proper transplanting, mulch, and more: linda chalker-scott’s horticultural myth-busting
THOUGH 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
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.
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.
A. 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.]
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
- Linda’s garden myths page
- On The Garden Professors blog
- On The Garden Professors Facebook page
- Join The Garden Professors Facebook group (and ask a question yourself)
(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.