Wood mulch is a fantastic way to add nutrients to garden soil. What’s more, it can also help safeguard soil from the very worst of extreme weather conditions.
But not all types of timber are a suitable choice for wood mulch. For example, treated wood has been saturated with chemical preservatives. So, using treated wood chip mulch is likely going to do more harm than good to your vegetables.
But, what about certain untreated natural timbers… such as Redwood?
Well, in this post, we explain why the carbon to nitrogen ratio of untreated wood mulch matters for your garden plants. You’ll also learn which natural Redwood compound can slow down the decomposition of Redwood mulch.
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What’s The Best Wood For Vegetable Garden Mulch?
Cedar wood mulch is probably the best option for a vegetable garden.
That’s because Cedar tree oils are an insect-repellent. Their oil works like a pesticide, thanks to a chemical they contain, called ‘Sesquiterpene Hydrocarbons’.
Any Cedar mulch that contains a high concentration of sesquiterpene hydrocarbons, is capable of driving away all manner of bugs.
What About Redwood? Does It Make For Good Wood Mulch?
Yes it can. Like all wood mulch, Redwood can help to regulate the conditions of your garden soil. And once it decomposes, it will add nutrients to the soil as well.
What’s more, studies have found that wood mulch can increase your vegetable gardens overall yield. So, if you scatter natural untreated Redwood around your garden, it will decompose.
However, it’s worth noting that Redwood has a fairly slow-rate of decomposition, (compared to most other types of timber). That is because it has a high concentration of a chemical compound that actively fights off rot and decay.
These chemical compounds are called Tannins, and they’re a natures-own fungicide, working to stop bacteria from growing on tree bark.
And What Are Tannins Exactly?
Well, Tannins are a compound found in most plants and trees. Yet, in certain trees, such as Redwood and Western Red Cedar, the level of Tannins in their lumber is very high.
So high, in fact, that sometimes the timber from these trees can ‘bleed’ Tannin, forming brown stains on the surface.
OK, So It Takes Longer For Redwood To Decompose…Why Does It Matter?
It matters because the longer wood mulch takes to decompose, then the more nitrogen it will leech from the ground.
Nitrogen Is Important For Decomposition
The bacteria responsible for breaking down and decomposing wood, need a lot of nitrogen.
Now, these bacteria first use up all of the nitrogen found in the wood mulch. But, wood mulch isn’t a very nitrogen rich material.
You see, ideal composting material needs to have a C:N (Carbon to Nitrogen) ratio of 30:1. Which means that for every 1 part of nitrogen, that material also contains 30 parts of carbon.
But wood chips have much less nitrogen in them than that, with a C:N ratio of around 100:1. In that for every 1 part of nitrogen, they also have 100 parts of carbon.
In short, they don’t have much nitrogen for bacteria to work with.
Now, if there’s very little nitrogen in wood mulch, then bacteria have no choice but to take it from elsewhere. And, in the case of wood mulch, that nitrogen will likely be leeched from the surrounding soil.
This is called ‘Nitrogen Drawdown’, and it can leave nearby plants and vegetables with very little nitrogen to live off.
Does That Mean Redwood Mulch Shouldn’t Be Used In A Vegetable Garden?
Not quite. Sure, decomposing wood can use up lots of nitrogen. But, once it has finished decomposing, it releases all of that enriching nitrogen right back into the soil.
Now, typically, it takes wood mulch anywhere from 3-6 months to complete decomposition. However, in the case of Redwood and Cedar, this can take much longer, due to their natural rot-resisting Tannins.
There is a balance to be struck here though. Redwood may take much longer to decompose, (due to those Tannin compounds). However, it won’t have as many decomposing microbes actively using up nitrogen anyway (due to those self-same compounds).
Still, to prevent Redwood mulch from using up too much nitrogen, simply place a layer of compost down first. That layer of organic compost will give decomposing Redwood mulch all of the extra nitrogen it needs.
So Why Is Cedar Wood Better Than Redwood If They Both Contain Tannins?
First off, Cedar wood will also take much longer to decompose, just like Redwood. And Cedar can also have lower-than-average microbe-activity will it decomposes too.
But, since Cedar wood also has the advantage of being an insect-repellent, you probably won’t mind if it takes longer to decompose. Especially if your garden has a bug-problem, and you don’t want to use chemical insecticides.
To Wrap Up, Here Are The 3 Key Takeaways From This Post…
- 1). Redwood contains a high concentration of Tannins. This is a natural kind of fungicide that helps Redwood to resist rot and decay.
- 2). Redwood will decompose more slowly than most other types of wood mulch. This can lead to Redwood mulch leeching a lot of nitrogen from soil.
- 3). Place a layer of organic compost down before adding Redwood mulch. The compost will give the wood mulch extra nitrogen, and protect your soil from nitrogen drawdown.
Gruda, Nazim. “The effect of wood fiber mulch on water retention, soil temperature and growth of vegetable plants.” Journal of sustainable agriculture 32.4 (2008): 629-643.
Carbon-to-Nitrogen Ratio | Science Direct
Wenninger, John A., Ronald L. Yates, and Meyer Dolinsky. “Sesquiterpene hydrocarbons of commercial copaiba balsam and American cedarwood oils.” Journal of the Association of Official Analytical Chemists 50.6 (1967): 1304-1313.