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

Is “lasagna gardening” really worth the effort?

This week I got a complimentary copy of Urban Farm, dedicated to “sustainable city living.”  The cover story is Lasagna Giardino – follow this recipe for a lasagna garden that grows perfect plants – Italian or not.

This is not a new idea, but was popularized several years ago as a way of preparing soil for planting.  The article relates the steps:

1)  Prepare the ground by mowing the lawn
2)  Dig up the first 12″ of soil (double digging)
3)  Place a layer of “noodles” (paper and cardboard are popular) – the low nutrient material
4)  Place a layer of “sauce” (the green material)
5)  Repeat as often as you like and “let it cook”

I like the first step of this.  But my second step would be:
2)  Add a thick layer (12″) of arborist wood chips and “let it cook.”

Double digging the soil 12″ isn’t necessary: we do it because it’s hard work, and we think we need to put elbow grease into the project.  Making layers of noodles and sauce isn’t necessary: we do it because appeals to us -lasagna is a tasty comfort food.

There’s a lot of damage that this “recipe” can cause.  Double digging the soil 12″ destroys soils structure. Don’t do it. The layers of noodles and sauce (especially the sauce) can create an overload of plant nutrients. Furthermore, the “noodle” layers – the sheet mulches – impede water and air movement.  They’re not needed to keep the grass from growing through. Wood chips do this just fine on their own.  And don’t worry about that initial 12″ of chips.  Within a few weeks it will settle to about 8″.  Let it sit for several weeks.  Then pull aside some of the chips and take a look.  If the process is done, the grass and/or weeds will be dead and decomposing – a natural compost layer.  You can then plant whatever you like.  Reuse the chips somewhere else in your garden.

It doesn’t look like lasagna, but it’s a heck of a lot easier and more closely mimics a natural mulch layer than lasagna does.

Looks like larch!

Great debate on the identity of Friday’s mystery plant! Several of you guessed larch, and this particular one is contorted larch (Larix kaempferi ‘Diane’), a cultivar of Japanese larch:

(As an FYI to a side discussion in the comments, tamarack is the common name for American larch, Larix laricina.)

And the eagle eye award to those of you who saw that YES! WE FINALLY HAVE A SEARCH FUNCTION on the blog! It’s still being tweaked so that the results pages will be a little more sophisticated, but it works. Now you don’t have to sort through hundreds of posts to find that one of Holly’s on pee bales. Or Jeff’s on drunken slugs.

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.

Heck, it’s hellebore!

You were too smart for me this week.  Though my husband was convinced that Friday’s photo was not enough to help identify the plant, JRR, Foy, and an unnamed commenter all recognized hellebore:

Always a welcome sight in the spring.  And don’t be fooled by those old wives’ tales that hellebores don’t like wood chip mulches. These beauties have been in wood chip mulches all their lives, and not only do they do great, their seedlings do as well.

A salt bath for your tomatoes?

This morning I got an email from one of my gardening colleagues, wondering about the wisdom of watering tomato plants with salt water.  He had a link to a UC Davis website which tacitly endorses spraying tomato plants with 10% salt water “to increase their nutritional value and taste.” Unreferenced “worldwide studies” are mentioned, along with the “major potential benefit of providing irrigation for crops in areas with freshwater restrictions.”

Before we deal with the impracticalities and out-and-out harm of using salt water for irrigation, let’s look at why this practice would work on tomatoes.  By training I’m a plant stress physiologist (and I’m well versed in the primary literature on this topic).  Watering tomato plants with a salt solution imposes a drought stress on the entire plant, as less water is taken up under these conditions.  So leaves and fruits are smaller and they may produce stress-induced biochemical compounds in response.  The upshot is that you have smaller tomatoes with a higher concentration of various solutes, some of which might be tastier or more beneficial to humans.

Guess what?  You can do the same thing by decreasing irrigation during fruit set!  Less water means smaller fruit and increased concentrations of sugars and other plant compounds, and voila!  So you can skip that step of adding salt water and just cut back on irrigation to induce a mild drought stress.

So…why in the world would you dump salt water on your garden soil?  The article blithely dismisses this:  “Many are still concerned about salt causing soil degradation and rendering some seawater-treated tomatoes inedible, but scientists cite that plants thrive in balanced soil containing both macro- and micronutrients.”  Sorry, but sodium is NOT a micronutrient for most plants and does NOT contribute to a “balanced soil” in one’s vegetable garden.

An ironic twist to this whole article is that most of the research that’s been done is relevant to arid parts of the world (the Middle East, primarily) where saline soil conditions and limited water are common.  I can’t imagine what they would think about people who would deliberately contaminate good soil by adding salt water to it.

Wet trunk – the whole story

We had a flurry of discussion on this over the weekend. The diversity of possible answers shows you how difficult it is to do diagnosis with only some of the information available. That being said, several of you (Gail, Tom, Dave and Jimbo) all had portions of the problem identified. Here’s the whole picture:

As both trunks of this double-leadered tree have continued to expand in girth, they’ve created the perfect conditions for disease to occur in the narrow constriction between the trunks. Though I’m not a pathologist, I would agree with an initial diagnosis of slime flux.

In addition to the poor structure and disease issues, the location of this tree – next to a street, sidewalk, houses, and across the street from a school – simply shouts for removal before it inevitably fails and causes damage or injury.

Nice job – and thanks for doing your homework over the weekend!

Friday puzzle: wet trunk

Another diagnostic question today.  Below you can see the lower portion of a tree trunk whose left half is obviously wet:

What is directly causing the wetness (in other words, what environmental factor), and can you guess what led to this problem indirectly?

I’ll eliminate some of the obvious possibilities:  it’s not from dog pee, nor is it from a directional sprinkler.  And the answer to the second question is not in this close up photo, but will be revealed on Monday.

Why do I torment you like this?  Well, if nothing else, it gives you an idea of what distance diagnosis can be like – which is what we tend to do more and more, thanks to the internet and digital cameras.  Someone will send a photo of a plant problem and expect us to figure it out, but the answer may lie outside the field of view.

In any case, have fun!