Category: Cool Research

Our visiting professor digs into tomato planting depth

With Ray’s recent photos of the peach, crabapple, and hydrangea planted too deeply, a discussion of tomato planting depth arose in the comments. I’ve seen the prolific adventitious roots start to form near the base of tomato plants, and I plant tomatoes to the cotyledon or deeper, but tomato planting depth not an area I have extensive research experience in. So I did a little literature search.

It seems that the practice of planting tomatoes is more than just friendly garden folklore. There’s some evidence that it works. Or, at least it works sometimes. Early in the season. And maybe not everywhere. Oh these darn scientists, with their wishy-washy caveats.

There just isn’t a whole lot of peer-reviewed research on the subject. One of the recent papers (from 1996) cites a “Crockett’s Victory Garden” (circa 1977) as a source of information for Northern gardeners about the benefit of deeply-planted tomatoes. Jim Crockett was no horticultural slouch, but just because successful gardeners do something doesn’t mean it’s sound practice (but like I said, I do it too…).

Southern tomato growers, on the other hand, have some peer-reviewed evidence to go on. It seems that in some years and some locations in Florida, tomatoes planted to the first true leaf were able to be harvested earlier (Vavrina 1996) than if they were planted to the top of the root ball. The overall trend was for a bit more and a bit larger fruit per plant. The paper is pretty sparse on detail (statistics and design), but that’s what they conclude. Similarly, a study of fall-planted tomatoes in Louisiana (Hanna, 1997) showed a benefit of planting to the first true leaf instead of to the top of the root ball in two years at one location. In that study, treatments that lowered the root zone temperature tended to increase yield. If you can get an extra 3 pounds of medium or larger tomatoes from a 50 square foot plot in some years, what’s the harm? Deep planting certainly helps prevent lodging in tender young plants, so there’s a clear benefit there if the supports aren’t adequate.

Most of the work suggests that, like mulch, deep planting helps to moderate root temperature, and fluctuating or high root temperatures are stressful to the plant. I can get behind that I suppose, but it’s hot in Florida and Louisiana in the summer. What about stuff planted in Minnesota, where I live? We use plastic up here to get warmer soil temperatures at planting, not cooler temperatures!  Well, I didn’t find any work on that.  But interestingly, some of the yield benefit seen in planting peppers to the cotyledon or true leaf in Florida (equivalent to 5 extra pounds per 50 square feet, in 3 of 4 years, with commercial spacing and fertilizer rates; Vavrina, 1994) don’t show up in Massachusetts (Mangan, 2000). But deep planting did help prevent lodging, which allowed for faster crop maturity in both places.

So the verdict on deep planting tomatoes? It doesn’t hurt, it helps sometimes, and it helps prevent lodging, so why not? One caveat: don’t plant grafted tomatoes deep. The scion will make roots, negating some of the benefit of the rootstock.

(And an addition from Linda:  here’s a diagram from TAMU demonstrating planting depth for tomatoes; this link will take you to the article itself.)

Bridging research and reality

This summer, I’ll be giving a seminar on “Arboriculture Myths” at the ISA conference in Portland, OR. I’ve been quizzing arborist-types for a few months now to find out what myths they would most like to see debunked during my talk. Intermixed with the suggestions of dubious products and questionable practices there was this question: “How often do the results from research with limited scope get over-extrapolated?”

I like the question a lot, because this is the fine line that we Garden Professors walk in bringing you the newest scientific information we can find.  As a rule, I tend to hold back on recommending anything that has only been tested in a lab situation.  I like to see field test results, where environmental variation will quickly swamp anything with marginal effects.  In other words, if something can make it through an experimental, replicated field test, I can get excited about it.

Which brings me to a recent article in Arboriculture and Urban Forestry (2012, Vol. 38, Issue 1, pp. 18-23. And no, I can’t post it on the web). Briefly, the article describes an experiment where water evaporation was measured in pots filled with various substrates, which were either left uncovered or mulched with about 3” of pine bark.  The results showed little difference between the mulched and unmulched containers.

As the authors point out in the discussion, it’s an artificial system that includes no trees, nor any way for water to move through the soil except from the top down.  And I really don’t have a problem with the methodology, or the data generated, or even most of the discussion. What bothers me is a single sentence at the end of the abstract:

“Given the minor reduction in evaporation, and reported disadvantages of mulch application close to the trunk, landscape managers might consider changing mulch application practices for newly planted trees.”

Wow. How did we get from a series of containers with no plants in them to this recommendation?

Every gardener knows the value of mulching – a perception that’s substantiated by hundreds of publications. Since I’ve written about mulches on the blog a number of times I’m not going to belabor the point. But I will refer readers to a short Ecological Restoration article I published a few years ago that most definitively linked mulch application to plant survival in restoration sites; Bert also published an article on the benefits of mulching and lent me a few photos to illustrate. And Jeff has even more data on the topic, including some that may radically change the perception that mulch against tree trunks is a bad thing.

Mulch increases soil moisture

 

Which plot would you rather have in your garden?

Those of you who read scientific journals probably read the abstract first – I know I do. If it interests me, I’ll read the entire article. But sometimes the abstract is the only thing you can find online. And for this reason, the peer-review process in many of the journals asks whether the contents of the abstract are justified by the results. Honestly, I don’t think this article meets that standard.

What I did on my Christmas vacation

The week between Christmas and New Years’ is usually pretty laid back around here.  But not this time!  Along with 22 volunteers, 3 family members, and 1 graduate student, I spent that week putting in 80 trees for a long-term experiment.


My long-suffering family and I installing the last of the 80 trees on the fourth day of hell.

My intrepid graduate student Cindy Riskin obtained 40 B&B Japanese maple (Acer palmatum) and 40 containerized mugo pine (Pinus mugo).  Half of each of the trees were installed conventionally, meaning the root balls were not significantly disturbed, and half were bare-rooted by root-washing methods I’ve discussed on the blog previously. Roots that circled or had other flaws were pruned as needed. Over the next several years, we’ll be assessing tree health and comparing the two root preparation techniques in terms of tree establishment.


Installed maple


Installed pine

Look at some of the surprises we uncovered during root preparation!  I will say unequivocally that these were the WORST quality trees I’ve ever seen coming out of a nursery.  And they weren’t cheap.


Yes, that’s a 4" pot still covering the roots inside this "gallon" mugo pine.


The duct tape is where the top of the burlap was in the original B&B.  Every one of the B&B trees we bare-rooted was buried too deeply in the clay and burlap.


Multiple trees?  Multiple messes!

Stay tuned for more…

Phosphate toxicity and iron deficiency

Bert’s post yesterday reminded me of some work one of my graduate students did about 10 years ago.  We were curious to see whether a transplant fertilizer containing phosphate was correlated with foliar iron deficiency, which is visualized as interveinal chlorosis:

 What Scott did was to plant 10 rhododendrons per treatment into pots containing containing a name brand azalea, camellia and rhododendron food (5-5-3) at 0, 0.5, 1.0, and 2.0 times the recommended amount. Here are some of the results of that study:

 
Total number of chlorotic plants

Total foliar iron vs. fertilizer treatment

Chlorosis as a result of phosphate fertilizer. 1= Normal (green leaves), 2= Light chlorosis in young leaves, 3= Moderate chlorosis, 4= Severe chlorosis, young leaves white

 For gardeners, the take home message might be that the control plants – those without any transplant fertilizer added – did the best. Don’t add phosphate to your landscape and garden soils unless you have a verified deficiency.  And only a soil test will tell you this conclusively.

You can’t fly by the seat of your pants on this one, folks.

Today in Cucurbit News…

Cucumbers are one of the most widely-grown vegetables in the world.  Baker Creek Heirloom Seeds (a great place to buy unusual and international veggie seeds) lists 51 varieties from North America, Southeast Asia, China, India, Mexico, and Europe.  Dark green ones seem to be in the minority – yellow, white, orange and red skins in shapes round to elongated dominate.

Cukes traditionally have a few nutrients including some Vitamin A from carotenoids and beta carotene, but have never had the reputation as nutritional power house. Watery and gas-inducing, yes.

Researchers with the USDA have recently released a cucumber high in beta carotene.  No "frankencuke" this; all the crossing was done by traditionally breeding methods (including bees and self-pollination).  Lots and lots of crosses with a warty, round-ish chinese cuke (Cucumis sativus var. xishuangbannanesis) and some standard pickling cukes has resulted in a stable cultivar that has the smoother skin and proper proportions of marketable pickling cucumbers (there are lots of marketing standards associated with most fruits and veggies).  But the big news is the orange interior, specifically the endocarp (the jelly-like stuff around the seeds) and the mesocarp (the fleshy part that is the whole point). It’s orange because it’s full of beta carotene  (mesocarp is 2.7 micrograms per gram of fresh fruit compared to 0.02 micrograms per gram with a traditional white-fleshed variety.  Even more impressive is the jump in endocarp beta carotene – from 0.16 micrograms per gram to 7.5 micrograms per gram).  I don’t believe the USDA is going to release this particular line directly to the public, rather they’re offering the genetics (two recessive genes control the beta carotene content) to other breeders.  This means other breeders can use it in their own breeding program to bring more nutritional value to their specific lines, at which point varieties will become available to growers/gardeners. Orange tzatziki!!!

from Staub, J.E., P.W. Simon, and H.E. Cuevas.  2011. USDA, ARS EOM 402-10 High β-Carotene Cucumber. HortScience 46:1426-1427.

(Linked, but my guess it won’t work if you don’t have a HortScience subscription or institutional access, sorry) 

An unusual company

This week I’m in Charlotte, NC as a guest of Bartlett Tree Experts.  In addition to providing tree services, this company also maintains the Bartlett Tree Research Laboratories and Arboretum. The latter includes over 300 acres of tree collections and ongoing research trials.  Here’s a sampling of the tree research we had a chance to observe:


Demonstration espalier pruning…


…and pleaching


Comparison of root barrier materials.  This area was covered with a sidewalk for a number of years and then exposed to observe tree rooting patterns.  The purpose of the research was to find which barriers were most likely to prevent sidewalk lifting and cracking.


A control – no barrier, lots of roots!


Black plastic – lack of rigidity allows roots to grow over (and through) the plastic, then under the sidewalk.


18″ rigid root barrier.  One of the more effective means of keeping roots out.


Removing circling roots before planting


A tree whose roots had been corrected before planting.  I think this had been planted in 2007, then lifted a few weeks ago.


A tree without root correction.  It didn’t grow any better than the corrected tree, and those circling roots are well on their way to becoming girdling roots.

This company employs a number of PhDs whose research is routinely published in arboricultural and horticultural journals.  It was fun to finally meet these researchers whose work I’ve been following for years.

Wouldn’t it be great if more companies put this much effort towards research?

So…How Much Pesticide Is Actually In Our Fruits and Veggies?

We have discussed the dirty dozen here before – those foods which a group called The Environmental Working Group (wow—fancy name – everything they say must be true!) has established contain more residues of different pesticides than other foods.  I’ve already stated my concerns about selecting organic foods instead of conventionally grown ones because of a fear of pesticides so I won’t restate that here.  Instead what I want to call your attention to an article sent to me by our visiting professor, Charlie Rowher.  This article runs down the amounts of pesticides that are actually in the dirty dozen. And the thing is….there just isn’t much pesticide of any sort on most foods and there is no evidence at all that eating these levels of pesticides would be bad for us in any way – even if we ate them in copious amounts day after day.

To be honest I think the authors of this article go a little too far – I do think that there is some potential for damage even from the ultra-small pesticide doses that we find on our foods.  But their points are well taken – the amount of pesticides in food is miniscule and less likely to be damaging to us than a great host of other things.  I’m much more concerned about certain segments of our population suffering malnutrition from avoiding conventionally grown fruits and veggies than I am about the larger portion of our population getting cancer from eating them.

Out of the lead frying pan and into the phosphate fire

A recent NYT post reports that adding fish meal to lead-contaminated soils will cause the lead to bind to phosphate found in fish bones.  As the article explains, this chemical reaction results in the formation of pyromorphite, “a crystalline mineral that will not harm anyone even if consumed.”

Given my concerns about excessive phosphate loading in urban soils, I contacted Dr. Rich Koenig, an urban soil scientist and chair of WSU’s Crop and Soil Science department.  I wondered, at least, if the article should have directed people to have a soil test done first to determine how much phosphate was already in the soil before adding more.

Dr. Koenig was likewise concerned that the application rate of fish meal was probably far higher than what plants would require, thus increasing the risk of phosphorus leaching and runoff. He also referred my question to Dr. Jim Harsh, a soil chemist in his department familiar with the process described in the article.

And as Dr. Harsh pointed out, it’s important to make lead less susceptible to uptake by people and other animals exposed to contaminated soils.  However, he’s unconvinced that phosphate is the best choice; in fact, research by Dr. Sally Brown (cited in the NYT article) and others (including Dr. Harsh) have found that compost containing iron is also able to bind and immobilize lead.  The advantage of high iron compost is that iron will not leach into nearby waterways, nor cause the same kinds of plant toxicity problems, as phosphate can.

Thanks to both Dr. Koenig and Dr. Harsh for their quick and informative responses on this topic.

Helium Makes Kudzu Float Away

As promised — some happy news:  There’s this kid in Valdosta, GA (close to Tifton where I spent a few years as a graduate assistant), who has been experimenting with ways to kill kudzu.  Here’s the video.

To see this kid work on something like this at such a young age is fantastic and gives me hope for the future.  I wish the kid were here so he could come to the University of Minnesota – I think he has a lot to offer and he makes me slightly more optimistic about where horticulture ends up.

For those of you who choose not to view the video, what this kid does is to inject helium into the soil around the root system of a kudzu plant.  After the injection the plant apparently dies.  The exact reason why isn’t known, but one person who was interviewed said he suspected that the helium smothers the plants roots thus killing it.

I’m a little bit suspicious about that explanation, and I’m also a little bit skeptical about how much more economically feasible it would be to use the helium instead of more standard herbicides.  I’m also very interested in any other gasses that he might have tried to kill the kudzu – I wonder, for example, if he tried propane?  It might work, but I’d say it was too dangerous to try.

I’m suspicious about the helium smothering the root system of kudzu because kudzu has such an extensive root system and because the helium should dissipate pretty quickly, especially in sandy soils like they have in Southern Georgia.   It’s also very unlikely that the helium itself is acting as a poison because helium is an inert gas.  It just doesn’t react with anything.  What I think is more likely is that, by finding the site where the kudzu’s stem enters the ground, this kid has found a “weak spot” on the kudzu which is susceptible to damage.  Then I think that the helium acts a refrigerant when it is released and actually freezes the stem of the kudzu.  However it works though, it’s a neat trick!

Harvesting an Experiment

This has been an exciting week for me.  On Monday we started cleaning off 72 rootballs of various tree species that had been planted 5 years ago for a study.  These trees had been planted in containers and become potbound at the nursery from which we received them.  We treated them in one of three ways.  Either we did nothing (in other words we just dropped the pot bound tree in a hole), we used the standard methods that Universities recommend for slicing potbound roots (Four deep slits down the sides and a deeply cut X across the bottom), or we cut off all of the circling roots by cutting the pot bound root system into a box shape.

A root ball cut into a box shape

The plan was to harvest after 4 years to see what happened – we decided to wait 5 – and boy did we see some interesting stuff.  At this point our results are preliminary – we need to run statistics before we can say anything conclusively – but this is what my eyes tell me.

  1. Trees that had their roots cut into boxes suffered reduced growth the first few years, BUT, their root systems look as good as any root systems that I’ve seen – almost no circling.
  2. We planted our trees with the surface of the soil at the same level as the surface of the media in the containers – which is too deep in most cases.  For trees with circling roots this created a severe problem as the circling roots often surrounded the stem – potentially causing suffocation of the tree later in life.
  3. Root systems that were cut using the 4 slit method didn’t look much different from those that weren’t cut at all.
  4. The number of large roots emanating from all of the treatments appeared to be about the same (we’ll need to run the stats before I commit to this one).  This is particularly interesting because many people expect large roots that are circling to continue circling — but that isn’t what usually happens (unless the hole where the tree is planted has hard sides which can force the roots to circle just like the container did).


This root system was from a control — no root pruning at all, but still plenty of large roots.

No matter what the results/statistics end up saying there will be more questions.  For example, all else being equal, how damaging are circling roots to the health of a tree if the tree is planted properly (no stem tissue under the surface of the soil) and the circling roots are under the surface of the soil?  If the answer is that circling roots under the surface of the soil aren’t very damaging (after all, there’s no stem tissue for them to crush) then why are we bothering to try to root-prune pot bound plants at all – what we should really be concentrating on is planting at the proper depth.

All the above is hypothetical though – I just enjoy thinking about this stuff as the data starts to roll in.  As we get more definitive answers and start to run the statistics I’ll let you know more.