proper transplanting, mulch, and more: linda chalker-scott’s horticultural myth-busting

Linda Chalker Scott booksTHOUGH GARDENING is part art and part science, we shouldn’t take artistic license with the science part and defy the kind of sound information that can help us succeed. Linda Chalker-Scott has the science nailed, with a PhD in horticulture and a double minor in biochemistry and botany–and a continuing passion for pursuing the latest developments in all of the above.

She is an Extension Urban Horticulturist with Washington State University, and an associate professor of horticulture and landscape architecture there—and joined me this week on the radio podcast to talk (and debunk) popular garden myths.

You know, like whether you should dig a really big hole for trees and shrubs and amend the soil before backfilling. Or whether gravel in the bottom of a container helps drainage, or bone meal is a must (or a bust) for bulbs. Or whether landscape fabrics are really the miracle they claim to be—that has so many people using them as “weed block.”

Linda has been scouring the scientific literature; testing horticultural products under controlled, scientific conditions to support or disprove marketing claims; and basically challenging and often busting garden myths for years, including in her award-winning 2008 book “The Informed Gardener,” and the 2010 sequel, “The Informed Gardener Blooms Again” (affiliate links).

You may know Linda Chalker-Scott as one of the team of esteemed collaborators on the popular Garden Professors blog at extension.org. They share a Facebook page called The Garden Professors, were Linda and her colleagues post stories and start the discussion about all things garden-science related. In a related Facebook group, more than 1,000 members can begin a new topic or ask questions.

Listen in to Linda’s and my recent chat, or read on, or both.

my q&a with linda chalker-scott

LCS1 - Copy (426x446)Q. You’re definitely not one of those people who just accepts package claims or marketing hyperbole, are you?  Do you remember the first horticultural myth you busted?

A. I’m not, and it doesn’t make me very popular with some people. [Laughter.]

Q. Do you remember the first myth you “busted,” the first horticultural thing that didn’t sound right to you that you challenged?

A. A lot of the myths are things I used to do because I just blindly followed what I thought was right. The first myth I challenged was the one about putting gravel in the bottom of a container to help with drainage.

It makes a lot of sense—a lot of these myths do, because they just sound logical. But it turns out if you put gravel in the bottom, it actually makes the drainage worse.

Q. So from there, you just couldn’t help yourself—you had to challenge everything. [Laughter.]

A. I couldn’t, because so much of what I had learned getting my PhD really wasn’t grounded in good landscape horticultural science. It was pretty much agricultural-production horticulture that had been more or less misapplied to gardens and landscapes.

Q. So it’s fall—which I think of as a good time for planting certain things, but maybe I should first ask if that “fall is for planting” idea is a myth, too? I read a “myth” paper you wrote on how Arbor Day, for instance, which is also promoted as prime planting time, really isn’t in some areas of the country (such as the West).  

A. Fall is actually the very best time for planting—especially trees and shrubs. The reason is that the roots never go dormant. While the upper part of the plant is asleep, the roots are busy getting established, which kind of gives them a jumpstart on spring. So when the buds are ready to open and the leaves flush out, you’ve got a good root system established.

Q. In my area, the Northeast, fall is usually also often moister, which I think is good for transplants—not so stressful.

A. A moist soil is better for root establishment, plus it helps buffer temperatures. A dry soil tends to freeze faster than a moist soil will.

Q. At Arbor Day, by comparison, in April—that’s when soils in many areas are heating up and getting drier, no?

A. On our side of the country, in the Northwest, we have a very droughty summer, so planting in spring here, for instance, can set you up for failure. It’s getting drier and drier as the root system is trying to establish.

Q. Let’s talk transplanting woody plants—which has many conventional wisdoms attached to it, depending who you ask; things like whether to backfill with amended soil or not, and whether to prune roots or score the rootball, and so on.

A. With trees and shrubs, these are things that you’re not going to be harvesting, and that will last decades if not centuries. What you want to do is get those roots right in there with the native soil.

Even though we used to have this mantra, “a $5 hole for a 50-cent plant,” adding all this stuff really isn’t the best way to get things established. It sounds good, to put in all these amendments, all this fluffy soil and fertilizer. But what that does is set the plant up for failure, because the roots get into that and they don’t want to leave. You’ve got to take the tough-love approach.

Q. Remember all the bales of peat moss that we were told to buy—bring home the shrub, plus a bale of peat to plant it with?

A. If you’re doing a container, it’s different, because it’s separate from everything else. But when you’re planting into a landscape where you’ve got a native soil, if you start making that soil in the planting hole different from the surrounding soil, roots won’t pass that boundary line. They pretty much stay in the amended area and circle and circle around.

Q. Not good!

A. No. Circling roots are a bad thing. [Linda’s video below shows what can result from circling, girdling roots.]

Q. So we’re not going to give the tree or shrub a false sense of security in this extra-cozy space with phony amendments. What about preparing the roots before we lower the tree or shrub into the unamended hole?

A. This gets into a real controversial area, and the science is evolving. But the evolving science tends to support more and more that we don’t just leave the rootball alone; that we don’t just handle it tenderly and put it in. What we want to do is disturb the rootball.

This means taking off anything that is not plant-related. Off comes the burlap, and the wire basket, and the pot, of course, if it’s in a container. And then getting those roots loosened up so they’re not going in a circle. This can be done with water, or your fingers—you need to get all that stuff loosened up; a lot of the soil falls off.

Then you’ve got a rootball that hopefully looks like spokes on a wheel [photo below] rather than a circling mass. Then they’re going out in the right direction, and it gets them right next to native soil that they need to get established in.

The more you can disrupt, the better it does—and again, it’s one of those things that doesn’t sound like it makes sense, but when you think about how plants respond when you start pruning the roots—like bonsai people do—and it stimulates rooting. When you’re disturbing the roots and some of the finer ones break off, that actually stimulates new roots.

OLYMPUS DIGITAL CAMERAQ. So with woody plants, this is very important.

A. It really is, because they’re there for years and years. With a lot of herbaceous things, that really don’t get that big, it maybe isn’t as much so.

And another issue with the woody plants: With the mass of woody tissue in a tree, if you amend that hole, what happens is that the weight eventually causes the hole to sink, so you’ll see a lot of trees planted with a heavily amended soil, they end up in a pit.

Q. And there’s nothing that a tree or shrub loathes more than sitting sunken in a hole.

A. No, because that’s a great place for water to sit. Unless you’re planting a bog garden, they really don’t like that very much.

Q. What about washing the roots?

A. That is the most extreme thing to do, and personally what I do and have been doing research on. I call it “taking it all off”—the bare-root method. You just use water to get everything away from the roots, and the reason is that with plants that have been left in a liner pot too long, for instance, you’ll sometimes discover some circling woody roots that will never straighten out. So getting in there and pruning those out means you will have a healthy root system.

You also end up having a much smaller hole to dig, because you’re not planting a bunch of clay, or [potting] media—you’re just planting roots.  Establishment is much faster.

Q. What about with bulbs: I have to confess, I never add any bulb food or bone meal. Is bone meal a myth, or a science-based must, or somewhere between?

A. Bone meal is a great source of Phosphorus, which is why it gets used. So when you look at it as a transplant fertilizer for bare-root plants or bulbs, you’ll see something about “stimulates root growth,” and “bigger, better blooms” and all that.

And again, there’s a grain of truth to this, because it turns out that in the absence of a lot of other things, the phosphate does stimulate root growth, but it does so at the expense of the plant.  So what happens if you add too much phosphate and the soil is overloaded, the mycorrhizae won’t grow.

Gardeners know that mycorrhizae are their best friends–those beneficial fungi that help plants take up extra water and nutrients. But mycorrhizae hate Phosphorus. If you put in bonemeal or any other high-phosphate fertilizer, the mycorrhizae won’t connect with the roots, and the plant has to grow extra roots to do that extra work.  So it’s not really a benefit to the plant, but it does it just because it doesn’t have its mycorrhizal partners there.

Q. So I can keep skipping it!

A. Absolutely. This whole thing of adding Phosphorus comes from fertilizers that have the N-P-K listed—and that’s all agricultural. It’s one of those agricultural things that’s kind of hung over into our gardening, and really doesn’t fit very well.

Q. I’m already starting on some of my garden cleanup, truthfully, as we’ve had a colder-than-normal season. I want to talk about putting the garden to bed, and the myths attached to that. I know the topic of mulching is positively loaded with them—such as bark vs. sawdust vs. wood chips.

A. Woody mulches in general are really great for landscapes, because they’re low in nutrient, and prevent a lot of the weeds from getting established, while your plants are still getting nutrients from the soil.

That being said, the second most important thing is the texture. When you have a really fine-textured mulch like sawdust it may look pretty, and uniform, but it doesn’t allow oxygen and water to permeate very well. You end up getting standing water, or compacted mulch, and the roots aren’t getting a lot of oxygen exchange and water infiltration.

So the big, chunky, coarse things like woodchips—my personal favorite—or even bark mulch, which I am not as wild about. But it has that 3-dimensional texture, and those are the best. [Linda’s garden-myths page about woodchips as mulch, and a pdf on how to use them.]

Q. So how 3-dimensional does it have to be? I use a medium-textured wood-based mulch that’s made from composted stable bedding. What I love the most is that besides doing the job of mulch, it gradually breaks down into the soil—so I can dig with my hands in my beds, after years of doing this.

The big bagged things—which I call baked-potato mulch, because the chips are sometime so big they look like baked potatoes to me…I don’t think of that as very in keeping with soil care. A big giant chip doesn’t appeal to me.

Tree skirtA. That’s one of the drawbacks of using the arborist chips, because you do tend to get some bigger chunks in there. That’s where the aesthetics come in, and I tend to pull the bigger chunks out myself.

You’re absolutely right: the size does affect the rate of decomposition, and that’s the great thing about wood mulch. If you use a wood mulch you’ll find it breaks down and creates better topsoil beneath it—it’s a wonderful soil-conditioner and soil-builder.

With the bark it doesn’t happen so much. A lot of these bark products are so waxy that they tend to mat into a plate type of structure and don’t break down very quickly. Things that have more wood than bark tend to break down faster.

Q. Do you like to keep a pile of chips, and age it, or use it right away?

A. There is some concern about disease transferring, and I’ll say right now that diseased chips don’t have an effect on a healthy soil. I have to say I get a pile of chips in my driveway and use them as soon as I can.

And if you use it right away you’re getting the Nitrogen, because that’s the first thing to go when the stuff’s composting is the Nitrogen, so if you get that onto the beds, then it’s leaching through into the soil rather than just onto the driveway or wherever the pile is.

Q. Though we may prefer different textures—I only use larger chips on rougher areas at the perimeter of the garden, not on perennials, for instance—I think we both agree that getting mulch onto your beds that’s organic (as in from a living or formerly living thing) is critical. And not “mulch” from things like shredded, recycled tires.

A. It’s the worst. I had a call from a woman in tears, who had about $3,000 of the stuff put down at her place, and it smelled so bad in her hot climate, she couldn’t go outside…

Q. …and not to mention that shredding up bits of rubber so that birds and animals and who knows who’s going to pick it up. [Linda’s garden-myths page about rubber mulch.]

100_5010Q. What about so-called geotextiles? I loathe landscape fabric.

A. I do more than loathe them; I hate them outright. They are the worst thing to use. They’re called “textiles” and “fabrics” because supposedly they breathe, but if anyone has ever seen them after awhile, once soil gets on them, they don’t breathe, and water doesn’t go through them. What happens is that weeds grow on top of them.

I call it “weed fabric” because it grows weeds really well…

Q. …as opposed to “weed-blocking fabric,” which is what the manufacturer would like you to call it.

A. If people don’t believe it, take a piece of the stuff, and rubber-band it over the top of a peanut butter jar, and try to pour water through it. Water doesn’t move through it very quickly, if at all, and if you put soil on top of that, it just stops [above photo].

Q. And I keep asking people: Why would you want to bury something like that in your soil? How much do you disregard your soil that you want to bury waste products in it?

A. Absolutely. [Linda’s garden-myths page about landscape fabrics.]

Q. Thanks so much for all this, Linda.

more from linda chalker-scott

(All photos from Linda Chalker-Scott.)

prefer the podcast?

LINDA CHALKER-SCOTT was my guest on the latest radio podcast. You can listen anywhere, anytime: Locally, in my Hudson Valley (NY)-Berkshires (MA)-Litchfield Hills (CT) region, “A Way to Garden” airs on Robin Hood Radio on Monday at 8:30 AM Eastern, with a rerun at 8:30 Saturdays. Get it free on iTunes, the Stitcher app, or streaming from RobinHoodRadio.com or via its RSS feed. The September 22, 2014 show can be streamed here now. Robin Hood is the smallest NPR station in the nation; our garden show marked the start of its fifth year in March, and is syndicated via PRX.

  1. Sharon Molnar says:

    My goodness. A lot of the stuff I’ve been doing out of sheer “laziness” is apparently correct. I hardly ever amend my soil, and if I do it’s with whatever leaves and mulch were still lying on top of the spot I am digging in.

    I really don’t baby my plants – I do a lot of research to try to only buy drought-tolerant perennials that will survive in zone 6 clay/shale soil with a heaping tablespoon of critter depredation. Once it goes in the ground, I often don’t touch it again except maybe to cut back dead foliage at the end of the season.

  2. heidih says:

    Thank you so much for the conversation and the links – bookmarked. Very interesting about the agricultural slant of research. Particularly love the planting info on prepping the roots and not amending the soil for the woody perennials. How much of that rubber tire mulch have I hand-picked out of people’s beds when replanting? – evil evil stuff.

  3. Cheryl Alloway says:

    AMEN! Thank you Margaret & Linda for sharing your research and wisdom. I’ve been telling my gardening clients these same things for years, sometimes to no avail. I guess some folks have a hard time letting go of old habits. At the risk of sounding like a broken record, I keep reminding them…. fall is the best time for planting, landscape fabric does not belong in the garden and gravel does not belong in the bottom of your planting pots! Your points for planting trees and shrubs is perfect. I’ll be forwarding this on.

  4. Lacey says:

    Two things make me curse out loud in the garden : landscape fabric and land planarians.

    I HATE landscape fabric. I’ve lived in this house for 4 years and I’m still digging and ripping it up. In some places it is 4 -5 layers thick through 18 inches of soil. It is a nightmare to get it all up.

    Thanks for busting the myth about fresh wood chips! I can get loads for free dumped in my driveway!

    1. Sharon Molnar says:

      I’m still pulling up quite a bit of it too, under about 12″ of soil. I only find out where the previous owners put the crap when I want to put in some new plants in an area where something else had died… often looking like it was still in a pot because they didn’t know you had to tease the roots out.

  5. Therese says:

    Great article and information! Our customers are often astounded when we stress there is no need to amend soil when planting and that peonies actually love clay soils.

  6. Ina Saltz says:

    Really loved all the mulching debunking! Read every word with great interest & since I have giant fresh piles of woodchips from taking down some trees recently, I’ll get out there and start using them this weekend. Also I realize I have not been mulching to the recommended depth so this info is especially helpful.

  7. Louise says:

    Thanks especially for the root information. I often saw a bush thrive and another of the same kind fail a few feet away. When I pulled one dying bush the roots still had the rounded shape of the pot.

    Cleaning the roots and pruning makes so much sense! Thanks so very much for the great conversations and links.

  8. Usha says:

    This article is very useful to me at a time when I am wondering if all the advise about 1) leaving fallen leaves on the soil to let them compost under the shrubs;2) adding lime chips and bone meal around Hellebore etc; and using lot of compost on soils in the Fall is good.
    I had heard wood ships are not good for the garden soil so I have been paying them to be removed when I have my large trees pruned.
    I am wondering if infestation of spider mites and lace bugs on my azaleas in PNW under which I have lot of leaves is due to the decaying debris? Any helpful advice on how to eradicate these pests organically and if leaving leaf mulch is a good practice or not will be most appreciated.
    Thank you

  9. Tom Brophy says:

    For many years I have been using sections of newsprint (black/white only, not colored) laid shingle fashion for weed blocking mulch in the pathways between raised beds. This material is covered by the leaves I collect in bags people put out for collection. The soil beneath stays cool and friable. Earthworms, fat, glistening, and sassy love it! Unlike artificial mulches, the paper and leaves disintegrate yearly, and need to be replaced. But it sure beats having to pull up landscape fabric choked with weeds. The leaves form a lovely forest-floor cushion underfoot, keep the paper from drying out, and look nice. Chopped straw mulch goes between the plants, which they all seem to like.

    1. margaret says:

      I do the same, Tom, and use either a medium-texture wood mulch (composted stable bedding) or shredded aged leaves on top in beds, and in paths I put somewhat larger chips on top. Thanks for saying hello.

  10. Ed Morrow says:

    We all need to buy copies of Linda’s books and hand them out to our friends. After all, what are friends for?

    I’ve got a couple of coast redwoods in y yard that are the wrong trees planted in the the wrong place – the work of the previous homeowner. In this recent California drought they were beginning to die – brown tips at the end of the branches.. Based on Linda’s observations and recommendations about mulch, I water the areas surrounding the trees and put down four inches of wood chips. After a couple of months the tress are beginning to show new green growth on the branch tips. My next problem – a decade away – is what am I going to do with these big trees.

    You can’t beat science.

    1. margaret says:

      Hi, Ed, and thank you for the first-hand success story. I am no scientist, but what I see in the natural forest on the floor is a duff layer of organic matter naturally deposited year after year after year — which is why mulching woody things (and even herbaceous shade plants) makes total sense to me, and always has.

    1. margaret says:

      Nice to see you, Matt — and yes, they are fascinating, and fun, too. (Of course as she and I talked about, I like my wood mulch less coarse…but you know how picky I can be.) :)

  11. Michelle says:

    I love moments of good timing. I’m about to plat a red twig variety Dogwood to be viewable from the living room’s picture window. My purchase was largely inspired by having just finished Backyard Parables a week ago. Your words must have been singing in the background as I wandered the garden center with nothing particular in mind. Garden centers would do themselves a favor by stocking your book : )

  12. Rita Wilson says:

    I loved this information. I planted a lovely crab apple tree in July. Have kept it watered faithfully. I did not do a thing to the roots and it has been bothering me, so I’ve been reading up on that. Now, this informative article convinced me that I needed to remove that little tree from the hole and fix the roots. I was planning on it anyway, so had been watering extra. Here is what I found – DRY ROOTS!!! They were so packed in there that only the edges were taking any water. The first inch was wet, the rest of the root ball was dry, dry, dry. I used a hose and sprayed it wet and as clean as possible. Separated the roots to the best of my ability and replanted. Now, I can’t prove it, but I think there is a big, damp smile on that tree!

  13. Beverly, zone 6, eastern PA says:

    I went to Linda’s Myth debunking site hoping to find information on using Pine Needles as mulch. I wanted her to tell me in print that they do not acidify the soil as time goes by. I could not find anything on that topic there. I did find a lot of other fascinating information, though.

    I favor Pine Needles in my beds for their free availability, ease of application, lightweight nature, uniform color and the way rain sinks right through them to the soil below. I am hoping they are not injurious in the long run.

    1. joanne says:

      It’s been my experience that the needles are acidic while green but have lost that as they age and finally when dead are not acidic at all. Use them.

  14. I stopped using bone meal years ago but only because it seemed to attract skunks. (I do think skunks are startlingly beautiful, but…well, you know…) What myths we live with. Thanks for dispelling so many with the help of Linda. The info about circling roots is really great – now I feel i have license to yank them around and chop ’em off without worry.
    On the other hand, back twenty years ago before I knew better I did use peat moss when planting my trees and shrubs, and dug the big hole, and was tender with the roots – and now these plants are enormous, tall and vigorous. Didn’t you sometimes plant things “wrong” back when and look at your garden! So what do you think?

  15. SusanName says:

    Terrific conversation. I would, however, like to comment on the wood chip mulches. There are wood chips and there are wood chips: composted wood chips, un-composted wood chip stable bedding, whole-tree with leaves or needles, planer chips, and “waste-wood” trunk/limb (no leaves/needles). These vary enormously in their N-P-K composition, pH and even alleopathic characteristics, all of which depend on the species of wood, whether the chips include green leaves/needles, exogenous sources of nitrogen – e.g. urine/manure. Stable bedding brings the enormous benefits of high-nitrogen urine/manure, so will compost readily and without the nitrogen-depletion period. Nitrogen, which is bound in plant proteins, I am pretty certain does not “leak” from the fresh, chips, but is taken up initially by the colonization and growth of bacteria and fungi that break down the chips. It is released later through their feces (bacteria not fungi) and later death and decomposition. If the wood chips are free of a sufficient nitrogen source for the colonizing bacteria (no manure, green leaves, etc.) they will take it from the surrounding soil – yielding the easily recognizable signs of nitrogen depletion. A well-composted wood-chip pile is balanced with nitrogen uptake and release from the bacteria/fungi. Whole-tree chips include nitrogenous proteins contained in the green leaves/needles, so will heat up and compost quickly, again potentially with minimal or no nitrogen-depleting period. If this is incorrect, I welcome comments/corrections.

  16. Marilyn says:

    Thank you so much for the information and the links. I love hearing that someone is actually researching some of the old gardening “wisdom” that has always been taken for granted as being the Bible of gardening.

    I mentioned your article and Linda’s research at a lecture given by my local County Extension Master Gardeners program, and whoo -eee! You would have thought I was a gardening TERRORIST for daring to question bone meal with every bulb, or using fresh arborist wood chips instead of partially decomposed triple shredded hardwood mulch.

    Guess I will just keep experimenting in my own yard, use what works for me, and learn to keep my thoughts to myself in those lectures from now on.

  17. Sue L'Hommedieu says:

    Finally, I can point people to an expert who states what I’ve been saying all along about landscape fabric. Sounds like a good theory, but is actually a bad idea in practice. Thank you!

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