Plant containers – does size really matter?

A few days ago I got a question from Cynthia about “potting up.”  For those of you for whom this is an unknown phrase (and no, it’s not a euphemism for a certain herbal activity), it refers to the practice of moving plants into ever larger containers.  She was wondering if there was any “real science” behind the practice – in other words, why not just start out with a larger container?

Hah! I needed no further encouragement and spent several days collecting and reading decades’ worth of research. And there is a LOT of research on this topic. As you might guess, it’s geared towards production nurseries and greenhouses.  But the good part is that it’s been done on just about any kind of plant material you could want.  Vegetables.  Annuals.  Perennials. Grasses.  Shrubs.  Native plants.  Ornamental, fruit and forestry trees.  Seeds, seedlings, cuttings, big plants, little plants.  Ahhhhh…data!

Almost without exception, you get better growth on plants grown in larger containers, whether you’re measuring height, number of leaves, leaf area, stem diameter, shoot and root dry and fresh weights, whole plant dry and fresh weight,…you get the idea.  This isn’t surprising, because with a larger root zone you can support more roots, which in turn support more above-ground growth.

The only parameters which tended to diverge for some species were flower and fruit production.  Restricted roots can stimulate sexual reproduction in plants, possibly because poor growing conditions spur the plant to reproduce before it dies.  Other drawbacks include increased probability of circling root systems, and higher ambient soil temperature, compared to plants in larger containers.

Smaller containers might be considered desirable when one is trying to limit above-ground growth – the “bonsai” effect.   And they require less water than larger containers – which brings us to the bottom line, as far as production nurseries are concerned.

Larger containers take more space.  And water.  In at least one study, water costs were shown to be “prohibitive for larger container sizes.”  Furthermore, smaller containers are preferred by production nurseries to “optimize production space.”  Another economics-based study found that “the smaller of these was the more economical.”

But most of you probably aren’t interested in the economics of plant production – you want to know what’s best for your own container plants, whether they are houseplants or pots of herbs or punches of annual color on your patio.  The science is clear:  it’s best to pot up plants in small containers quickly into their final destination, rather than making several (pointless) intermediate transplants.

Girdling roots: The source of all evil in the world?

One of the most widely discussed topics in arboriculture and tree care these days is the problem of girdling roots.  Virtually every conference or workshop on tree care has a speaker or speakers on how poor planting technique or poor nursery practices lead to girdling roots and their subsequent correlation with poor tree performance, tree failures, global warming, the soaring Federal budget deficit, and the batting average of the Seattle Mariners.  Before we go any further let me state categorically that I do not think girdling roots are a good thing; nor do I think any of the consequences mentioned are a laughing matter – especially the Mariners’ batting average.  I do think, however, that we often see a rush to judgment as soon as girdling roots are found on trees that are declining or have died.  I attribute at least part of this to the increased availability of air spades for excavating tree roots.  I have nothing against air spades; they are useful tools and a great way to non-destructively examine and treat roots and even move trees.  The problem is that often when people see a tree in decline they examine the roots, see a girdling root, conclude that was the problem and blame the person that planted the tree (unless they were the person that planted the tree, then they blame the nursery).


Example of oak tree with leaf scorch (Photo Phillip Kurzeja)

A recent study here at Michigan State presented by Phillip Kurzeja at the recent Arboriculture Society of Michigan ArborCon, points out the importance of looking beyond girdling roots in assessing tree problems.

The problem:  Oak trees at several locations on the MSU campus have been suffering severe leaf scorch.  In some cases virtually 100% of the leaves on the trees are affected and growth has been severely affected.  Examination of the trees by a pathologist ruled out bacterial leaf scorch, suggesting that the problem may be abiotic.  The researchers looked at a battery of variables including degree of leaf scorch, number of girdling roots, planting depth, soil compaction, foliar nutrition, leaf water potential, and leaf photosynthetic function.  Most importantly, they looked at these traits on trees without scorch as well as trees with scorch.

Evaluating girdling roots (Photo Phillip Kurzeja)

The results:  Trees with mild or severe scorch leaf scorch had girdling roots.  At this point one might have leapt to the conclusion that the girdling roots were responsible for the leaf scorch.  But girdling roots were also found in trees that did not have any leaf scorch.  In fact, in some cases the healthy trees had more severe girdling roots than trees with the worst leaf scorch.  So, what factors differed between trees with scorched and un-scorched leaves?  The researchers are still working on the analyses but the most obvious differences were that trees with leaf scorch were consistently planted deeper and had lower levels of foliar manganese than healthy trees.


The presentation I saw did not include data on soil pH or soil nutrient levels, so it’s impossible at this point to establish causal relationships among planting depth, foliar manganese, and leaf scorch.  But, for those who have to answer the ‘what’s wrong my tree?’questions, this study does point out the importance of keeping an open mind and looking at a variety of factors and not leaping on the first defect to appear.

Planting trouble: multiple trees in one hole

[I enjoyed Jeff’s Valentine story so much that I thought I’d stick to the theme of togetherness…for better or worse.]

A week or so ago a reader asked about the practice of planting three or four fruit trees in the same hole.  Having not heard of this before, I checked on the web and found many “how to” pages geared to home gardeners who either want a longer harvest of a particular fruit (early to late) or a mixture of different species.  Doesn’t it sound just great, especially for smaller urban yards?

One of these sites has these written instructions: “Plant each grouping of 3 or 4 trees in one hole at least 12 to 15 inches apart.”

Now, I’m sorry, but this is just asking for trouble down the road.  Readers of this blog know that root systems extend far past the drip line, and that roots from different trees are going to compete with one another.  You’ll end up with three unhappy trees, all jostling for space and resources, just like kids in the back seat during those long car rides.

But wait! you might say.  There’s research on high density tree planting, and it’s been shown to increase fruit yield on a per acre basis!

Yes, in fact there is a lot of planting density research on many different species of fruit trees.  What’s considered by researchers to be “high density” varies, but it rarely exceeds 2698 trees/acre (6666/ha for our international readers).  Optimal and sustainable levels of high density planting are also variable, as they depend on not only species but rootstock and the crown architecture; 1214/acre (3000/ha) might be a mid-range number.  This can be converted to a per-tree requirement of 36 sq. ft. or a 6’x6’ planting area.

How does this compare to the 12-15” recommendation given earlier?  If we’re generous and use the 15” recommendation, this translates to 6.25 sq. ft. per tree or 6970 trees/acre.  The 12” recommendation would lead to a whopping equivalent of 10,890 trees/acre.  (And no, it doesn’t matter if you’re using dwarfing rootstock or not; most of the higher densities in the literature are for dwarfing rootstocks.)

You don’t have to be a math whiz to see that these densities are totally out of line with reality.  Sure, you can probably keep overcrowded trees alive with lots of water and fertilizer, but they’ll be under enough chronic stress so that pests and disease might take hold, and fruit production will likely be poor.  And it’s about as far from a sustainable practice as you can get.

Big Trees for Crime Reduction

Like Linda, I believe that we don’t plant enough bare-root trees.  Trees that are harvested and sold bare root tend to establish better and recover faster from transplant shock than trees sold in containers or as B&B (balled and burlapped) stock.  But, in general, trees that are purchased in as bare-root stock are smaller than the other two styles, with B&B generally being the production method which yields the largest trees.

I disagree with Linda that, as a general rule, B&B stock should have its roots washed off prior to transplanting — I’ve done it and I’ve lost trees.  Most of the B&B trees that I know of where root washing has been successful have been small, relatively easily transplanted stock.  Once we have a few nice, big, long term studies that shows that B&B trees with their roots washed perform comparably to, or better than, normal B&B trees I’ll start to believe.  (I will note that, as a rule, it looks like B&B stock is dug and cared for much better here in Minnesota than Washington!)

I’m not going to go into the nuances of the arguments here — we’ve done it before if you want to check the archives.  But what I am going to point out is that a new study in Oregon has shown that bigger trees might help to deter crime.  Yet another reason for the people of this country to demand larger stock.

Despite what all of the research shows (that it’s better to plant smaller trees — preferably bare-root) people want big trees — they want an instant landscape.  They want it because to them it looks nice — and now its a way to protect your family too.

Historically this big stock comes B&B and is very expensive, cumbersome, and not the easiest things to successfully plant.  We need a new, cheaper way to grow large stock.  A number of researchers are working on different methods to produce large stock (special containers, bare root from a gravel bed) but nothing has worked out perfectly yet.  It’s going to be interesting to see how all of this shakes out in the future — especially with the loss of ash trees in the Midwest.

Newsflash: trees will die if their roots can’t establish

I’ve blogged before about the importance of getting tree roots in contact with the landscape soil during transplanting (you can find those posts here, here, here, here, and here). My advice to bareroot woody species upon installation is often ignored in favor of the quick-n-easy methods so often showcased on HGTV (“A complete landscape makeover in a weekend!”). And of course everything looks great…for a while. Let’s see what happens after a few years.

Below are photos of a pine tree, several of which were installed in 2007 at my children’s school (The Bush School in Seattle):


Not only is this pine tree planted too deeply (you can’t see the root flare, so it’s too deep), but the twine and burlap were not removed, leaving the roots encased in clay.  Furthermore, we’re not sure how great a root system this tree has since we can’t see it.  Even more horrific, the orange nylon twine is beginning to girdle the trunk.  What’s been planted is a big ball o’ trouble.

I sent these photos and my concerns to the administration and advised them to have the installers (low bid, of course) redo the planting before the one year warranty expired.  My advice was ignored, and here we are three years later:

This particular tree has declined to the point that the foliage is chlorotic and the uppermost needles are dead.  It’s symptomatic of a root system that has failed to establish, which is what I predicted would happen.  But it’s long past the warranty period, so if this tree is replaced the school will have to pay for it…again.  (Though it’s hard to see in this compressed photo, the pine next to this one also has top dieback, and I’ll continue to follow its decline.)

Many professionals, including some of my fellow GPs, disagree with the bare-root approach.  But based on this evidence, how could one argue that bare-rooting would not have been preferable to decline and death?

Dig it up and give it another chance!

Too often I’ve come across relatively young trees, shrubs, and vines that are surviving, but not thriving.  Every year they struggle gamely to put on a few new leaves, grow a few more inches, but something’s fishy and it’s not fertilizer.   Today I’m going to try to convince you to give these languishing woody species a second lease on life.

Long-time readers of this blog will remember some of the root horrors I’ve (literally) uncovered in containerized and balled-and-burlapped plants.  Poor root quality, improperly amended soil, roots swaddled in multiple layers of materials, and root crowns sunk far below grade are some of the most common reasons why roots fail to establish after transplanting.

Fall can be a great time to correct these problems.  For deciduous species, it’s best to wait until the leaves have fallen so that water needs are reduced.  You can find basic instructions on how to install and care for woody plants on my web page.

There are other reasons that plants might not establish, too.  You might remember my long-suffering Clematis, two which had been planted in an area with a perched water table. The lack of oxygen both retarded root growth and created an iron toxicity problem.  I dug them up and transplanted them into containers (during which I had even more fun with overmixing the soil with water and then allowing the undersides of the leaves to sunburn).  They were pretty sorry looking back in July – most of the leaves fell off after being burnt – but here they are just two months later:


So while you’re out putting your landscape to bed for the winter, take a close look for stragglers.  Give their roots another shot at survival – you’ll be glad you did.

Why bother having trees?

Sorry to be late with my post this week – I was away reviewing grant proposals.  It was interesting and useful work, but really drains your brain.  So with that being said, my post is long on pictures and short on words.

One of the things that bugs us GP types is poor plant placement.  Why bother planting a tree if you’re not going to allow it to grow naturally?  Here are some photos to mull over the weekend.  While I have lots of bad pruning pictures, these ones are chosen specifically because the trees were obviously poor choices for either site usage or size.

Because my sense of humor seems to have been left at the grant reviewing venue, I can’t think of amusing captions for these pictures.  But I’ll bet you can!  Just submit them in the comments sections, and I’ll repost the photos later next week with your contributions.

Photo #1

Photo #2

Photo #3

Photo #4

Photo #5

Restoration ecologists – you need us! Part 2.

Last month Linda posted on the need for horticultural knowledge for those trying to restore native habitats or at least establish native plants. There seems to be a pervasive notion that if we plant natives all we have to do is stick them in the ground and walk away. They’re native, right?  Don’t need irrigation; don’t need fertilizer; all that good jazz.   Well, often there is lot more to it than that.


A case in point.  Over the past couple of years I’ve been watching an unintended experiment near the State Capitol grounds in Olympia, WA.   The State opened up a vista so that the south end of Puget Sound and the Olympic Mountains were visible from the Capitol campus. It’s a lovely view.   As part of the development, a switchback trail was established on the steep hillside to connect the Capitol grounds with the park surrounding Capitol Lake below. A great idea.  Along the trial the hillside was planted with an array of native plants such as Oregon grape, salal, alder, and western redcedar.  Another fine idea.  Now comes the problem.  Near as I can tell, there was no plan for maintaining these native plants.  In fairly short order the hillside has become overrun with grasses, dandelions, and Himalayan blackberries – not exactly the desired effect.  And therein lies the rub.  Everyone is on board to plant natives but who’s on board for the hard work to maintain them.  Keeping weedy species from this planting by hand would take an army volunteers.  Burning is likely out due to the proximity of the Capitol and probably wouldn’t promote the desired species.  The answer?  Most likely a combination of hand-weeding and herbicides.  It is interesting that when the end justifies the means, herbicide is not such a dirty word anymore.  So there you go.   In order to effectively establish and maintain native plants, not only do we need to know about Mahonia aquifolium, Gaultheria shallon, and Alnus rubra; but it also helps to know about glyphosate, flumioxazin, and triclopyr.

Restoration ecologists – you need us!

Restoration ecology – the science of restoring degraded ecosystems – is another branch of applied plant sciences.  Oddly enough, very little plant science makes it into the scientific literature of this field.  This has driven me nuts for a number of years after reading an endless stream of papers where no mention is made of how plants are selected, installed, and managed.  Or worse, some ancient horticultural practices are used – like amending the backfill with organic material rather than using just the native soil.  Here’s what that will lead to:

Native soil discarded in favor of “lite-n-fluffy” amendment

The whole story

Restoration failures like this are often attributed to more esoteric causes, like lack of local plant gene pools in the plants used.  Believe me, even local populations aren’t going to survive poor installation techniques.

Thus, one of my recent graduate students conducted a meta-analysis of the applied restoration ecology literature to analyze it for horticultural content.  The results were not encouraging.  In Kathleen’s thesis abstract, she states:  “…careful selection and handling of planting stock, site and soil preparation, and rootball preparation, essential to increase survivorship of planted seedlings, are infrequently discussed in peer-reviewed restoration publications…Findings from this review support that restorationists either do not understand or are not providing important information to their peers, stakeholders, or the public on significant horticultural aspects of the restoration process.”

Now I know most of you are not restoration ecologists…but I’ll bet many of you are interested or actively involved in planting or maintaining native plant habitats, public greenspaces, degraded urban lots, etc.  The science behind gardening is just as applicable to these “wilder” areas as it is to home landscapes and gardens.

Failure of entire installation.  Note suckering from the roots – an attempt by the tree to establish a shorter crown.  (It’s easier to transport water to a short crown than a tall one, and suckers are often a symptom of root failure)

Five little lavenders – an update

Long-time readers of our blog might remember my August 12 column (linked here for your convenience).  You saw my giant lavender plant devolve into 5 small plants with tiny spiraled root systems that put Marge Simpson’s beehive to shame.  In any case, I promised to keep you up-to-date with their progress.

Transplanting in August is a risky proposition, especially when you prune out root defects.  Nevertheless, all but one of the five survived the summer and here they are earlier this spring:

The one that didn’t make it was in the upper left corner of this south-facing landscape, where there is a lot of reflected heat from the nearby bricks and concrete.  (You can see the empty spot where it was in this photo.)

If you haven’t tried corrective root pruning before, I’d encourage you to try it with an inexpensive shrub.  Your best bet will be older containerized plants in the “sale” section of your nursery.