Your Questions And Answers About Mulch

Have you ever used woodchip mulch? There are many myths and misconceptions about the right and wrong ways to use wood chips on your land. Learn the facts. Read about our experiments using it on our own Central Portugal garden and orchard, and be sure to check out our links to university research and scientific study results that may surprise you!


Is it true that you shouldn’t put eucalyptus woodchip on the garden because of the oils it contains?

No – this is a myth. Eucalyptus chip offers useful phosphorus and potassium and will decompose without any harm to your plants. As long as you don’t dig this – or any mulch – into your garden soil, it will slowly rot down into nutritious humus. Of course, the very best nutritional balance comes from a mix of chips from different trees. Take a look at: http://www.patwelsh.com/fertilizer/chipped-eucalyptus-wood-and-leaves-from-tree-trimmers/

Won’t the wood chip make my garden soil really acidic?

No, that’s not accurate. It is true that, at the lowest level of your chip, where it is dampest and interfaces with the soil or sheet mulch (cardboard, biodegradable or other material), the chip is acidic. But, contrary to what you might think, there is no scientific evidence that the acidity of the soil is increased. The University of Washington in Seattle is one of the world’s foremost research universities. Here’s an interesting pdf of theirs about wood chip. I’m sure you’ll find it interesting.

Can I use sheet mulch such as newspaper or cardboard and then wood chip on top around my fruit trees?

Yes! The trees will benefit, as long as you DO NOT pile the chip up in a “volcano” or allow the woodchip to lie against the tree bark. Keep it back at least 10-20 cm from the base of your trees, or you will a) encourage secondary tree roots to grow upwards into the moist chips, and b) you could get breaks in the tree bark and fungal infections. Watch this video: https://www.youtube.com/watch?v=fI12XNNqldA

What about fire risk? Won’t wood chips burn in a forest fire?

Because woodchip mulch actually inhibits evaporation from the soil, its lower layers are always damp and start decomposing right away – just as forest bits and pieces would do naturally. This means that once spread out on the soil (after the winter rains are the best time to spread woodchip in Portugal), fire risk is low. This pdf explains that coarse woodchip is actually the LEAST flammable of all organic mulches. http://cru.cahe.wsu.edu/CEPublications/FS160E/FS160E.pdf Of course, we hope you wouldn’t do something silly like putting a barbeque grill directly on top of woodchip, or use it next to your garden burning area!  

I’ve been told wood chips will rob my plants of the nitrogen they need. Is this correct?

This is a very common myth, based on a misunderstanding of how to use woodchip mulch correctly.
Remember, this is mulch, not compost. You leave mulch on top of the soil, you don’t dig it in. Nitrogen is being consumed by the wood chip only at the level where the chip touches the surface of the soil or cardboard because that’s where it starts to decompose. And that is why you always plant your plants below the chip and below any sheet mulch. A Cornell University 15-year study of woodchip use for vegetables showed that over time, nitrogen levels in the soil actually increased where chips were used. Read this: https://www.uvm.edu/vtvegandberry/Pubs/Wood%20Chips%20in%20Vegetable%20Production.pdf 

I’m interested in growing some edible mushrooms outdoors, and I understand that wood chip is great for some varieties. Any suggestions?

By far the best source we know is “Growing Gourmet and Medicinal Mushrooms” by Paul Stamets (available as an e-book), the internationally recognized “guru” of all things to do with cultivating mushrooms outdoors or in a lab – not just to eat or as medicine, but also to remove or microfilter out pollutants like oil and pesticides from water or soil. He tells about which edible varieties to try, with step-by-step photos showing the whole process and results. We will be trying three saprophytic (wood-eating) varieties in our horta this year: Shiitake (Lenticula edodes); King Stropharia or Wine Cap (Stropharia rugoannulata); and Enokitaki (Flammulina velutipes). Stay tuned for results!

When the loggers came to cut our trees, they left a hell of a mess on the ground – tree crowns, broken branches where they crushed other trees, etc. Should I leave as is, or chip?

If you want to maintain a fire-safe and healthy woodland, definitely chip your woodland waste! You see, unless wood debris is broken into smaller pieces with much more surface area, it could take decades to break down and return nutrients to your woodland – or what’s left of it. Beneficial fungi seek out and invade fallen debris where it comes in contact with the soil. They form a vast network of thread-like mycelium beneath the soil and set up a symbiotic micorrizal (Greek mykes = fungus + rhiza = root) relationship with the trees. Trees and shrubs give the fungi sugars they need (because they can’t photosynthesize), and the fungi give back water and mineral nutrients. REMEMBER: if you take away trees and don’t chip remaining debris to be decomposed, beneficial fungi begin to retreat from the tree roots, deeper and deeper, getting hungrier until they die. Result: less water in your forest soil, fewer nutrients for the trees and woodland plants, depleted soil that’s both more likely to erode and less likely to support a vigorous ecosystem.