For Mulch

Posted by Bert Cregg

Just a quick note up front that today’s post is a little data heavy, so if you’re still adjusting to this weekend’s time change; be advised.

A few weeks back Jim Urban wrote a post entitled ‘Against mulch’ on the Deep Root blog. The principle reasons he cited for his position were: 1) Mulch floats and can clog drains and releases “lots of phosphorus” as it breaks down, and 2) work by Gilman et al. that suggest that mulch does not reduce evapo-transpiration. We discussed the Gilman et al. paper ad nauseum here already so I’ll stick to the other points.

Most organic mulches float, it’s true. However, if mulch is repeatedly washing from a bed into a drain this suggests a problem with the design as much as anything. Second, I’m not sure what constitutes “lots of phosphorus”. Branch and stem tissue of hardwood trees is about 0.1% P. If we use just the bark as mulch, the P concentration is about 0.2 to 0.3%. Is that ‘lots of phosphorus”? I don’t know. I suppose if you put enough it down and allow it wash into a drain it could be.

So let’s stick to what we do know about landscape mulch. Linda has written the most comprehensive review of mulch out there and it demonstrates the benefits of mulch. Nevertheless I’d like to add some recent observations of my own to the discussion. These come from follow-up measurement on some studies that we have already published on shrubs and conifers. But I think our new data are important because they demonstrate the long-term benefits of much on tree and shrub growth.

2006 Conifer study. In 2006 we installed a trial to compare several different weed control strategies for newly planted conifers. Weed control, either by hand, Vis-pore mulch mats or 3” of coarse wood chips, dramatically increased tree survival.
swmrec mulch survival

After 8 growing seasons, trees that had the wood chip mulch or mulch mats had significantly greater caliper than trees that were not mulched.
swmrec mulch caliper

2004 shrub study. In another trial we compared the effect of various mulch types (wood chips, pine bark, hardwood bark) on growth of common landscape shrubs (golden globe arborvitae, Runyan yew, ‘Tardiva’ hydrangea, cranberrybush viburnum, and arrowwood viburnum). We re-measured heights of the shrubs study a couple of weeks ago (nine growing seasons after installation). To keep things simple here I’ve lumped the mulches together and simply compared mulched vs. un-mulched.

After nine years mulching increased height growth for all shrubs except the arborvitae.
mulch 2013 ht

Even more interesting is that the growth benefit of mulch extends beyond the establishment phase. If we start at age 4 and look at the relative growth rate for the past five years (i.e., growth increment for past 5 years / height at 4 years) we see that mulch continues to provide a growth advantage for all shrubs except the arbs.
mulch RGR

As I said at the outset, a little data heavy today but I think this is an import point. There is a lot of discussion these days about proper planting techniques but I think after-planting care often gets overlooked and mulching is an important part of that. That’s why I’m for much.

Powers of the Mind


A couple of days ago I read a journal article which seemed to show that certain individuals could, using some sort of mind powers, called biofield treatments, influence the growth of plants.  You can read the article here.

In case you were wondering what goes through my mind when I read something like this, let me tell you:  The first thing that enters my head are skeptical thoughts.  I try to get rid of these quickly though, because I believe that, as a scientist, it is my job to critically evaluate the science behind the paper without letting my own preconceived notions influence me.  It’s also important to remember when reading a paper like this, which challenges preconceived notions, that this paper has gone through a significant review process.  This process does not guarantee that the paper is perfect, but it does mean that some other scientists somewhere have concluded that the paper is worth something.

OK, so now you know what goes through my head.  Next question, after reading the paper am I convinced that powers of the mind can actually make plants grow bigger and have greater yields?  The simple answer is no.  There are a lot of things that are going on here that are just odd and which raise questions, and without answers to these questions I find it difficult to believe that everything is occurring exactly as indicated in the article.  Yes, something appears to be going on, but whether it is due to “biofield treatments” isn’t clear.  To begin with, I’d like to have soil tests showing the nutrient status of the soil prior to the experiments.  I’d also like to see a nutrient analysis of the foliage of the plants at the conclusion of the experiment.  It is odd to have added the nutrients that the researchers added to test plots and to see no effect – unless a biofield treatment was used.  It also seemed odd to me that plants wilted when there was drip irrigation there.   And it seemed odd that the fertilizers used weren’t described better.  There were other things I was interested in knowing too, but I won’t bore you.

Another thing I noticed is that one of the authors of the article is actually a member of the foundation which paid for the research to be conducted — and is, in fact, the corresponding author (in other words, the author who you should contact should you have any questions).  This isn’t “against the law” or anything, but it is odd.

As a scientist it is my responsibility to acknowledge the possibility that these biofield treatments had some effect on plant growth, but to actually convince me that they did you need to write an article that is rock solid with no opportunity to say “But what about….”.  Right now this paper just doesn’t do that.  Too many odd things going on.

Are natives the answer? Revisited

I started to leave a comment on Linda’s Friday post regarding Seattle Public Utilities proposed building codes regarding “Healthy Landscapes” but decided I’d weigh in with a regular post.  Linda honed in on the 75% native requirement but there are lots of things to make one scratch their heads in the proposed codes.

Existing invasive plant species shall be removed and no invasive species planted.
No mention of how invasive plants shall be removed.  Heavy-duty herbicides? Armies of school children forced into slave labor? Slow-moving ground-fire? Goats?

75% of all new plantings will be native to Western Washington.
So where did 75% come from?  Sounds like a number that was pulled out of the air.  How is 75% defined?  75% of plants? 75% of the area?  And how does this foster “Healthy Landscapes”?  If I have a 2 acre landscape and plant an acre and half of salal or Oregon grape I’ve met the requirement of 75% but have I increased species diversity or structural diversity or contributed to a “Healthy Landscape”?

A vegetation plan must be submitted for review.
By whom?  What happens if they (whoever ‘they’ are) don’t like it?

Existing native plant species shall be protected whenever possible.
Sounds reasonable but what about existing non-invasive non-natives?  Could a homeowner be required to cut down a 40-year-old red maple?

And on and on we could go.  Let me state clearly, I’m not against native plants.  Quite the opposite – I grew up in western Washington and have a passion for PNW plants since my high school days.  Since moving to Michigan I’ve written articles and given talks promoting natives here as well.

Nonetheless, I think many in the native plant movement hurt their cause by parroting the same old lines without ever critically thinking about what they’re saying.  Repeating a lie often enough times does not make it the truth.

Let’s critically look at some of the reasons for planting natives according to the Washington State Native Plant Society:

Native plants are adapted to our climate of wet winters and dry summers.
True. But so are lots of non-natives.  Adaptedness is a function of the environment in which plants have evolved; whether it’s native or exotic.  There are many climates around the world that are similar to the PNW and can produce similarly adapted plants.

Require less water than most non-natives once they are established.
Once again, adaptations such as drought tolerance are a function of the climate under which plants evolved.  There are many exotic species that are more drought hardy than western Washington natives and likely to use less water.

Resist native pests and diseases better.
Sometimes. But unfortunately the days of worrying only about native pests are in the distant past.  Exotic pests are here and they are here to stay.  Dutch elm disease, white pine blister rust, emerald ash borer, chestnut blight, Japanese beetle, the list of exotic pests is long and getting longer.  Native does not mean pest-free.

Improve water quality by needing less fertilizer and no pesticides.
OK, here’s where I get confused.  The reasoning in Doug Tallamy’s book, Bringing Nature Home, is that native insects don’t feed on exotic plants, therefore if we plant exotics, native food pyramids will collapse and it will be the end of life as we know it.  So… if native insects won’t feed on exotic plants, why would exotics require more pesticide use?

Save resources and encourage a sense of Stewardship.
Ok, now maybe we’re getting somewhere.  Not sure why stewardship is capitalized here but if they mean a ‘sense of place’ or a ‘connection to the natural environment’ then I can buy it.  Many native activists, including Tallamy, run away from this argument – apparently it doesn’t sound scientific enough – but it’s one of the best we have.  Washington state has some of the most incredible plants anywhere.  They should be celebrated and promoted and planted.  In my mind, the biggest reason for planting natives – along with carefully selected non-natives – is to increase overall biodiversity.  When I mention biodiversity I am speaking broadly; species diversity, structural diversity, age-class diversity, and landscape diversity.  When we look to the future we have no idea what lies ahead. We don’t know what new, exotic diseases or insects are looming on the horizon. Most of us expect climate will change but no one can say with certainty how.  Plants cannot evolve as fast as climate will change or as fast as new pest will be introduced. The only way to deal with this uncertainly is to spread the risk through diversity – this includes natives, exotics, and even interspecific hybrids.

The Wrong Message

Every once in awhile I’ll see a new garden product that really speaks to me.  Something that promises spectacular results on some garden problem that I’ve had to deal with before and attacks it in a novel way.  Then I’ll read the advertising materials for the product and be let down before even trying it.  Such is the case for a new product called Liquid Ladybug (which, by the way, is one of the niftiest product names that I’ve ever seen — so there’s a win for the company!).

According to the manufacturer Liquid Ladybug is a spray-on product which kills spidermites, evaporates quickly from the plant, and which has organic plant oils as its main active ingredient.

So far so good — and even believable.  Plant oils can kill spider mites.  Of course simply wiping the plant with a cotton swab soaked in isopropyl alcohol can do that too — or you could easily make up a soapy spray to spray on the plant which can do the same thing.  Still, the claims don’t seem too bad so far.

Here’s the part that I have a problem with — you can, and are all but encouraged to, spray this stuff with no protection (like gloves).  See the website here .  Is this a bright thing to advertise?  Many plant oils don’t agree with eyes, mucous membranes, or beneficial insects, and let’s not even get started with allergies!  In my opinion this is reckless, foolish advertising.  Pesticides, organic or not, need to be respected.  Without that respect we inadvertantly put ourselves into bad situations. Another problem with this products is that it is likely to kill any predatory mites or other soft bodied beneficial insects just as readily as it kills bad mites.

And check out the price of this stuff!

My advice, skip this product and use insecticidal soap, or, if you’re anxious to try something new, try a beneficial insect such as the big eyed bug or minute pirate bug.

It’s That Time of Year

This time of year is tough for folks who do work with plants.  It’s the happiest time of year because the world is turning green again, but it’s also the busiest time of year because we need to be inside teaching, outside planting, and also on the road since spring talks are finishing up.  Honestly I’m having trouble finding an hour to myself to work on writing up papers and articles.

That said, let me leave you with something that’s been bugging me the last few days (based on an article I read a few years ago).  If a person were in inhale a small seed, would it grow into their lungs?

What do you think?  Real or BS?

What can CO2 do for you?!

Well, it looks like the climate change skeptics are starting to hedge their bets.  Global climate is not changing.  But if it does change, it’ll change for the better.  At least that’s the gist of a book by the Center for the Study of Carbon Dioxide and Global Change entitled The Many Benefits of Atmospheric CO2 Enrichment”.  The book documents 55 different ways that increasing global CO2 will benefit the world.  Most of this is built on studies documenting increases in plant growth and/or photosynthesis associated with increasing CO2.  If you’re interested you can look at a preview of the book at:

While CO2 enrichment can benefit plants and trees in the short term, it’s less clear how they will respond over the long term.  For example, nutrition or water may soon become limiting such that the full CO2 ‘fertilization’ effect is never realized.  Also, it’s likely that certain plants will benefit more from increased CO2 than others: Will exotic invasives gain an additional advantage over natives?  And, of course, if rising CO2 results in increased global temps (which the Center denies) then all bets are off.


You can learn more about the Center for the Study of Carbon Dioxide and Global Change at their website  There is a tab on the homepage where you can donate to support their cause.  Why not?  Exxon/Mobil already has.

Building a House of Straw — With Brix!

Before I get into the meat of this issue I wanted to mention that, a couple of weeks ago, Purdue came out with a statement which basically supported a post that I had previously written – one where I stated that there’s not a lot of good evidence that using Roundup leads to sudden death syndrome in soybeans. Yeah me! Well, not really – that’s not the way science works. While it is nice that other scientists agree with my conclusion, that doesn’t mean that Round-up isn’t affecting soybeans – it just means that there’s no evidence of it right now. Science doesn’t stop because a few scientists agree.

Same with compost tea – Linda and I have both posted about compost tea and we agree that, at this point, there’s no reason to use it. But science doesn’t stop, and it wouldn’t be completely surprising if, someday, someone comes up with a compost tea type product which is actually reliably useful.

Now on to the flavor of the day! Brix. So, what is brix you ask? Brix is a measurement of solids in a water (usually these solids are sugars). It’s easy to test brix by using something called a refractometer which measure how light bends when it passes through a thin film of water. We use brix in foods to tell how sweet they are. We just take a little sample of sap or juice, put it on a refractometer and bang, we know about how much sugar we have (actually, as I mentioned earlier, any dissolved solid, not just sugar, can alter measurements, as can non-solids, like alcohol. But the dominant thing affecting the brix measurement is usually sugar content).

Though it isn’t exactly mainstream, there is something out there called brix based gardening. Basically, the goal of brix based gardening is to increase the brix of the food we eat. Increasing the brix means increasing the sugar. The theory is that the higher the brix of a food the better it is – in terms of taste, resistance to insects, resistance to disease, healthiness.  The list goes on and on.

The biggest problem with arguing that gardening based on brix is a bad idea is that there is a nugget of truth in brix based gardening. Moreso than compost tea (in my opinion), brix has proven itself useful in certain situations.  Particularly in wine making, brix is used to quantify the sugar content of your grapes so that you can predict the sweetness of the wine you will produce (it will also help tell what level of alcohol you’ll get). As time goes on at the end of the season grapes increase their sugar content, so grapes are picked according to when the brix is right for the wine you want to produce.  Another grain of truth is that with higher brix you’ll get less insect pressure. This stands to reason, at least to some extent, because insects are usually looking for nitrogen rather than sugar.

But along with the little grain of truth comes some BS. For example, the idea that eating a food with a higher brix reading means that you’re eating a healthier food is just silly. It just means that you’re eating a sweeter food. The idea that higher sugar levels mean a healthier plant is also silly. In fact, one of the most significant things which can make brix go up is putting the plant under drought stress. Under drought stress, with less water, the concentration of sugars in sap naturally increases (because there’s less water to dilute it). Raisins are sweeter than grapes! Furthermore, the variety of the plant which you grow has an extreme effect on brix. Chardonnay grapes may have a low brix (around 21), a late harvest Riesling may have a brix of 42!

I have seen an inordinate amount of gobbledygook about mixing different fertilizers to get the perfect ratio of nutrients to increase brix. First, it’s important to realize that fertilizers can alter sugar content. For example, fertilizing heavily with nitrogen will increase growth of the plant but will usually decrease sugar concentration (hence brix). Indeed, from what I’ve seen, nitrogen seems to be the biggest player in sugar content.

That said, calcium and phosphorus based fertilizers seem to be the favorites among brix based gardeners – but from what I can find research hasn’t actually shown that these fertilizers increase brix on anything approaching a reliable basis. Another common recommendation is a molasses based fertilizer – once again, research on molasses doesn’t seem to show that it can do much to increase brix. Honestly, it looks to me like those recommending high brix as necessarily a good thing for us and our plants and then offering methods to do it are putting a scientific veneer on witchcraft – at least until further research comes along. I am sure that those favoring brix-based gardening will disagree with me – if you do and you read this I would welcome seeing some published papers which support your claims.

Almost But Not Quite

Today I was reading an interesting gardening website with a wide variety of advice, some good and some not so good.  As I analyzed the website in my mind to figure out why some stuff was good and some stuff was bad it occurred to me that the problem was that a lot of the stuff that the author was recommending was based on testimonials.  And then it occurred to me that many of you out there might not know what a testimonial is and why recommendations based on testimonials shouldn’t be recommended — and then viola! I had a blog post.

A testimonial is testimony presented by one person about their experiences with something.  It’s like when a friend tells you that dryer lint controls slugs in a flowerbed. Don’t laugh, this is a real suggestion on one website!  This person decided that it was appropriate to put dryer lint around their garden, and when they did, slug damage appeared to be reduced.  Good for them.  But is it good for you?  The answer is maybe.

The problem is that this person is missing the two things that we need when assessing whether a particular thing works.  First, we need a a control, and second, we need replication.  Let’s use the dryer lint example.
We have no idea whether, if we hadn’t put out the dryer lint, the slug population might not have dwindled anyway.  To find out whether it might have we need to treat only a portion of our garden with dryer lint and then see if the treated portion has more or less damage than the untreated portion after a few weeks.  The untreated part of the garden is called a control and it is necessary for a good experiment.  But it isn’t the only thing necessary for a good experiment, so is replication.

It is possible that the part of the garden which was treated with the lint had less slug damage than the control portion of the garden for some reason besides the lint — for example, perhaps the area where the lint was applied happened to be further from the sprinker than the other section — and slugs like it moist.   So to combat possible problems like this you need to conduct your experiment more than once.  In other words, you need to replicate your trials.  This might be done by doing this experiment over multiple years, by having other people in your gardening club try it too, or by dividing up sections in your garden into six or so equally sized sections so that three randomly selected sections get treated with lint and three don’t.  Or, best of all, do all of these things — multiple years, multiple gardens, and multiple plots within a garden!  If we did more testing like that I have a funny feeling that we’d have fewer crappy products for sale.

Testimonials are interesting, but don’t get fooled into thinking that they prove anything.  They don’t.  You need control and replication to demonstate that something really works.

Permaculture – my final thoughts

We’ve had some good, vigorous discussion about permaculture, specifically around the book Gaia’s Garden.  I’ve pointed out some problems with the author’s understanding of relevant plant and soil sciences and will wrap up this week with a look at the glossary and bibliography.


The glossary contains a number of scientific-sounding words and phrases with unscientific definitions; for example:

“Buffer plants: Plants placed between guilds or between allelopathic species. They should be compatible with the trees in each guild and should have a positive effect on one or both of the guilds to be linked.” (“Buffer plants” is a phrase legitimately used in ecological restoration where plantings separate wetlands or other natural areas from human activity.)

“Guild: A harmoniously interwoven group of plants and animals, often centered around one major species, that benefits humans while creating habitat.” (The term “guild” is ecological and refers to groups of species that exploit the same types of resources.  It has been hijacked and redefined for permaculture.)

“Narcissistic: Plants that thrive on the leaf litter of members of their own family, such as the Solanaceae, or nightshade family.” (In this case, this is an unscientific term given a scientific-sounding – but nonsensical – definition.)

“Polycultures: Dynamic, self-organizing plant communities composed of several to many species.” (Polyculture is an agricultural term referring to the planting of multiple crops. It’s a cultural strategy in Integrated Pest Management.)

“Sectors: Areas where outside energies such as wind, sun, fire and so forth enter a site. These energies can be mitigated, captured, or otherwise influenced by placement of elements in the design.”


There are only two books I would consider scientific; one soils textbook from 1996 and the other is Odum’s classic text Fundamentals of Ecology (1971). I’m disappointed in how scarce and dated these references are, given the wealth of more recent articles and books that are both relevant to urban gardens and scientifically sound.

The bibliography also includes many books on design and I’m not including them in this critique. Of those that remain, the bulk are nonscientific and in many cases pseudoscientific. Examples of the latter include The Albrecht Papers (Albrecht, 1996), Weeds and What They Tell (Pfeiffer, 1981).

And this last criticism embodies what permeates much of Gaia’s Garden: pseudoscience. In the glossary, we see scientific-sounding terms or definitions that are ultimately meaningless or incorrect. Furthermore, we see scientifically legitimate terms such as guild used incorrectly. Both of these practices are characteristics of a pseudoscience.

I think this is unfortunate. I’ve mentioned before that I agree with much of the philosophy behind permaculture. But dressing up this philosophy as science both misleads nonexperts and alienates scientists.

So here’s a challenge – why not write a new book on permaculture and collaborate with a scientist? (I know a few who are writers!)