Be a Part of History!

Here at the Garden Professors we pride ourselves on being on the cutting edge of technology.  In fact, we’re so tech savvy we didn’t even whine when FaceBook foisted a new homepage format on us for no apparent reason.  So it’s only fitting that we offer you, Mr. and Ms. Garden Professor Blog reader, an opportunity to participate in the first ever landscape horticulture research project designed by social media.

 


Here’s the deal.  My current research project on water and nutrient management of trees in container production has left us with over 100 ‘Bloodgood’ London planetrees in 25 gallon containers.  What I need from you are ideas for a study plan on what to do with the trees next.

 

Of course, as with any major research project, the first step in the rigorous scientific process is to come up with catchy acronym for the study.  I propose “the SOcial MEdia DEsigneD TRansplant ExpErimental Study” or SOME-DED-TREES for short.  Needless to say, I am willing to consider alternatives.  In any event, we have a unique opportunity to investigate post-transplanting growth, development and physiology of landscape trees.

 

So here’s what we have: Approximately one hundred,  2” caliper trees, grown in containers in a standard mix of 80% pine bark and 20% peat moss.  Trees have been grown for two years in essentially standard nursery culture – daily irrigation and 60 grams of Nitrogen per container.  The subject of the original study was fertilizer source; half the trees were fertilized with Osmocote and half received the same amount of nutrients from organic fertilizers. After two years we have not seen any difference in growth or foliar nutrients between the treatments. Nevertheless, I will need to include the prior treatment as a blocking variable to eliminate any potential confounding effects.  Beyond that it’s wide open.  We could have 6 treatments x 2 blocks x 8 trees = 96 trees.  I strongly suspect in the final analysis the block effect will be non-significant and we can consider there to be 16 replicates, but life is full of surprises.

 

So, what tree establishment or tree care question is burning a hole in your brain?  “Shaving” or “butterflying” container rootballs?  Fertilizing at time of transplant?  The latest biostimulant?  Crown thinning at time of transplant?  Frequency of post-planting irrigation?  Width of the planting hole – how wide is wide enough?  Send me your suggestions and we will set up a poll to vote for the top choices.

Podcast #8 – Water Works

We’ve finally gotten our summer here in the Pacific NW and it’s been pretty hot for a few weeks. The plants weren’t really prepared for this, so we’ve had to irrigate quite a bit to keep all that lush foliage happy. So the topic of this podcast is Water Works – focusing on how water moves in the soil and through plants.

One of the more interesting tidbits I found this week is a recent USDA study on growing more potatoes with less water. Sound impossible? Listen to find out the one single, simple thing that increased water use efficiency by 12% and reduced fertilizer runoff as well.

I also debunk the common myth about using drainage material in container plants. Research from 100 years ago demonstrated that water won’t cross textural barriers – so putting gravel in the bottom of the pot will actually create a bathtub effect rather than helping drainage.

The interview this week is with my garden – primarily the sunny south-facing side. I thought I’d take you on a tour to see what’s happened in the last 11 years. The photos below will help you visualize the interview.


The front yard in 2003. We’ve started taking out the turf and moving around trees and shrubs.

The front yard in 2003 from another angle. We’ve removed the second driveway and covered the area in wood chips. By the garage you can see two of the roses I dug up from the shady back yard and moved to the sunny front.

The new front yard, with fencing, more plants, a pond, and no turf.

The rhody-hydrangea corner in front of the arbor vitae hedge

The new street garden, with a new retaining wall to hold back the soil that used to wash into the street.  Everything not covered in plants is covered with wood chips.

This is the last podcast of the first “season” of The Informed Gardener. We’re going to take off for about a month before starting the next series. If you’ve got ideas about future topics, you can email me or post a comment here. In the meantime, you can listen to archived podcasts found on this blog; just click on “podcasts” on the right-hand menu.

Hot weather and not-so-hot advice

Today I was sent a link to a posting on “droopy leaves.”  Essentially, it suggests that droopy leaves are a means to conserve water on hot days and that watering these plants causes more problems than it solves because the roots don’t get enough oxygen.  A link to the science of transpiration is provided.  The advice is to wait until the evening and if the plants perk back up, then they didn’t need water after all.

This is one of those maddening articles that has enough science in it to make it sound reasonable, but is ultimately incorrect in its assumptions and advice.  It’s worth looking at the topic in a little more detail.

Some plants are adept at conserving water in hot weather.  Their leaves tend to be small, thick, with a heavy layer of waxes protecting the surface.  Leaves can also move to limit their sun exposure and thus reduce the heat load.  But wilting is not a method of conserving water.  Instead, it’s a sign that water loss (evapotranspiration through the leaves) exceeds water uptake from the roots.  And if you ignore wilt, you do so at your own peril.  Once terminal wilt is reached, it’s all over for that part of the plant.


Wilt.  Sorry it’s a fuzzy photo.

Large, thin leaves, common in many of ornamental, annual and vegetable species, do not conserve water.  Tomatoes, zucchini and black-eyed susans, the plants specifically mentioned in this article, are not water conservers.  Chronic wilting of these and other can eventually cause leaf tip and margin necrosis (or tissue death).  It also reduces growth, so that your yield of tomatoes, zucchini and black-eyed susans will be decreased.

Leaf tip and marginal necrosis from chronic drought stress

So yes, do water your plants if they are wilting in the midday heat!  Use mulches to conserve water!  (You’ll notice in the photograph on the linked site that the plants are in bare soil.)  Fine root systems are generally near the soil surface, and keeping these hydrated keeps them alive.  You won’t see an instantaneous response to watering if plants are already wilting, but they will recover – much better than if you don’t water them at all.

Rain barrels

A few weeks ago one of our readers, landscape architect Owen Dell, sent me a link to his blog where he takes on rain barrels. It’s a great analysis of the (im)practicalities of rain barrels and it got me to wondering how many of our readers (and my GP colleagues) use these as supplemental sources of irrigation water?

I have two in our back yard that were made from old olive oil containers retrofitted for collecting and dispensing water. They’re hooked together so that when one fills, the rain is diverted to the second.

We use this water pretty much for watering container plants, especially those on our south-facing front porch that require watering every other day during the summer. The barrels each hold 55 gallons and are always full during the winter and spring. We drain them almost dry over the summer, but even a brief rain results in several gallons collected.

So I think they’re a pretty good deal, since we use relatively little water from the hose to keep our container plants happy. But Owen brings up some valid points in his analysis, as do commenters on his blog.

What do you all think?

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.

My Long Suffering Basil

Sometimes I am not such a good garden professor.  That’s because, when I get home, I sometimes (OK — often) don’t give my plants the attention they need.  It’s also because, when we leave for vacation, I often forget to tell whoever is watching the animals to keep their eyes on the plants too.  Now, really, you would think that someone who saw a plant on the back porch in full wilt would think “Hey, Maybe I should water that!”  But still, I blame myself for not spelling it out.

What this all leads to, of course, is that we had a group of really nice basil plants in a container on the back porch which weren’t watered for a week while we were on vacation.  There was little rain over this week, and subsequently the basil was in full wilt for at least 2 days (perhaps as long as three).  We watered these plants soon after we arrived home (my wife loves caprese salad) and the plants perked up a bit, but the leaves were obviously damaged and now our salads will be a little less flat.

It’s not just basil that is affected by an incidence of drought.  Most plants, including trees, will actually suffer for a considerable period of time after the event.  Sometimes growth won’t return to normal for as much as a year or two.  This is a really bad situation if you’re buying trees grown in containers.  If the grower, or retailer, didn’t know what they were doing and let the tree wilt severely prior to selling it to you then buyer beware!

Need a lift?

One of the topic groupings for our posts is titled ‘Cool research’.  The subject of today’s post has actually been around for a few years but I still think it’s pretty cool.

 

When we think of interactions between plants we usual think of negative interactions such as competition for water and nutrients or maybe allelopathy.  But there are cases where plants can benefit each other.  One of these is a phenomenon known as hydraulic lift.    Hydraulic lift is the passive movement of water from roots into soil layers with lower water potential, while other parts of the root system in moister soil layers, usually at depth, are absorbing water.  In essence, plants will large, deep root systems (usually trees) bring soil water from depth to the surface where it can be used by other plants.  Hydraulic lift has largely been observed in arid and semi-arid ecosystems, though it can occur in wetter systems as well.  For me, the research that went into discovering hydraulic lift is as fascinating as the process itself.

 

One of the key lines of evidence for hydraulic lift comes from studies of stable isotopes.  As you may recall from college or high school chemistry, atoms of each chemical element have a certain number of protons and neutrons, which give it its mass.  A small portion of each element has extra neutrons resulting in a ‘heavy isotope’.  In the case of hydrogen, approximately 1 in 6400 atoms is heavy hydrogen or deuterium (2H).  Interestingly, the amount of 2H in water can vary depending on the source of the water; this ratio is termed an isotopic signature.  By comparing the isotopic signature of ground water and rain water in a given location, researchers can actually tell where certain plants are getting their water.  One of the classic studies in this area was conducted by Todd Dawson at Cornell in the early 1990’s.   Dawson analyzed isotopic signatures of groundwater, rainwater, and water in various plants around sugar maple trees and determined that many herbaceous plants contained a high proportion (up to 60%) of groundwater.  But how do shallow-rooted plants obtain groundwater?  The neighboring maple trees bring it to the surface from ground water as they are hydrating overnight.  An efflux of water from the maple roots results in a localized increase in soil, which can be utilized by other plants: hydraulic lift.

 

How important in hydraulic lift in most landscapes?  Probably not very.  Demonstrating significant hydraulic lift requires the proper hydrology (shallow ground water accessible to trees or shrubs but not smaller plants) and limited rainfall.  But the importance goes deeper (no pun intended!).  Prior to the advent of stable isotope techniques, many would have been dismissive of the concept of hydraulic lift.    Since 1993 over 100 papers have now been published on the subject.  To me, the ultimate value of Dawson’s work and related studies is showing the importance of keeping an open mind and being receptive to new ideas. 

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