Advancing the science of gardening and other stuff since 2009
Author: Linda Chalker-Scott
Dr. Linda Chalker-Scott has a Ph.D. in Horticulture from Oregon State University and is an ISA certified arborist and an ASCA consulting arborist. She is WSU’s Extension Urban Horticulturist and a Professor in the Department of Horticulture, and holds two affiliate associate professor positions at University of Washington. She conducts research in applied plant and soil sciences, publishing the results in scientific articles and university Extension fact sheets.
Linda also is the award-winning author of five books: the horticultural myth-busting The Informed Gardener (2008) and The Informed Gardener Blooms Again (2010) from the University of Washington Press and Sustainable Landscapes and Gardens: Good Science – Practical Application (2009) from GFG Publishing, Inc., and How Plants Work: The Science Behind the Amazing Things Plants Do from Timber Press (2015). Her latest effort is an update of Art Kruckeberg’s Gardening with Native Plants of the Pacific Northwest from UW Press (2019).
In 2018 Linda was featured in a video series – The Science of Gardening – produced by The Great Courses. She also is one of the Garden Professors – a group of academic colleagues who educate and entertain through their blog and Facebook pages. Linda’s contribution to gardeners was recognized in 2017 by the Association for Garden Communicators as the first recipient of their Cynthia Westcott Scientific Writing Award.
"The Garden Professors" Facebook page - www.facebook.com/TheGardenProfessors
"The Garden Professors" Facebook group - www.facebook.com/groups/GardenProfessors
Books: http://www.sustainablelandscapesandgardens.com
One of the great things about doing a multi-author science blog is that there will be topics about which colleagues will disagree. One of those topics revolves around the best way to prepare woody rooted plants (trees and shrubs) before planting them. This is an area in arboricultural science that is evolving. A search through our blog archives will find many of these posts and for convenience’s sake I’ve linked one from each of us here.
Rather than belabor the points that Jeff, Bert and I have already made in our posts, I think I can sum up our major difference here: I like to bare-root trees and shrubs completely before planting (so I can correctively prune all flawed roots) while Bert and Jeff prefer a less invasive approach. What we do agree upon, however, is the deplorable condition of the roots of many trees and shrubs that end up in the nursery. Because I do practice bare-rooting trees, I thought I’d use today’s post as a rogue’s gallery of trees that should never have made it to the retail nursery. (All of these trees were ones that I bare-rooted and root-pruned myself before planting – and all are thriving.)
Ginkgo “knee” rootInterlocking redbud rootsJunky Japanese mapleThe duct tape around the trunk is where the burlap bag began. While the roots are in pretty good shape, root-washing revealed the root crown of this tree (which the “at grade” part of the tree) 10″ below the top of the bag
I get versions of this question often. You have something in the garden, but this year it looks a bit different than it did before. There are a bunch of different things that could have happened to cause this change, and I’ll attempt here to make a complete list of them.
Rootstock
Trees and shrubs, including things like tree peonies and roses, are often grafted, so the part with the pretty flowers or delicious fruit is stuck onto the roots of a different variety, often not as pretty/useful/tasty but more vigorous and/or easier to propagate. Sometimes shoots come up from that rootstock and take over the plant. The best sign will be to see if the shoots that look different are coming up from the very base of the plant. Cut all the shoots with the off-type flowers or fruits as completely as possible to allow the desirable parts of the plant to continue on.
Reversion
This is most common with variegated plants that are chimeras, though there are a few chimeral varieties which have multicolored flowers rather than multicolored leaves. I wrote about what a chimera is here, but the short story is: Many variegated plants are made up of two different cell types living together. If one cell type starts taking over, you loose the bi-colored effect of the variegation. As soon as you notice them, cut out the reverted shoots to keep them from out growing the multicolored parts.
A variegated sport on a lilac
Sports
Sports are chance mutations, and sometimes a flower will just up and change color. Usually, a sport will only change one aspect of a plant, most often color, while size and shape and everything else stays the same. New variegated varieties almost always come into being as sports. Sports will almost always be isolated to just once branch or section of the plant while the rest of the plant maintains the original color. If you like the sport, you might want to try taking cuttings. If it is an attractive sport of a popular plant, it might even be valuable.
Seedlings
Sometimes it isn’t that the original plant changed, but rather that it had some babies, and the babies look different. Confirmed self-sowers like phlox and aquilegia are notorious for this. You get a nice variety, but in a couple years, it has died out and replaced by a few seedlings which usually aren’t as nice. To prevent this, dead head after flowers fade to prevent seeds from developing.
Soil
Hydrangeas famously will change flower color depending on the pH of the soil in which they are grown. However, this is rather an anomaly, and soil conditions have no impact on the look of flowers for the vast majority of plants.
Climate
Some flowers will change their look, sometimes radically, depending on climatic conditions, often getting darker or lighter depending on temperature and the amount of sunshine. So if a plant looks different in a year when it has been unusually hot or cold or cloudy or sunny, it will probably go back to the normal coloration once your weather returns to normal.
Nursery conditions
Often plants you get at the nursery will look quite different once they’ve spent some time in your garden. Many plants from the nursery will have been treated with chemicals called plant growth regulators which are used to keep the plants short and compact. As those compounds wear off, the plants will grow taller and looser. Also, commercial greenhouses are usually warmer than your garden and often exclude UV light which can chance the coloration of a lot of flowers.
Contamination
If your neighbor gave you a clump of their favorite old bearded iris, but after a couple years the iris they gave you is replaced by a different color or a different plant altogether, they likely accidentally included a bit a different, more vigorous plant along with the one they gave you. Your best option here is to lift up the whole clump, wash all the soil off the roots so you can see exactly where one plant ends and the other starts, and just replant the ones that are true to type.
A whiff of glyphosate caused this iris flower to bleach out.
Herbicide drift
If you are spraying something like glyphosate (active ingredient in roundup) sometimes a bit of it can drift onto plants, not enough to kill them, but enough to cause them to be deformed or discolored. Glyphosate drift in particular will often cause flowers to be bleached out, white or nearly white. In this case, just be more careful next year, and everything should return to normal.
Forgetfulness
Okay, let’s just admit it. We don’t always remember that we moved something or what we bought or exactly what a new plant looked like last year. Sometimes there is no great scientific mystery beyond the fact that we’re all busy and don’t keep as good records as we know we should.
Though psychology was the subject of this study, you shouldn’t assume the results were unique to that particular field. The are plenty of reports of similar failings and the so-called ‘Decline effect’ in other scientific disciplines.
So why is that? There are a lot of reasons. Research can be poorly designed, based on flawed assumptions, and sometimes an unlucky flukes can create false positives. It is also the sad fact that science is done by humans, and humans are complex things with a lot of motives besides the pure quest of knowledge.
I think the general public often fears that scientists are swayed by money from corporations and/or special interest groups, but my experience in academia is quite different. I’ve never heard anyone concerned they might loose a corporate grant. I have heard lots of people, more-or-less continuously, worrying that if their experiment doesn’t work out they won’t be able to get their PhD, land a job, or get tenure. There is enormous pressure to find something significant, to find an effect, and it matters not at all the political ramifications of that effect. So if you are worried about Monsanto buying off scientists to say GMO are safe to eat, don’t be. Convincing data that GMOs are somehow unsafe to eat would be of enormous significance, completely rewriting what we know about genetics, and would come with huge professional rewards. In my opinion, you should be more concerned that some new study showing that X, Y or Z makes plants grow bigger or yield more is actually the result of fervent, wishful thinking on the part of a grad student desperate for publishable data.
So what’s the solution? There has been a lot of talk in the academic community about making it possible to publish negative results and provide funding to regularly attempt to replicate previous studies. I hope these changes go into effect, as they could make an enormous improvement in the reliability of new findings.
In the mean time, you, as a concerned gardener, should take information supported by only a single, isolated study with a big grain of salt, particularly if it seems to contradict findings from other research. If you go to scholar.google.com and start searching around, make sure you read as much of the research on the topic as you can, so you can differentiate between the intriguing new research that may well be proved wrong and reliable findings that have been sustained by several independent researchers. And always remember that while the scientific process is far from perfect, it is still the best we’ve got.
The end of August brought an unseasonable rain- and windstorm to the Puget Sound region. We had some spectacular tree failures which I missed seeing as I was out of town. But one of our Facebook group members, Grace Hensley, was on the ball and took some great photos of a fallen purple-leafed plum. The first thing you see is the complete lack of a stabilizing root system.
“Rootless” purple leafed plum
Now look at the base of the trunk, which is actually a massive circling root that has girdled the trunk over time.
A big wooden doughnut
By now you must be able to see the orange twine extending from the base of the tree to the soil. Yes, those are the remains of the balled-and-burlapped clay root ball that was planted many years ago. Commercial landscapers will assure you that tree roots can grow through the burlap and establish. And this is sometimes true, as in this case.
But what doesn’t happen when the whole B&B mass is plopped into the ground is that circling woody roots aren’t discovered and corrected. Over the decades what started as a small circling root grew bigger and bigger, slowly squeezing the trunk and preventing it from developing girth at that point. It’s kind of like a blood pressure cuff being pressurized but never released.
Trunk growth was prevented by the girdling root. The broken part here used to be in middle of the wooden doughnut.
In time, the constricted point becomes so unstable that the tree breaks. Look are how small the trunk that’s still in the ground is compared to the trunk of the tree itself. Windstorms are often the final push these failing trees need.
How long before this neighboring tree fails, too?
Commercial landscapers say it’s too costly to remove the twine and burlap and clay surrounding the roots, not to mention doing any of the corrective root pruning that might be needed. It’s easier to just plant the whole thing and cross your fingers that the tree lives past the warranty date. This is what happens when you consider a tree as just another design element rather than a living organism.
As a homeowner, however, you can insist that your trees are planted correctly (if you have someone else do the work). Or you can do it yourself. The bare-root method (sometimes called root washing) is an emerging science and it requires thoughtfulness, but it’s certainly better than the conventional approach in terms of long term tree health.
Doubling of flowers — the development of extra petals — is a common mutation, and often beloved by gardeners. Sometimes double forms of flowers become so popular that gardeners hardly recognize the single flowered, wild-type. Wild roses, for example, have just 5 (or, in once case, 4) petals and look totally different than the extra petal flaunting varieties familiar from gardens.
Rose glauca, like other roses before human breeders got their hands on them, has only 5 petals
Doubling usually happens when gene expression gets mixed up and bits of cells that were destined to develop into anthers develop into extra petals instead. Sometimes a single mutation makes a dramatic change all in a go, but more often, the path to a double flowered cultivar starts with something like this:
Iris x norrisii with three ugly little petaloids full of potential
Here we have a flower of Iris xnorrisii (formerly known as x Pardancanda norrisii) with the usual six petals, and three “petaloids” — anthers that are stuck in an ugly transition between anther and petal. This is a seedling in my garden this year, and I’m going to grow out lots of seeds from it — hopefully some of them will get past the petaloid stage to full on extra petals and hey presto, a double flowered variety will be born!
If you’ve spent enough time around flowers, you’ve probably seen this. It isn’t exactly common, but it happens, and is so distinctive that you’ll almost always notice when it happens, as I did on one of my gladiolus the other day.
Everything else is as normal, but a chunk of the flower is white instead of the usual soft peachy white.
What we have here is a sectorial chimera. Chimera means an organism with two (or more, I suppose) genetically different cell types, and a sectorial chimera is when there is one distinct section of the plant made up of a different cell type.
And why is this showing up in my usually pink gladiolus? Well, somewhere early in the development of this flower spike, there was a chance mutation in a cell. That mutation stopped those cells from producing the usual pink pigment, so the mutant cells make white flowers. The new mutation and the original cells continued to grow and divide, so some of the flower is from the newly mutated white form, and some is the original cell type.
Now, when people hear the word “mutant” they either think x-men or nuclear fallout, but the fact is mutations are a perfectly common, normal part of everyday life for organisms, and of course are critically important to continuing evolution.
This type of bicolored flower is cool looking, but certainly a one-off. Sectorial chimeras are very unstable. Next year, most likely, the flowers will just be pink again, or possible a branch will send off pure white flowers. So when you see a sectorial chimera in the garden, take a picture, put it on facebook, and enjoy it because it probably isn’t coming back.
Salvia azurea (maybe my favorite salvia in the world — sky blue flowers in late summer/fall, hardy to zone 5) is blooming in the garden, and the bees are all over the flowers. But while some are poking their heads into the flowers to drink nectar and transferring pollen as they do so, others are up to something more sinister.
This carpenter bee looks like it is pollinating. It isn’t.
The evidence of what they are up to is clear if you look closely at the side of the flowers after they leave.
The little hole at the base of the flower is clear sign of an illegal break in.
See the little hole in the base of the flower? That is where the carpenter bee bit a hole in the flower to get access to the nectar instead of going in the front of the flower as one would expect.
This phenomenon is called nectar robbing because it is an evolutionary betrayal of sorts. Flowers have evolved nectar to lure bees and other pollinators into the flower, so the bees will pollinate while getting their sugar fix. When the bees nectar rob, they’re getting the payment without doing the actual pollinating.
So why bite a hole in the side of the flower instead of just going in the front? Well, many flowers have evolved flower forms that make the nectar hard to reach by anything but their preferred pollinator, the species that most effectively moves pollen from plant to plant. In the case of this salvia, the nectar is down at the base of the flower, and only accessible to bees with long tongues, like whatever species normally pollinates it in its native range in the US plains. In other words, it is out of reach of the carpenter bees to save it for another bee, probably a bumble bee. Which works great to avoid wasting nectar on sub-optimal pollinators… unless, of course, those bees become robbers.
So next time you see bees on your flowers, take a look… they might just be robbers, not pollinators.
I spent last week in Orlando at the ISA annual meeting (that’s the International Society for Arboriculture). It’s a great venue for networking with colleagues and hearing about the latest tree research. And once in a while I’ll have a WTF moment. (That stands for Why Trees Fail in case you’re wondering.)
My WTF experience this year revolved around some new terminology and techniques. I learned there are now “environmental arborists” who practice “retrenchment pruning.” In the last few days I’ve tried mightily to find some standard definitions from reputable sources. I don’t know what an environmental arborist is, since it’s not a certification (like an ISA certified arborist) nor is it a university degree program (like urban forestry or environmental horticulture). It seems to be a self-anointed title.
This is what a mature urban oak should look like.
But the real WTF issue is retrenchment pruning. I looked in vain for published research through my usual data bases and found nothing – other than two articles in Arboricultural Journal (which is not the same as ISA’s journal – Arboriculture and Urban Forestry). Neither of the articles presented experimental evidence to justify this radical approach to pruning trees. Instead, they are more philosophical in nature, with a smattering of ecological theory.
Fortunately, retrenchment pruning methods are easily found on the internet, along with horrific pictures illustrating the results. As described on various websites, retrenchment pruning imitates the natural process of aging. Practitioners remove live branches or partial trunks to reduce the size of the tree and prevent future failure. These aren’t clean cuts, either: they’re “coronet cuts” or “natural fractures.” The rationale described in one of the Arboricultural Journal articles is that these jagged broken branches and trunks “promote specialist habitats and enhance colonisation rates of niche species.” In other words, this technique creates large wounds that are easily colonized by various insects and microbes.
An example of natural fracture pruning (http://www.countytreesurgeons.co.uk/veteran.html)
So apparently we’re expected to ignore the well-established field of woody plant physiology (which happens to be my specialty) and related practical bodies of knowledge (e.g., formal and informal pruning techniques of said woody plants) and start hacking away at mature trees. In doing so, we’re removing live tissue and creating large wounds. This has the effect of both reducing photosynthetic potential of the tree as well as opening it up to possible pest or disease invasion. But nowhere are these possibilities discussed as part of the “natural aging process.” Nor was there mention about how to manage the epicormics shoots that result from improper pruning. And they do need to be managed.
These are epicormic shoots resulting from topping this tree.
I saw some very angry arborists at the ISA meeting who were incensed at the idea that we should deliberately malprune trees. But others seemed quite excited with this new philosophy. To paraphrase one of my plant physiology colleagues, “Give a bad arboricultural practice a catchy name and it magically becomes legitimate.”
Sometimes where you shop matters, because if you buy plants at the wrong place you run the risk of importing new problems into your garden. Those cheap plants you grab on a whim sometimes are far more trouble than they are worth.
I learned (or relearned…) this the hard way last year when I grabbed a bag of super cheap gladiolus corms at a box store. They were cheap, and pretty… and infested with gladiolus thrips.
White marks on the flower are signs of an early infestation of gladiolus thrips. As the summer goes on, the damage gets worse and worse.
At first, they just caused a few small white marks and discolorations on my flowers, but by the end of the season the population was so high that almost every bloom was totally deformed and ugly.
The solution? Getting all my corms this year from a specialty grower who sends high quality corms which have been properly treated with cold temperatures to kill all the thrips. This particular thrips can’t survive my cold Michigan winters, so by buying clean stock this year my garden is totally gladiolus thrips free.
As an added bonus… the specialty growers have MUCH cooler varieties. Like this Gladiolus ‘Huron Mask’ which has become one of my all-time favorites.
I’m not a fan of using corrugated cardboard as a mulch, which like other sheet mulches creates problems for the underlying soil. Long-time readers of this blog may remember several previous posts (1, 2, 3 and 4) on this topic and I won’t belabor the points made in those posts. Instead, today I’m doing to focus on cardboard itself.
Cardboard mulch under wood chips
First, cardboard is a generic term that can refer to many types of manufactured paper. The box you see delivered to your front door is more properly called corrugated board or containerboard. It consists of two layers of linerboard sandwiching a layer of accordion-like fluting material. The linerboard is made from sheets of pulp that may be coated to improve smoothness (more about this later). The finished linerboard is laminated using adhesives to both sides of the fluting material.
Corrugated boxes are built tough
These boxes are made to withstand rough handling and to protect the contents from the external environment. It’s tough stuff: while you might be able to bend a piece of corrugated board fairly easily, it’s more difficult to tear it in half. The more heavy duty the box, the more difficult it is to bend or tear its walls.
So let’s now consider using this tough material in your garden as a mulch. It may be coated as mentioned earlier to improve smoothness. That’s going to prevent it from absorbing moisture. The coating also reduces the ability for gases to move between the soil and the atmosphere. In fact, smoothness is measured using an air leak method – the smoothest materials have the least air leakage.
Photo credit: vizpix at Flickr
A garden or landscape mulched with cardboard (or heaven forbid several layers of cardboard as part of the science-free lasagna mulch method) is now covered with a tough, relatively gas- and water-impermeable material that will take some time to break down. It’s hardly a mulch that’s going to nurture soil life.
But cardboard mulch fans swear that they find more earthworms under cardboard than anywhere else in their garden. This is almost always the first response I get from gardeners who don’t believe that cardboard causes problems. And this is where it’s important to consider earthworm behavior.
Photo credit: Kurt B. on Flickr
We’ve all observed that earthworms crawl to the soil surface during heavy rains; this is due in part to water filling their burrows and reducing oxygen availability (Chuang and Chen demonstrated this nicely in 2008). Likewise, the reduction in oxygen movement from the atmosphere into cardboard-covered soil would cause worms to crawl upwards in an effort to find oxygen at the soil surface.
So don’t assume your lasagna mulching draws earthworms to your garden. It’s more likely that you’re smothering their habitat.
***An update on cardboard gas permeability. We’ve just published an article comparing diffusion rates of different mulches. You can find the article here but it is behind a paywall. Here is a graphic comparing diffusion rates of various mulches. This is a logarithmic scale. Now, until cardboard proponents publish evidence to the contrary, it’s pretty obvious that cardboard mulch interferes with gas diffusion.
***And another update on how our blog works. This post, by far, is the most popular. It generates a lot of comments. All comments must be approved before they’re posted, and I don’t approve comments that are derogatory or promote a belief in the absense of supporting science. If you want your comment to be published, be polite and provide evidence to support your statements. Otherwise, you are wasting your time.
***Another update on cardboard in your garden: A recent paper reports on PFAs (aka “forever chemicals”) in various products used for poultry bedding (among other things). Cardboard was one of the worst. The article is behind a paywall but I have access to it and was able to find the table shown here. So if you need yet another argument to NOT use cardboard as a mulch (like in “lasagna gardening), maybe “keep forever chemicals out of your garden” will do it.
Look at the top line for shredded cardboard and then the last column which has the PFAs data. Not to mention all the other contaminants listed.
