Wood Chip Myths, & There Are Many, Put Many Gardeners Off Using Chip. That’s a Pity As There More Good Than Myth In This Versatile Product. This Article Explodes The Myths.
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Wood chip myths, or as it is sometimes written woodchip myths, frighten many gardeners and they miss out on this versatile natural product that can enrich any garden at minimal expense. In some cases it can even be used as a peat or other compost substitute for seedlings and module grown plants.
Whichever spelling you use the myths abound. Everything from wood-chip should never be used as it depletes soil nitrogen to coniferous wood chip will acidy your soil. Ohh and wood chip is toxic.
Like most myths there is a shred of partial truth in many of these myths. It is however often misconstrued, misunderstood or just plain wrong.
Let’s start with a simple one .. if such a thing exists.
The “Coniferous Wood Chip Acidifies Your Soil” Myth
There are two parts to examine here. Firstly is coniferous wood chip acid and, secondly, if so will it make your soil acid?
Below is a chart showing the pH of various woods (and Coca Cola). My chart making skills are inferior to my gardening skills but the pH of each species is the difference between the max and min fires indicated. Eg. Oak has a pH of between 3.3-3.9. That’s more acid than Scots pine which is between 4.3-4.6.
Strangely enough no one ever suggests that oak is going to acidify the soil, but they worry about coniferous species such as Scots pine, larch, Sitka spruce etc.
The reality is that most woodchip is acidic. In this list only elm is close to neutral.
But these figures don’t mean their chip will seriously acidify soils. Even if we put 4-5 inches of woodchip on the soil it is ON the soil, and it is th wood that is acid and that acidity doesn’t leach into the soil to any extent. What little does is buffered by the soil itself. Plus the quantity of compressed wood (ie before it was chipped and made less dense) is small compared with the depth of top soil in most gardens.
Lastly, many people used to grow seeds and plants in peat. Peat moss has a pH of around 3.0 – 4.0 when dug. Over time this drops to around pH5.5. No one using peat complained that peat was too acid for most fruit and veg sowing. So even if the woodchip alters the pH slightly, does it matter unless we are trying to grow a very narrow range of plants that need very alkaline soil? I don’t think so!
Woodchip Depletes Soil Nitrogen; Myth or Fact?
Technically this is true to a v very limited extent but not enough to matter if you use it properly. And of course once the chip starts to break down it contributes nutrients, including nitrogen, back into the soil.
The nitrogen used in decomposition comes primarily from soil fungi, from within a few inches of the chip, and bacteria, from very close to the chip. It doesn’t come from all of the soil.
So where the roots of plants go deep in to the soil they will not be suffer nitrogen depletion. Only those with roots in the “compost zone” will be affected. That is partly how wood chip inhibits weeds. It reduces nitrogen very close to where they germinate. If you want to germinate seeds then just pull the woodchip back a few inches and sow the seed in the soil, not on the woodchip.
There is another factor to be taken into account. Chip size and hence surface area.
The larger the chip the smaller the surface area of the pile. And that means it breaks down much slower and hence uses less nitrogen in any given time. It’s that simple. If you want chip to break down fast, ensure it is finely chipped. If you want it to break down slowly, use large chip. When all other factors are equal, chip size determines decomposition rates and nitrogen requirements.
finally, don’t forget that once the chip is decomposed it contributes nitrogen so helps to balance the equation.
The “Woodchip Is Toxic” Myth?
The dyes used in some coloured wood chip could be toxic to plants and animals. B ut most are made from iron oxide, carbon or vegetable-based colours are are quite sage (even if they often look garish and ugly in my view). Personally I’d never use dyed woodchip in my garden, so it’s not an issue.
As for normal woodchip, with the exception of a few species that don’t grow in the UK there is no evidence of toxicity.
The only exception I know to this that you might encounter in the UK is when walnut is grown. Walnut produces juglone. And juglone, due to its allelopathic properties, is toxic to seedlings and some marine organisms. So, too much walnut chip could init seedlings. But it will not damage mature plants, so could be usefully used in some situations.
I’ll write more about juglone in another post as the situation is not black and white.
The “Woodchip Spreads Diseases” Myth
Some trees are cut down because they are diseased. So theoretically they might be carry disease spores. But most spores spread by the wind so you could get them anyway. If the disease is one that need living wood then it will die once the tree is cut down. So the problem is much reduced.
We also need to consider if the possibility of the chip carrying disease is a practical problem. Tree diseses aren’t a problem of many of the other species we might be growing. For example if I use woodchip on my veg garden paths which of my veg is at risk? Probably none of them as they Arte from totally different species.
Wood Chip Piles Can Cause Spontaneous Combustion: Woodchip Myth Or Fact?
If we pile woodchip to a depth of 30 feet (10 metres) in a long and wide windrow there is a chance that it might spontaneously combust. A slim chance. But the sort of piles see in the UK where a lorryload is tipped and left a short while isn’t an issue. To burn, wood needs to reach several hundred degrees Fahrenheit and this temperatures are highly unlikely in smaller heaps as the heat readily dissipates.
The above applies to the UK, I can’t vouch for much hotter climates.
It is not unusual to see wood chip piles “smoking” on caller autumn morning. But that “smoke” is almost always steam.
I’ve researched reported cases of woodchip fires in the UK and there are a few where bulk storage of woodchip has been involved. The most serious involving a 200 ton store of woodchip. In none of the cases is spontaneous combustion cited as being the cause.
Does Walnut Wood Chip Kills Plants?
As I report above, juglone in the walnut trees can inhibit seedling germination and growth on other species. But I’ve yet to read a report of mature plant problems from walnut chip.
The Best Type of Landscaping Fabric To Use Under Wood Chip
There really isn’t one. Cardboard or other organic materials can be used as they break down reasonably quickly. Most landscape fabrics are however made of plastic or related products and can cause no end of problems island under woodchip.
Lightning Strike Can Ignite Wood Chip: Fact Or Woodchip Myths
In theory very dry chip could be ignited by lightning. But would it then continue to burn if in a thin layer on the soil surface? It is unlikely but not totally impossible. In some countries, where wild fires are common, the fire authorities ban woodchip near houses and other buildings. As far as I know this has never been the case in the UK. But as the weather warms with global warming maybe this will change!
Woodchips Are A Fire Hazard
I refer you to my comments above on lighting strike and spontaneous combustion.
Wood Chip Doesn’t Stop Weeds Growing: Myth Or Fact?
On its own no. Especially if used in a very thin layer. It has to be laid properly. But why would we use it badly if we want it to stop our weeds?
If a few inches of wood chip is applied to the soil it excludes light from the soil surface. This is inhibitory to weeds. So is dry chip. Plants need moisture to grow.
And if we add a cardboard barrier below the woodchip there is less chance of seed in the soil germinating.
Deep and loss woodchip is not a good environment for weed seedlings. But in the same way that plants can grow on pebble beaches it’s not impossible for a few weeds to struggle through. but hey are loosely anchored and very easy to pull by hand.
More About Woodchip & Woodchip Myths
There is a lot more about woodchip in Ben Raskin’s book, The Woodchip Handbook which is reviewed on this website.
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