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Composting Down Under


Composting Down Under

Q. Ruth in Australia writes: "G 'day Mike! I'm writing from the leafy suburbs of Sydney, and my question is whether the leaves of gum trees (aka eucalyptus trees) might respond well to being composted in the same way as your leaves from North American trees.

As you may already know:
1) Gum trees store oil in their leaves. Concentrated eucalyptus oil is a powerful anti-bacteria and anti-fungal chemical. It's up there with raw garlic & can kill off fungus like athlete's foot overnight.

2) Gum trees emit allelopathic chemicals into the nearby soil. These chemicals create an unpleasant situation for other plants, which either can't start life or fail to thrive if they're planted too close to a gum tree. Would these yucky chemicals be in the leaves too? {Mike: "As with our North American black walnut trees, the answer is unfortunately yes."}

3) Gum trees shed a LOT of leaves ALL YEAR. Can we gardeners create a coffee-laced "gum leaf mulch" or use shredded gum leaves as part of our ingredients in a hot composting system?

If so, what about all those naturally occurring oils & chemicals? Will gum leaves play nicely & move smoothly from being leaves to becoming compost or mulch? Or will gum leaves remain stubbornly in their original form, even if physically shredded? Will adding gum leaves to compost damage the compost's rich community of bacterial and fungal colonies?

Sorry to hit you with such a tough question, Thanks again & please continue all your good work. Love your podcast; so many excellent practical ideas! Be of good heart my friend...

Follow up:

Eucalyptuscompost1

I ignored a pile of leaves behind the shed for 6 months or so. I've sieved it, separating out any "soil" that's been created. Not bad, not bad at all.

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One of the nearby trees. This species can grow to about 100 feet tall (30 meters). There are about 1000 species of Euc that generously shed leaves all year round.

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One of my hot composts, built around a central "steam chimney" of PVC pipe with holes in it. Temperatures easily hit 140 degrees F for weeks; I even have to cool it down sometimes. Ingredients are: regular garden prunings (shredded), vegan food leftovers, coffee grounds (for nitrogen), shredded paper, shredded branches, deciduous leaves, the soil that I capture from under a wood-chip pile after 6 months of decomposition, and water (crucial!!!).

PS: I also asked one of the Botanical Gardens in Australia for their opinion. Here's a short version of their response:

Can eucalyptus leaves be composted in a hot composting system?

Our advice would be not to do this. It is likely to have a negative effect on the bacteria in the compost system (eucalypt oils have antibacterial properties and may slow the growth of or kill many species of bacteria) and the resulting compost may have negative effects on the plants you spread it around.

Can they be composted alone in a leaf cage?

Composting eucalypt leaves in a leaf cage will take an extremely long time, as the leaves have a thick waxy cuticle. You could decrease the break-down time by mulching the leaves, but they will not behave the same way as leaves from deciduous trees to make leaf mould.

We think the riskiest use of eucalypt-based mulch/compost would be around vegetables vs. the safest being around Australian native plants, particularly under species naturally found in ecosystems with eucalypts. The ANBG [Australian National Botanical Gardens in Canberra]

A. As you worried about in your emails, I knew almost nothing about using Eucalyptus tree leaves in a traditional composting system. But I went deeply down the rabbit hole of research, because I DID know that many gardeners in locations with extended dry times like yours are desperate for 'dry brown' ingredients with which to make compost. So first I will digress a bit. When the late and great Robert Rodale and I wrote a book about famine prevention called "Save Three Lives" for the Sierra Club back in the late eighties, one of the things we recommended for dry climates was to make compost below ground in a pit to conserve moisture (a technique that may have originated in Australia).

Excavate the desired area, line the bottom with a sturdy tarp with just a few holes punched in the bottom to handle your rainy season excess (December through March) and do your best to fill it with the right mix of dry brown and wet green material minus eucalyptus leaves. Don't use shredded paper; it contains zero nutrition. Heavily shredded wood chips would be fine; as would things like shredded straw, cat tails and dry brown cornstalks. No kitchen waste other than spent coffee grounds and tea bags. Dispose of your other kitchen waste in a worm bin and use the nutrient rich worm castings on your veggies. As you note, keep the pit watered during dry times.

Otherwise, I basically agree with the Botanic Garden: segregate your eucalyptus leaves in their own pile, shredded as many times as you can stand. No matter their age they are technically 'green material', so add only shredded woody material, and, as your national botanic garden suggests, use any resulting compost only on native plants.

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