Guess what - wood chip mulch is okay after all!
April 8th, 2008 
Susan Harris
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.
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April 8th, 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.
April 9th, 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.
April 10th, 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
April 10th, 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?
April 10th, 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!
April 13th, 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.
April 13th, 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?
April 13th, 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!
April 14th, 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.