australian landscape must surely be through their toxic load. Allelopathy not only subdues competitive trees, it controls the forest. It allows trees to mature without competition from their own seedlings. This seems to be true of the long lived white-trunk gums, but not so much of the messmates and other stringy barks and box, whose seedlings can grow in the shadows of their parents and produce thick growth.

So while Eucalypts will be somewhat immune to their own toxins, they will be affected in the most toxic soils below very mature trees.

It might then be hypothesized that Eucalyptus use allelopathy as a kind of signalling system for their next generation. While the soil remains toxic, the seeds remain dormant in the soil. When the trees die, or bushfire comes through, the toxicity may be reduced allowing new trees to germinate.

Some of the best research into Eucalypt allelopathy comes from China, where researchers have looked at the effects of leaf and root exudes on growing vegetables.


Effects on vegetables

( Allelopathic effects of leaf litter and live roots exudates of Eucalyptus species on crops. Allelopath J - Cheniu Zhang, Shenglei Fu )

These results seem to suggest that the main toxic load comes from the leaves. The effects vary for different types of vegetables. with cucumbers seeming to be least affected. (A result consistent with the type of vegetables I have had most success with).

It is (was) my belief that the main toxic load is associated with the tannins in the leaves and is water-soluble. There are oils and waxes that Eucalyptus produce, as well saponins which also affect soils in different ways, but the growth inhibition is mostly from a water soluble compound that leaches from the leaves. (this theory I have since discarded - see conclusions below)

When fresh gums leaves are left in water, they release tannins that stain the water brown. It is my hypothesis that these tannins are inhibitors, or are associated with them.

Questions that I want answered as a gardener and hobby farmer are:

How long do these toxins remain in the soil?

Do these chemicals break down in the soil, or are they simply dispersed by leaching?

Are there ways of minimizing or negating these effects?

Is there a way to 'repair' areas of highly affected ground without chopping down trees?

To answer these questions I will do some simple scientific controlled growth experiments myself to understand far better the nature of Eucalyptus allelopathy.




Experiment 1 will test the hypothesis that the growth inhibitor associated with Eucalyptus allelopathy can be extracted from gum leaves. The experiment will test if wood-fire ash and trace minerals plays any role in alleviating the effect, and if old gum leaves still contain the inhibitors.


After 17 days of subjecting chicory to several combinations of gum-leaf affected water, I have come to the conclusion that there is no water-soluble allelopathic chemical in gum leaves. Small growth inhibitory effects are seen from using water that was in contact with fresh gum leaves, but this effect is most certainly small, and can be explained by the acidity produced from lacto-fermentation of the gum-leaves.

The effects that I was looking for included slower growth, along with yellowing and even purple leaf discoloration. Proneness to water-deficiency stress was also a possible symptom, but more likely in grasses than in chicory, as field observations of chicory under gums tends to show they can still hold on in dry conditions. (Chicories ability to show yellowing without dying is the reason I have chosen them for experimentation, as it allows me to test ideas for correcting the problems without having to start again all the time)

It is true that research papers can be found on the web, where similar methodolgy was used, and similar effects were noted. It seems possible that these researchers were not experienced with the producing home-made sauerkraut, and therefore failed to realise the acidity of the solutions they produced.

However, some important lessons have been learnt from this experiment. The fresh gum leaves did lacto-ferment in the water, indicating a high level of sugars in the fresh leaves, while the old gum leaves continued to putrify and produced a very unpleasant sewerage smell indicative of fats being broken.  The water associated with old putrified gum leaves was also harmless to the seedlings. Knowing now that the allelopathic chemicals are most likely associated with the oils, the fact the the putrification of the gum leaves failed to release toxic allelopathic chemicals into water is valuable knowledge. It tells us that gum leaf mould is not only a safe mulch to use given sufficient break-down time, but also suggests the possibility that a means of treating gum-affected soils lies in promoting this putrification process.

The idea of putrifiying gum leaves and using the by-products as a soil treatment is now an idea for a future experiment. It will take many months to produce large quantities of putrified gum leaves, and hopefully breed up bacteria and enzymes that can break down gum-leaf oils. I will also begin collecting koala droppings in the hope that these may contain some super-charged bacteria that will help break down Eucalyptus oils.

In the short-term, the next experiments will deal with trying to reproduce the allelopathic effects using fresh gum leaves themselves, and comparing with eucalyptus oil.




Experiment 2

Experiment 2 found that Eucalyptus oil in a reasonably high dose does indeed stop germination and growth. In particular it significantly reduces root development, which would explain how small plants under gums tend to be more sensitive to drying out.
No yellowing effects were seen, and this is probably because these particular plants do not respond in this way.
There is certainly some indication here that Eucalyptus oil could be the transport mechanism for Eucalyptus allelopathy, however there were also some results that make it not so straight forward a conclusion. Firstly we should note that the pot with fresh pulverised gum leaves had little effect on the barley and chicory growth, but it did attract a white mould-like growth similar to the pot with the high levels of oil. This means that it must require a significant amount of leaves to carry enough toxins to have an effect. Also, as the experimented progressed it did seem that the oils were breaking down and the toxic effect was diminishing.
If Eucalyptus oils are the source of Eucalyptus allelopathy, then we may need to add another mechanism to explain how it can accumulate in certain soils without breaking down.



Experiment 3

It will be necessary to determine if indeed the pot with the high levels of eucalyptus oil in the previous experiment did de-toxify over the course of the experiment. To test this we will remove the seedlings in the pot and start with new seeds. We will run this pot against a new pot with fresh Eucalyptus oil. We will also look at the role tree-sap plays in allelopathy.

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