Susan Harris
All about gardening the eco-friendly way, by Susan Harris and 22 other garden writers and experts.

Guess what – wood chip mulch is okay after all!

April 8, 2008 · 29 comments

 Sometimes it’s damn hard to keep up with the current best thinking on gardening practices, and the question of whether wood chip mulch is good or bad for our plants is a case in point.   After what thought was a lot of research, I came down against it in my page about mulch and mulching, recommending instead the use of shredded pine or leafmold mulch.  (Around plants, that is.  Wood chips on paths are indisputably okay.)

Now here comes Master Gardener Magazine with an article about wood chip mulch by Linda Chalker-Scott (Ph.D., Extension Urban Horticulturist and Associate Professor, Washington State University) to contradict conventional wisdom.  (Those durn scientists can be SO annoying, with their namby-pamby reliance on the scientific method and peer review and all that stuff for sticklers.) 

Here’s what she has to say:  In tests (something that apparently scientists are still doing) wood chips perform with the best of all possible mulch materials for moisture retention, temperature moderation, weed control,sustainability and enhanced plant productivity.  And what’s more, in urban areas they’re often FREE.

Drawbacks?  What drawbacks?

Referring to the reported drawbacks of wood chip mulch, she calls them "much ado about  nothing".  The concern that wood chip mulches can tie up introgen and cause deficiencies in plants, it turns out that studies show that it actually increases nutrient levels in soils and the foliage of plants.  "My hypothesis is that a zone of nitrogen deficiency exists at the mulch/soil interface, inhibiting weed seed germination while having no influence upon established plant roots below the soil surface."  For that reason, she recommends against wood chip mulch around plants with shallow roots – annuals and vegetables.

Even if you remain unconvinced by the research, you can still use wood chips on top of a more nutrient-rich underlayer (say, of compost).  This "mulch sandwich" approach mimics what you’d see in the mulch layer of a forest.

How deep?

Here’s what surprised me – her recommendation that 4-6 inches of the stuff be used.  That’s because "A review of the research on coarse organic mulches and weed control reveals that shallow mulch layers will promote weed growth and/or require additional weed control measures."  Again, a divergence from lots of other writers recommending 2-4 inches, or even a maximum of 2 inches. 

But what about soil structure?

Now here’s something the article didn’t addres – the impact of mulch on ability of the soil to hold moisture and other benefits of what we call good soil structure.  Do wood chips decompose fast enough to improve soil within, say, a year?  I’ll shoot the link to this post to the good folks at Master Gardener Magazine and see if we can get a response. 

Photo credit.

{ 29 comments }

1 Christopher C NC April 8, 2008 at 10:57 pm

Susan, I have been using wood chip mulch fresh from the trimmers truck for twenty years with only positive results, even on annuals and veges. I strive for a minimum of four inches of thickness. In Hawaii where temperatures were just right for decomposition 24/7, a layer of mulch that thick would be gone in one year. I will see how long that takes in NC. You can see my recent post on my load of chips here:

http://outsideclyde.blogspot.com/2008/04/grand-entry.html

The chips vastly increased the soils ability to hold water in my desert climate there and improved the tilth of the soil over time. Really they built a soil over time.

I think the main difference here will be a matter of temperature and the rate of decomposition. The eventual results are going to be the same.

The only drawback was if you got a load of chips filled with viable seeds. What you get is dependent on what the arborist is trimming. If they trim a tree when it is setting seed you get those too. It just meant for the first few weeks you had to weed out the sprouting seeds, which in a thick layer of mulch is a piece of cake. Let them go too long and it could be a problem.

It is also preferable to get a load that is more wood than leaf because it will last longer as a mulch.

2 lkwagnersc April 9, 2008 at 9:28 pm

We’ve been using ‘pruning’ results that are free for the asking from the local tree services for some time in our home garden, in mostly large mulched areas that we’ve converted from lawn to woodland and shrub borders. So I’m not surprised to learn that it’s a good thing. I never could figure out the nitrogen problems, since it seemed like it would gradually decompose. Not putting fresh wood mulch on vegetables makes sense, but why not use it under trees, etc. I thought Christopher’s comment were right on target.

Actually, we’ve had more issues with weed seeds coming in with leaf mulch from the city trucks. Leaf mulch is gold, certainly, but with bermuda grass seed or winter annual seeds, ugh!

Thanks for providing the latest research on this subject.

3 Gail April 10, 2008 at 7:13 am

Susan,

I did read with interest your earlier post on mulch. I have always used what the trade calls soil conditioner, very finely ground up pine bark mulch not the sand in some bad ‘soil’ from the big box stores! It has the benefit in my opinion of being a lot more attractive than big hunks of lumpy mulch and it doesn’t over whelm small plants. The downside, it is twice the price of hardwood mulch. I would the hardwood bark on my paths.

Gail

4 Layanee April 10, 2008 at 10:01 am

Wow, 4″-6″ of mulch? If you have that much mulch around your plants then you have not got enough plants! Mulch should not be a design feature! I know, I’m ranting but let’s not give substance to those mulch volcanos! I’m still sputtering…four to six inches? Okay, in Hawaii but in a garden?

5 DebbieTT April 10, 2008 at 7:21 pm

Mulch for those areas where you are waiting for the plant material to grow enough to cover the soil would be one reason for that much mulch. I may just well be a mulch volcano using it to keep bare earth protected while plants are dormant, and especially for new beds. Mulch that eventually breaks down and adds humus to the soil, I say pile it on!

6 Barbara Pleasant April 13, 2008 at 6:04 am

One of the most interesting things I’ve learned from Daryl Pulis (now retired from GA extension) is the value of amending clay soil with chunky stuff like slow-rotting wood chips.

On another note, the folks researching organic control of urban kudzu in Spartanburg, SC, (http://www.kokudzu.com) like the performance of deep, deep wood chips on slopes. A big enough mound of wood chips will smother the stuff.

7 Benjamin April 13, 2008 at 3:01 pm

Ok. Confused. What about clay soil? You don’t need that much mulch to retain mositure in clay, and, Barbara P, is a LOT of mulhc good for conditioning clay? And don’t we need less mulch in shady areas and more in sunny?

8 bev April 13, 2008 at 3:05 pm

The observation about the mulch creating an interface which suppresses weed seeds interests me, because I have observed that very phenomenon this spring, in a perennial bed that I never have time to weed. Where I put a fairly thin layer of the wood chip mulch last year (free from a friendly arborist), that d___d chickweed is absent, but where the mulch is not present near the edges of the garden, the chickweed flourishes. (I don’t think it’s thick enough to just be smothering it in the mulched areas) Perhaps there is something to this!

9 Kathy, Washington Gardener April 14, 2008 at 9:48 am

If your goal is just yo block weeds – then ,yes, this thik layer of bark mulch will work fine. But what of the LONG TERM effects of hardwood bark mulch on your soil and root health?
Also along with Benjamin’s post I think this is NOT the answer for our local DC-area clay soils (or even those sandy shore soils). I know I’m not the only one to pull back several inches of bark mulch that was wet only down to 2-3 inches and see drought conditions below — check your local office park for these disaster conditions.

10 Chuck,Biggar Saskatchewan June 22, 2008 at 8:45 am

Zone 3 up here and tough to grow even a weed, I removed my 3 year old wood chips from between my raised garden beds , and used them to raise my hosta borders. The result is fantastic, just a fluke but think I tripped onto something, dark rotting chips prevent weed growth and my hostas grew to epic proportions very quickly!

11 John Mannion October 23, 2008 at 11:42 am

I am wondering if Sassafras wood chips tilled into the soil would be good for it. I have many sassafras trees and have made a lot of mulch from it. I am tentative about tilling it into the soil and the effects it could have on my plants and grasses in the fields.

12 bob February 8, 2009 at 4:05 am

Wood chips are the best. Check out the white mychorridia fungus that grows in it! I’m surprised research hasn’t mentioned that fact. ANother thing, mulch after a rain storm to seal in the moisture. Puting mulch on dry ground .4-6 inches high in the middle of summer may not do the job. Water the ground first then water the hell out of the mulch. That wull work if you get no rain. I’ve been using woodchips for years and have noticed dramatic improvements since before I used them.

13 Carolyn March 22, 2009 at 5:01 pm

Has no one noticed the phenomenon of shredded bark mulch forming a thick, hard layer similar to composition board? I have noticed it in many gardens and it is not only bonded like cement or composition board, but it is loaded with various colors of fungus. The soil underneath is dry because the water cannot seep through the thick, congealed material. People just add more bark mulch on top of the past years’ layers of bark much. I have read that bark mulch is acidic because it is made from conifers such as balsam, fir, etc. It seems that adding an acidic product to the soil is not good for plants that prefer an alkaline soil. In regions with four seasons, the new layer of bark mulch, if it is still loose, gets picked up when leaves are raked or blown off if a leaf blower is used. So, it seems tobe a waste of money. However, applying compost early in the growing season seems to make more sense since the compost will work into the soil through a natural symbiotic process. I have also read that bark mulch is sometimes made from diseased trees. That could bring disease to one’s garden, no? And for the environmentalist’s point of view, isn’t the industry cutting down forests just to make bark mulch? Do we know if the trees are coming from tree farms and not from the Rain Forest, for instance, where the environment is being threatened?

14 Carolyn March 22, 2009 at 5:15 pm

Do I have to join to submit a comment here?

15 Sharon April 4, 2009 at 4:16 pm

Does pine bark mulch that has not been cured for 1 year before using it present any danger to trees or schrubs it surrounds?

16 Sharon April 4, 2009 at 4:20 pm

Is there any danger to trees or schrubs by putting freshly cut pine mulch around them as long as it is not leaning against the tree or schrub?

17 Nathan April 19, 2009 at 10:16 am

I’m digging out new veggie beds out of clay and gravel. I’ve mixed as much compost and black soil I can find but feel I need something more substantial to condition it with. I have access to some really old rotted pines and was hoping to use it both as a conditioner and as a mulch. What do you think about the acidity problems?

18 Giselle April 27, 2009 at 12:33 pm

We have just bought a chipper and will be chipping eucalyptus plus various other trees, bushes from a very untended garden. I plan on using the newly chipped wood for flower and veggies bed. Any thoughts – from the previous posts it seems like a good idea. I am in Topanga, Ca. Hot and dry May – Oct.

19 Giselle April 27, 2009 at 12:34 pm

We have just bought a chipper and will be chipping eucalyptus plus various other trees, bushes from a very untended garden. I plan on using the newly chipped wood for flower and veggies bed. I am in Topanga, Ca. Hot and dry May – Oct.

20 Jean May 1, 2009 at 8:19 pm

I just potted a buch of starwberries and I used all compost. I then took it out and put a bunch of pine chips mixed all in the compost thinking that this would prevent the plant from not getting enough oxygen and burn. Is is Ok to put the pine bark on the bottom of the pot and all mixed in or will it hurt my plants?

21 Chris May 2, 2009 at 9:14 pm

I have clay soil and was wanting to know if I added wood chips to it would it help improve the soil an help it drain better, it is in a large field. do I plow it in or let it works iis self in ?

22 Jean May 3, 2009 at 9:47 am

Hello Chris, It sounds as if it is Ok to use as a mulch. I would not plow it in the soil. You need some worm castings, rock dust, and compost first and then put some wood chips on top.

However, I am not sure if I can use pine bark in potted plants as I have done. I mixed it all in and used a ton of it. If anyone knows about this I would love to hear back from someone.

23 ~~Rhonda June 30, 2009 at 11:58 am

Interesting post. I was googling as that is all we use and, in the past, I’ve read about the “nitrogen problem.” Was hoping to find some definite answers. The chips we use are from a tree trimming friend and include all sorts of trees, except walnut. Our current huge pile is a mix of pine and oak. We’ve used wood chips for years. We have 350+ daylilies in our garden, as well as many other perennials, shrubs, etc. Have never noticed any problems. Our garden grows like a weed, as they say…of course, the weeds grow like that too, if we don’t put down the mulch every year! It breaks down quickly. Our soil is soft, black and lovely. Not sure that contributes anything to the conversation, but just chiming in with our personal experience. We are in IL, zone 6b. Happy gardening! ~~Rhonda

24 willie July 2, 2009 at 2:52 pm

PLEASE HELP

i FOLLOWED THE ADVISE FROM THIS WEB SITE AND THOUGHT WOOD CHIPS WOULD BE GREAT FOR WEED CONTROL AND SOIL ERROSION, ON THE UPPER END OF MY PASTURE, i HAVE NOW RECIVED A LETTER FROM THE COUNTY SAYING THAT I WILL HAVE TO REMOVE THE WOOD CHIPS DUE TO THE EXTREME FIRE HAZZARD, I DON’T RECALL ANY OF THE COMMENTS LISTED ABOVE, SAYING ANYTHING ABOUT FIRE HAZARDS.

PLEASE I WOULD LIKE TO HEAR FROM ANYONE WITH INFROMATION I COULD SHOW THE COUNTY AGENT TO CONVINCE HIM THERE IS NO FIRE HAZZARD.

THANK YOU

25 Diane McClure August 30, 2009 at 8:03 am

Susan,
I teach pre-school and our playground has wood mulch in the areas not covered by sidewalk. Some of the pieces are two inches long and will penetrate the soles of my shoes. There seems to be a difference in opinion among the teachers whether it is safe for our 3 and 4 year olds to go barefoot in the mulch? Is the wood mulch treated with chemicals? It has various hues which I assume is the variety of woods used. Your anwer would be appreciated. Thank you, Diane McClure

26 William Lake November 20, 2009 at 11:29 am

I would like to know if I can use fresh (new) wood chips as a mulch.  I want to put it under my hydrangers and other shrubs.  They are from a tree just cut down and put into chips.  Thanks for your help.

27 Ryan January 24, 2010 at 11:44 pm

A quick question about wood chips. Two years ago I planted 3000 new trees as shelterbelts around our new yard. For these I used plastic mulch, wich you lay down after you plant the trees to aid in weed control and moisture preservation. I found that the plastic worked ok for weeds but not so good for moisture, it seemed to shed the moisture ( wich i know makes sence because its plastic) to much and we haven't had enough snow in the last winters to replenish it. I have another order for another 4000 trees this year and i'm wondering if wood chips are the answer to keep the moisture in and the weeds out. I live in zone three in central alberta.

28 dana mareshie April 27, 2010 at 10:37 pm

I had a 80+ year old dead white oak cut down and the trunk ground out. There is a significant amount of the ground out trunk mulch left over and I want to know if it is ok to use as mulch? It is ground quite fine but there is enough of it to cover a bed that I have. Also, should there be a concern with termites with untreated wood?

29 Tommy Graham May 18, 2010 at 7:16 am

I just had about 8 trees taken down in my yard. I got the stumps grinded down and have a ton of wood chips to get rid of. I was wondering if it would be ok to put them under my pool deck and all around the pool? A friend of mine said it would attract alot of bugs and ants.

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