The great wood chip debate goes on

My local gardening Yahoo group is debating the often-repeated warning that "wood chips will tie up all the nitrogen in the soil and will lead to an irreversible manganese build-up," with two professional gardeners agreeing and declaring they use it on paths only.  A third then added, "If used near plants, you have to add nitrogen to compensate...Heard this years ago from Dr. Gouin, U. of Maryland."

Now locally, this Dr. Gouin is considered quite the horticultural sage, but from across the continent comes advice from another Ph.D.-packing expert with an entirely different take on this subject.  Linda Chalker-Scott, a professor at Washington State University, writes in "Arborist wood chips mulches - landscape boon or bane?" that the notion that wood chips will lead to soil nitrogen depletion is wrong.  She notes that a thin zone of nitrogen loss exists where the chips touch the soil, but that for woody plants with subsurface roots, there is no nutrient loss. 

Now this IS confusing because I'm a big fan of Linda Chalker-Scott myself, as is sustainable gardening author and frequent Rant commenter Ginny Stibolt.  But then Washington Gardener editor Kathy Jentz wrote:

I've just returned from garden touring in Portland OR and speaking with nurseries there about their cultivation techniques.  The mulch in the Pacific Northwest is all pine bark -- that is what they have in abundant supply -- whereas our cheap/free mulches here are hardwood mulches.  So the real question is what "mulch" Linda Chalker-Scott at Washington State University was testing and working with.  I'm guessing she was using the pinebark one that is a staple of the Pacific Northwest and very probably did not consider that we in the Mid-Atlantic (the OTHER Washington :-) might not be using the same one.

Thanks, Kathy, for offering what might just be the explanation for these conflicting positions.  Now let's throw it open to GardenRant readers to weigh in and either clinch the question once and for all - or muddy it up even more. 

Oh, and how about that "irreversible manganese build-up"?  Do wood chips really do that and if they do, is it as dire as it sounds?

Photo by Tale Kinker via Flickr - because we just don't have enough cats on this here gardening blog.

Comments

[Becca]

Forget the chips and go with composted dairy manure! Not only does it ADD to the soil but it looks just fine as it settles in.

[Linda Chalker-Scott]

The arborist wood chips we use here as a landscape mulch in the Puget Sound region of Washington State is a mixture of species - hardwood and softwood. It depends on what kind of trees the arborist has most recently been trimming. It's not bark mulch (which has its own problems) and in our area rarely has much pine in it (since pines are not a large component of our urban landscape). That being said, there's nothing wrong with pine wood chips as a landscape mulch.
We've used arborist wood chip mulches for years on restoration sites, arboreta, and public and private landscapes. Not only is it a natural mulch source, it keeps materials out of landfills and is reuse of a local product.
From a scientific viewpoint, there are few drawbacks to using arborist wood chips but many benefits. It mimics what you might find in the duff layer of a forest - which is really what we should be shooting for in many of our landscapes that are based on trees and shrubs. I can say definitively that if wood chips are used as a topdressing and not worked into the soil they will not tie up nitrogen. We've demonstrated this in laboratory research as have others. For readers who would like a challenge, I'd invite you to read my review of the scientific literature comparing landscape mulches, published last December in the Journal of Environmental Horticulture. It also addresses many of the misconceptions held about wood chip mulches. You can email me for a free pdf of this article if you like.

[Linda Chalker-Scott]

I forgot to address the manganese build up. There's no evidence this happens to the soil under wood chip mulches. Where you can find buildup of heavy metals is in the leaf litter - and eventually soil - of orchards and other landscapes that have been treated with foliar fertilizers or pesticides that contain zinc and other metals. Manganese and other metallic nutrients are associated with plant enzymes, most of which are in the leaves. You wouldn't expect much to be found in the wood.

[John]

After being led to Linda Chalker-Scott's website and book from this website (thank you, thank you, thank you) I can say that I've altered some of my techniques and my garden has done great. Sometimes I feel like I need to just carry a copy of The Informed Gardener and both of Jeff Gillman's books with me everywhere I go so that I can dispute local gardening guru's and master gardeners advising garden newbies. As time goes on, new things are learned and it often flies in the face of what was conventional wisdom. Luckily most of the people that corner me for answers are the type that do want to know the science behind the action. Their eyes don't glaze over if the information takes a while to describe.

[Barry Prince]

I think we need to step back a bit on this matter and consider all the pros and cons of using wood mulch. For example, weed supression, how easy it is to obtain, reuse of a potential landfill item, etc. Also, I use hardwood mulches (note I did not say chips since I don't see too many hardwood chip mulches and prefer the shredded mulches) and often the mulch is hot in the bag since they are breaking down into humus (without nitrogen inputs leached from the soil) which suggest a more complex chemistry at work.

I don't doubt to some very small degree there is some nutrient exchange with the soil, but I believe that process is part of the larger soil management goal - living soil consuming additional organic matter.

Moreover, I feel if we focus too much on N-P_K debates rgarding garden soil, it is as counter-productive as discussing our diets relative to only a couple of vitamins (e.g. vitamin C & E) and iron supplements rather than the collective impact of all the foods we eat.

[joanne]

As a Master Gardener in Pennsylvania, I was also told by an extension educator that wood chips should not be used as mulch, but the caveat was that FRESH wood chips should not be used. Let them age for 6 months to a year. However, most of the (very quick - I am at work after all) research I just did seems to indicate that it is the decomposition process that might pull nitrogen, not the aging process. e.g.: "Microorganisms in the soil use nitrogen to break down the wood. Within a few months, the nitrogen is released and again becomes available to plants." (from http://cals.arizona.edu/yavapai/anr/hort/byg/archive/soilamendments.html) And the 4-5 references I glanced at didn't forbid the use, but did caution that you should keep an eye on the plants for nitrogen deficiencies (yellowing leaves) and fertilize if necessary.

I will definitely email Linda for a copy of her article - it certainly sounds like the most recent research (I have noticed it does take a while for the newest information to filter down to the MG volunteers).

[Gloria]

Maybe what is being attributed to nitrogen tie-up is actually not enough water. Enough water is needed to move nutrients into plant or plant will not be able to utilize.
Thick mulches, applied over newly planted areas where roots have not had time to expand throughout the soil, may be absorbing shallow watering and small rainfall amounts.
At tree keepers in Chicago we are advised to use woodchip mulch in a donut shape leaving space near trunk. The trees are watered well before mulching and watered when necessary for a year or two being sure water is getting under the mulch.
Studies at university have shown slower growth in mulched trees, where no additional watering is done, until the roots have penetrated the original planting hole. Maybe cornell?

[Steve A]

Lee Reich, soil scientist from New Paltz, New York, would concur with Chalker-Scott.

If the mulch is laid down on top of the soil (not dug into it) the nitrogen loss takes place only at the soil-mulch interface.

I don't mean to speak for Lee, but he has explained this to me on a couple of occasions. Whether he is right or not, I can't say. But he is a soil scientist and I'm not.

[Christopher C NC]

My twenty years of experience using fresh from the truck wood chip mulch is proof enough to me that the benefits are visibly observable in a short, couple of months, time frame and any negatives are hard to find. I was using tropical hardwood mulch. There are next to no pines or conifers in Hawaii at lower elevations, the source of my fresh, often steaming wood chip mulch piled onto beds at depths of four to six inches while minimizing depth around the crown of the plants.

The only thing that was a potential problem was a load of chips that had plenty of viable seed. Monkeypod at the wrong time of year or worse Haole Koa. As long as you pulled the sprouting seeds over the first few weeks before they could root through the chips into the soil it was fine.

I lived in the desert so everything was irrigated and as a matter of client comfort, light applications of granular fertilizer were used in landscapes once or twice a year. In Hawaii, the decomposition rate would have a six inch layer of mulch completly gone in one year. If there was any nitrogen deficiency it was not observable and was masked by the double rate of growth achieved using wood chip mulch versus not using it.

Since moving to NC and having bought, $200 for ten yards, and spread shredded hardwood mulch and getting a free, $30 tip to the driver, load of fresh wood chips, I have to say I prefer the wood chips much more. The uniform shredded mulch is far more apt to form a matted clump and act as a water barrier than the coarser chips. Linda Chalker-Scott did address this in her boon or bane article. Against her advice I have used fresh from the trimmers truck chips on shallow rooted annuals and vegetables with equally good results as on tougher shrubs and perennials.

In Hawaii tree trimmer chips were so highly valued they cost as much as $85 a load. Here they dump them on the side of the road or give them away for free.

[Ginny Stibolt]

Okay, let's get into the other side of this issue. Shredded wood mulches are often made from bald cypress trees and whole forests are being torn down to feed Americans' addiction to this mulch--dyed in many colors. This is not a sustainable practice. If you're purchasing mulch bagged or by the load, ask for wood that is sustainably produced. Here in Florida, millions of dollars are spent on removing invasive aliens species such as melaluca, Australian pine, or eucalyptus and you can often find these pest trees shredded for mulch. The production of bagged mulch, no matter what type, uses energy in the packaging and delivery.

For myself, I use the neighborhood arborists' chipped wood and feel naked garden-wise if I don't have a working load to use. As we reach the bottom of our current pile, I listen for the drone of tree chippers in the neighborhood. Just yesterday, we acquired a nice load from the local utility company's truck. This is one of the most sustainable mulching solutions because the tree trimmers save gas by dumping in the neighborhood, you save gas by not driving somewhere to get it or paying someone to deliver mulch, the landfills are not used, and you save money.

[Michele Owens]

Thank you so much, Susan, for adding the voice of a scientist to this debate and thank you Linda-Chalker Scott for being part of the conversation. I am always amazed at how little soil science there is behind a lot of gardening advice--such as be careful with wood chips because they rob the soil of nitrogen.

It was also extremely interesting to hear that Christopher C has successfully used wood mulch in a vegetable garden. I'm interested in this idea for weed control in mine. I am deliriously happy with the results I've gotten from my current mulch scheme: a layer of alpaca bedding (manure plus straw) close to the soil for the nutrients and then another layer of shredded fall leaves for weed control. However, I have apparently tragically created the perfect environment for ticks in the leaf mold.

Since three out of five members of my family got Lyme after working in the garden this spring, I need to try something else. My lawn guy, an old sage, says, "Put the wood chips you use on the paths on your vegetable beds."

Shocking advice, but maybe he's right.

[Ginny Stibolt]

I don't use woodchips in the vegetable gardens because it doesn't hang together and is not easy to strip off at the end of the growing season. I use pine straw, which I can easily rake it off the soil, and can prepare for the next crop. Later I redistribute the same pine straw back between the rows and the growing areas. Pine straw doesn't form a crusty layer as chips sometimes do so there is better moisture absorption into the soil.

I harvest the pine needles from the neighborhood roads. This tactic also keeps the storm drains clear and is better for the health of our lake not to have all that organic matter added to it.

[Donna Williamson]

I've come across a great remedy for ticks...my friend raises guinea fowl and they eat ticks. Watching them flock like sheep and fish is entertaining, you can enjoy their eggs and apparently they are delicious.

In addition, thanks to Linda Chalker-Scott for helping to put an end to bad gardening information being passed down in books and articles. Her research is first-rate and will really work to change the current climate of perpetuating poor and defeating practices in landscaping.

[Lisa Albert]

Minor correction to Kathy's original comments: It's not pine bark mulch that Oregon nurseries use, it's Douglas fir bark mulch. As Linda noted, we don't have an abundance of pines but Dougs? Oh, my, yes, we've got Dougs - it's our state tree and our primary lumber tree. I believe the Doug fir bark mulch is a byproduct from the lumber industry but I'll need to check for sure.

As an aside, the bark mulch is becoming scarcer as the lumber industry changes so costs are going up, which in turn drives up production costs, which result in higher prices for plants all along the line. It's been a topic of concern for our state's nursery industry and they've been working to find alternatives. I think a GRer posted about one of the alternatives in the past few months but danged if I can remember which one posted and what the post title was.

[Nancy]

For those of you who live in more temperate climates with active termite populations, beware of wood chip/mulches. Termites love 'em, and will happily move from your mulch to your buildings. If your garden is a good distance from your house, barn, shed, or fence, OK, but if not, better use pine straw.

[Linda Chalker-Scott]

I've not found evidence in the scientific literature about termites preferring wood chips. In the article Susan mentions, I state "Many wood-based mulches are not attractive to pest insects but are actually insect repellent. For instance, cedar (Thuja) species produce thujone, which repels clothes moths, cockroaches, termites, carpet beetles, Argentine ants, and odorous house ants. In general, termites prefer higher nutrient woody materials such as cardboard rather than wood chips."

Anecdotally, I certainly have seen no evidence of termites in wood chip mulches - and our entire home landscape is mulched with wood chips.

[greg draiss]

Fo well estblished trees and shrubs I see no problem with wood mulches. For shalow rooted plants, annuals, perennials and herb gardens I see a problem with nitrogen depletion.

I also see yet another inroad into an attack on garden mulches. First was the peat moss not to be used as a mulch. It never should be used as a mulch not for depleting peat bogs but beacuse it simply stinks as a mulch.

Now the tree hugging crowd says do not use wood mulch because it depletes the forest........
NO! wood mulch is a by product of the timber industry and the stuff would end up in landfills.

On the subject of landfills the biggest mistake in my opinion was the outlawing of grass clippings in landfills. Grass clippings provide a much needed source of nitrogen to break down all the carbon based products there.
And what is a landfill except a large compost pile?

The TROLL

[lepis]

It's not possible to the nitrogen to escalate upworth in the ground, so the mulch has no effect to the fertilization level that way at all. Only proplems that might come are with kalium during the first year or two, but they are not major ones. This applies if the mulch is used as a cover. It will effect to the nutrition level if blended with the soil. And that's where this belief comes from, early times when mulch was used as a fertilization at vegetable fiels.

[Karen]

This is a bit off the nitrogen/manganese topic, but my mom (garden sage) doesn't use fresh wood chip mulch for fear that it could come from diseased trees (maybe more likely to be chopped down?) and that the diseases could spread to her garden. Does anyone know if this is correct or not?

[Linda Chalker-Scott]

Karen, I also cover the disease issue in my review of landscape mulches and in the article Susan referenced from MasterGardener magazine. Short answer: it's probably not a problem if you have an existing healthy landscape and soil. Again, if this was a significant issue you would see entire forests die off when diseased trees fell.

[Christopher C NC]

Michele, yes indeed I used fresh wood chips as a mulch in my vegetable garden. I simply raked it away in six inch wide strips for seeding things and scooped out a small hole when planting starts. My post on that topic is here: http://outsideclyde.blogspot.com/2008/06/vegetable-gardens-and-woodchip-mulch.html

I did use granular ferilizer at one half to one quarter rate. I flung a little on things when the mood struck. No pesticides were used at all. The resident gardeners said it was the best vegetable garden in years and there was more than enough to supplement the table for three people.

Weeds were few and far between and mostly came up with the sown seeds. Now if it only kept the ravenous corn thieving stinking raccoon away, it would be perfect.

It is my intent to cover the garden for the winter with a fresh load of chips.

[Terry]

Just can't resist this one!

I simply can't bring myself to buy mulch when it's free for the taking from most municipalities and utilities. Heck, some municipalities will even deliver the stuff by the truckload right to your door!

I used nothing but double-milled City of Syracuse wood mulch (made mostly from brush set to the curb for collection each month) for almost twenty years in our lawnless landscape. While you need to pick out the occasional shredded plastic bag, car part, etc., it's FREE and does what a mulch is supposed to do - cool the soil, retain moisture, and return organic matter and (a little) nutrients to the soil as it decomposes.

For many years I've encouraged my clients to hire a trucking company to deliver similar mulch from our county's waste management authority for less than $10.00 per yard.

For more information, click on the following link:

http://tlehcs.com/Question%20of%20the%20Week/Archive%20Questions/Landscaping/municipal%20mulch.htm

[SJ]

Here's a point that has not been addressed in the mulch debate - the constant remulching of said beds in my opinion leads to depletetion of nutriets, rotting and general stunting of growth.

This in my experience is what I find detrimental. You cannot expect either perennials or trees/shrubs to thrive under a constant seasonal or every other year bombardment of mulch. It builds up which equals dead or stunted plants.

When I install or renovate an existing perennial/shrub garden 2 inches of a sustainable mulch (either the free mulch from the city, from a local arborist or purchased pine bark mulch) goes down for weed control and then that's pretty much it forever.

No shredded (which mats down and does not allow proper exchange of gases or water percolation) or cypress, cedar or dyed mulch is used. Think about it folks, they make rot resistent outdoor furniture out of this stuff so why would you want to put it down on your beds? It stunts growth and does not readily decay.

A properly thought garden should then knit together so that in a few seasons very little open ground is visible. Any top dressing can be done with either compost or leaf litter with no real additional need of mulch other then the occasional bag.

[chuck b.]

In San Francisco, all of Golden Gate Park, including the 55-acre San Francisco Botanical Garden, and I think every park in San Francisco County, is mulched exclusively with wood chips and has been for many, many years.

Everything is fine.

Lots of "weeds", but no diseases, no nitrogen deficiency, no manganese toxicity.

To avoid nitrogen depletion, don't incorporate unrotted mulch into the soil. This is very basic no-till gardening.

[Aunt Ida]

I'm not positive where the mulch in my neighborhood comes from, but I suspect it's from all the trees cut down to build mcmansions in Sun Valley and McCall. The wood chip mulch I put on my yard also contains chips of particle board and that worries me - not sure what might leach into the soil from glues, etc.

At any rate, I removed the thirsty grass from the entire front yard last summer and HAD to put something down to keep the top soil from blowing into the next county before the plants filled in.

This summer when my thermometer said 107 degrees and 13% humidity, the mulch kept the moisture in and the heat out better than if there had been bare soil, or worse, lava rock mulch.