Wood chips are just little pieces of wood, but their use can spark heated discussions on gardening social media. Why the fuss? Are they good or bad? 

The wood chips we’re talking about don’t come in bags. Bagged mulch is often shredded (not chipped) wood and bark, and frequently colored with dye. It’s not what we’re after here, and would be prohibitively expensive at large scale. 

Fresh arborist wood chips (top) and bagged dyed mulch (bottom).

The wood chips we’re talking about are fresh from the arborist service or landscaping company, and are the byproduct of tree removal and landscape cleanup after storms. In short, pieces of tree were fed into a noisy machine, which chopped them up, blew them into the back of a truck, and then got delivered to your home where they were dumped in the yard, hopefully not in front of your garage. 

Sometimes called arborist wood chips, they should be actual chips an inch or two long, not sawdust or the fine, small chips from a chainsaw. You may see claims that only hardwood chips should be used, but in fact, both chips from hardwood, softwood, or mixed sourcing are all fine to use. They won’t acidify your soil, and they aren’t toxic.

Wood chips are valued by gardeners as a huge source of carbon and soil organic matter. Soil carbon is a key indicator of soil health, directly enhancing soil structure, water retention, nutrient cycling, and microbial activity. 

Basically, we use wood chips to build soil on a large scale, while also providing immediate garden benefits. You might not be able to get a couple of tons of finished compost delivered, but you can probably get several tons of wood chips cheap, if not free. That’s a lot of material. Wood chips are a gardening gold mine of fertility if used correctly. Used incorrectly, they are a headache.

Why Use Wood Chips?

Wood chips are one of the best materials for weed suppression in the garden, surpassing straw, grass clippings, and even shredded leaves. They do a great job at helping the soil retain moisture and moderating the temperature swings on hot days (soil microbes don’t like to get cooked, and neither do worms). Best of all, they greatly improve soil structure over time.

Wood chips, like all organic materials, decompose. If you’ve had to replace mulch every year or even partway through a growing season, you’ve experienced mulch breaking down over time. That decomposed organic matter gets incorporated into the soil, improving soil structure and aeration, moisture, nutrient holding capacity, and fertility. 

Arborist wood chips are also sustainable and local. These chips, if not used by you, might very well end up in a giant pile at the side of a landfill, doing no one any good. Creating a valuable soil amendment and keeping weeds down while using a local and renewable resource? That’s a pretty good argument for wood chips. 

Where to Use Wood Chips

Wood chips are perfect for using as mulch around ornamental trees, fruit trees, a row of raspberries, or under your shrubbery. But they’re also great for garden pathways and between raised beds. 

While some folks prefer to keep them out of their veggie garden, wood chips work fine there too. The large area where you let the pumpkins sprawl out in summer is perfect for a nice layer of wood chips, which keeps the weeds down and gives your pumpkins a nice spot to sit off the soil surface. 

Using wood chips between rows, or all around plants (keep them away from the stems like with any mulch) are good ways to harness the benefits of wood chip mulch in any garden location and build soil health without much effort.

How to Use Wood Chips

You may have heard horror stories about gardeners using wood chips. I tried them, and nothing grew. Or, I had to abandon that plot and start over. These are all examples of the problems with improper wood chip use. On a recent social media foray, I found a gardener who had spread woodchips 18 inches thick over her entire garden. That’s about knee deep! In a dozen years, it will be a great spot. This year, not so much. 

When using wood chips, it’s important to think of them as a mulch, not a growing medium. We don’t plant directly into wood chips; we plant into the soil underneath them. Wood chips are a soil covering with great benefits, but they are not a substitute for the soil itself

Wood chips as mulch

By far, the easiest way to utilize wood chips is simply as mulch. Whether in the paths or in the beds, wood chips are very effective. They excel at weed suppression, are easy to rake or smooth back into place if disturbed by planting or weeding, and excel at improving soil moisture levels and keeping soil cool under hot summer sunshine. 

But the best benefit happens at the soil surface. Just like other organic materials, wood decomposes over time, and that’s precisely what we want. If we didn’t want to improve our soil, we could use plastic mulch. 

Wood chips decompose more slowly than grass clippings or shredded leaves, but they will decompose. Fungi, soil microbes, and even worms and insects all help, just as they do out in the forest. Wood chips are amazing at fostering active soil microbial life. They can even provide benefits when used in the paths, as some of that activity extends outward from under the mulch into your garden beds. 

When used as mulch, a 2-4 inch thick application is sufficient, although you can spread it on a little thicker for pathways. You’ll likely need to top off the wood chip mulch layer every year, as quite a bit will be lost to the process of turning into compost. Like any mulch, if it gets thin, that’s when the weeds get a foothold.

Wood chips as compost fodder

Search online and you’ll see people simply composting huge piles of wood chips. Whether cold composting or with fancy bio reactors to speed things up, they generate high volumes of compost for use in their garden. It isn’t always fast, but it is cheap and worth doing if you have the space. 

Remember that wood chips, like all composting materials, will break down faster with appropriate moisture. If your compost pile or chip pile is dry, the microbes aren’t very active, and decomposition will be terribly slow. Adding greens (kitchen scraps, coffee grounds,grass clippings, green garden refuse, weeds that haven’t gone to seed) will speed up the composting process. If you often have enough greens for your compost pile but struggle to find enough browns, a mound of wood chips in the back yard might be just the ticket.

Myth busting about Nitrogen tie-up

The biggest concern gardeners have about wood chips is that they’ll tie up the nitrogen in your garden soil, making it impossible for plants to grow. That isn’t true, but it isn’t false either. It depends on how they are used.  

Wood chips are a “brown” in composting parlance, and are a huge carbon source. The compounds in wood are difficult to break down, and the little soil microbes that do so need some nitrogen as fuel to help them along. When the microbes scavenge available nitrogen, those nutrients are not available to plants. It’s called nitrogen immobilization, and it is temporary. As the wood chips are broken down and microbial activity decreases, the nitrogen that was tied up in the microbes is again released and becomes available to plants. But this temporary nitrogen deficiency is what has given wood chips a bad rap. 

The nitrogen tie-up or immobilization happens at the location of decomposition. When chips are used as mulch, that interface is at the soil surface, which is not where the plant roots are. They’re down lower. Soil microbes don’t wander down six inches to grab some nitrogen and then commute back up to the surface. They use what’s available right there where they are.  

When wood chips are mixed or tilled into the soil it moves the interface deeper, down to where the plant roots are, and that’s when problems arise. Folks think they should mix all those wood chips into the soil as deep as they can to speed up decomposition, and to compound it, often add so many that the soil is half chips. Don’t mix wood chips into the soil. Leave the wood chips on top.

When used as mulch, wood chips do not cause a lack of available nitrogen for your plants.  The nitrogen immobilization will happen at the soil surface, but won’t affect nitrogen levels deeper in the soil. If you are worried, you can spread some blood meal or coffee grounds on the soil before adding the wood chips, but it isn’t necessary. 

Wood chips aren’t a fit for all gardeners

Wood chips can be a free or nearly free source of garden fertility, but they aren’t for everyone. Typically, they are delivered in very large loads, sometimes up to ten or more yards at a time.  Small yards, folks with picky HOAs, or those with smaller gardens may not be able to handle a pile of chips four feet tall and 15 or more feet long. The pile is often larger than a car. 

The idea behind wood chips is to allow nature and the soil microbial life to do their thing over time. If you need soil fertility this week, or are in the habit of tilling your garden every spring and fall, wood chips probably aren’t a good fit for you. 

Gardeners who have success with wood chips are building fertility year over year, adding more chips annually, encouraging natural processes, and gaining benefits over time. If that sounds like your plan, call around to local tree services and inquire. You might get a load of wood chips sooner than you think.