REGULAR READERS know that I preach the doctrine of, “Know thy weeds.” I’m a passionate believer that you cannot possibly subdue plants whose life cycles and survival strategies you do not know—and you can’t research clues to those workings if you don’t even know the plants’ proper names.

One of the best resources ever for those wishing to know their weeds is the book “Weeds of North America,” published in 2014 by University of Chicago Press, and co-authored by Richard Dickinson, with France Royer. Since its release, it is always at the ready here—with information about 500 species, plus photos of most every one at every life phase from seed to seedling to full plant and leaf and flower detail. There will be no mistaking weed from wildflower or garden plant again. 

Toronto-based Richard Dickinson has taught plant taxonomy for more than 25 years, and he joined me to talk about every gardener’s favorite—or is it unfavorite?—subject, weeds. I learned how they get so good at being weedy, and what their environmental impact is beyond taking space away from desired plants (spreading plant diseases, or harming monarch butterflies, for example). I also got a 101 on how gardeners can learn to “key out” one plant from another, using simple clues like a taxonomist does.

Read along as you listen to the May 16, 2016 edition of my public-radio show and podcast using the player below. You can subscribe to all future editions on iTunes or Stitcher (and browse my archive of podcasts here).

my weed q&a with richard dickinson

Q. What got your started with weeds? I have to ask. It’s an interesting path; I get when people are into plants—and of course weeds are plants. [Laughter.]

A. It’s interesting because our first few field guides were wildflower books. Then somebody said—a friend of ours who worked at [the department of] Agriculture—said, “You should do a book on weeds.” Most of us don’t know how to identify them. We sort of fell into this, and it has been a great ride.

Q. I suppose we have to start with that question that poets and other literary figures have philosophized about: What is a weed?

A. A weed is simply an unwanted plant. I know it’s pretty subjective, because what’s one person’s weed is another person’s wildflower. I know a lot of people like to grow Queen Anne’s lace, or wild carrot [Daucus carota], because it’s pretty, it’s beautiful. But it’s quite invasive once it takes hold.

I was thinking about this today when I planted my castor beans in the yard. They’re huge, tree-like perennials in Florida, where they’re a really bad weed. Where I live [in Toronto] it’s an annual, so I don’t have to worry about it as much.

It was sort of like: It’s my wildflower, but it’s their weed.

Q. Do you just grow it as an ornamental? One of the red- or green-leaved ones?

A. I like both of them, and they give a great tropical feel to the yard.

Q. Yes, they’re good. See it didn’t take me long to derail us off weeds onto ornamental things that give a tropical feel [laughter]. I love foliage plants.

So a weed is a plant in the wrong place, or an unwanted place. You said you and France Royer have co-authored some wildflower guides, so how do you distinguish the one from the other? Is it just how much of a problem it becomes?

A. I tend to think of the difference as weeds being non-native plants, or introduced species, and wildflowers as native plants. That’s sort of where I draw the line, although it can be pretty gray as well sometimes, I’m sure.

Q. Of the 500 plants covered in the book—and hopefully I don’t have all of them in my yard. [Laughter.]

A. I hope not, either.

Q. What do you suppose the ratio of native/non-native is, since not every weed is alien?

A. In the book we have about 90 percent that are non-native. The other 10 percent are native to North America. But it depends on what area of North America you’re in—a plant that’s native to California may be classed as a weed somewhere else in North America. It’s pretty subjective.

Q. If I want to become a successful weed, what qualities should I acquire?

A. I’ve got a list here. [Laughter.] Probably some of the most important ones are deep, fleshy roots. Like field bindweed [Convolvulus arvensis], where the roots can be 30 feet deep. The gardener is going to dig forever, and probably never get all of it.

Q. When I was a little girl, my mother would say: “If you keep digging, you’re going to go all the way to China.” I guess field bindweed goes all the way to China.

A. [Laughter.] Another characteristic would be seed dormancy. A lot of seeds can remain viable in the soil. Common mullein [Verbascum thapsus], they’re proven, can be viable in the soil for up to 100 years. If you let it set seed a couple of years in your garden or your field, there is going to be a seed bed there for as long as you live.

And high seed production is another characteristic. With purple loosestrife [Lythrum salicaria] large plants can produce up to 2.7 million seeds per season.

Q. Wow.

A. And they also float, so if you’re anywhere near a watercourse, they’ll float downstream.

Q. That’s a great dispersal method; you can really move a long distance.

A. For sure.

Another thing is allelopathy. I liken it to plant warfare. Some plants produce chemicals that kill off other plants, and that’s how they would get a stronghold in your yard or gardener.

Q. As a gardener for many years, there are many plants I grow—desirable plants—that I know and love, but nothing grows under them, like certain shrubs, and of course the famous black walnut. It has its own turf. I wonder if garlic mustard [Alliaria petiolata] has that quality.

A. It does.

Q. My observation visually would be that it does.

A. There has been quite a bit of research done in garlic mustard, and what they’re finding is that when it pumps out the chemical into the soil, it inhibits sugar maple seedlings. You often have a very old sugar maple forest, and no seedlings underneath it—due to the allelopathy of the garlic mustard.

Q. I read in the book that a large plant can produce 7,800 seeds.

A. I actually saw some old stems today, and it made me shudder.

Q. [Laughter.] I can’t say that I am too kind to that one; I’m always on the warpath.

A. Supposedly you can eat it, but I can’t imagine it tasting well.

Q. That extended seed dormancy is really a clever trait, because you think you get them all out, and then the next time the conditions are favorable, another generation emerges.

A. Yes. Especially if you till the soil, you bring the seeds up to the surface, giving them a chance to germinate.

Q. Getting beyond what’s a weed, the book is so helpful. I know you’ve heard this a million times, but the pictures: The fact that you have not just the plant description and the common names and quick ID tips and so forth, but also a photo of every phase in the life cycles [above, the images from the Canada thistle entry, Cirsium arvense] from seed to the full plant, with flower detail and leaf detail and seedling. So that no matter what age the thing is at, you can learn to identify it.

Now, you’re a taxonomist; you have well-honed tactics for keying out species. It’s all probably second nature to you. If gardeners want to develop such skills, though, it’s great that they have the book by their side—but where should they begin? How can they learn to look more closely?

A. I think of taxonomy as a puzzle; you’re getting all these little pieces from the plant you’re looking at, and putting them together.  When I’m out teaching, at an extension course or to whomever, the first thing I look for is what’s the leaf arrangement on the stem.

Are they alternating, or opposite? Are they all at the base of the plant, or appear in a whorl on the stem?

The other thing I look at is leaf complexity: Is it a simple leaf, or a compound leaf. A compound leaf has several leaflets, like clover would have three leaflets.

Other distinguishing characteristics are flower color, and fruit type. Is the fruit fleshy, or is it dry? How many seeds are inside?

If gardeners can home in on those first few characteristics, it really helps them out. The keys that we developed in this book—those are the characteristics that we used. It guides the reader through all the possibilities, starting at 500 and getting it down to five or six photographs that they can look at and say, “Oh, that’s the plant I’ve got.”

Q. When I began to realize that even though I could as a gardener visually recognize, “Ah, that’s a weed, I want to pull it out,” that I didn’t know their names, I started a practice to learn more of them.

What I would do is the same thing you can do if you want to have someone help you weed your garden: I would take a tray or baking sheet and pull up one of each one—dig it out, so I had all the parts—and I lay them on it. I took pictures of them. I found that forcing myself to really look really helped a lot.

I saw what their roots were like; which ones had rhizomes; I got to know what I was up against, and learned to name them.

A. It helps in the case that you just mentioned to see that annuals are going to have a taproot, and it’s going to be fairly short, just a few inches into the soil. Then you get into the rhizomes, the plants that spread like crazy and make you want to pull your hair out. So yes, looking at roots as well is perfect.

Q. Some of them have quite impressive arrangements going on below ground, and don’t want to be eradicated. [Laughter.]

A. It was funny, because when we were photographing the seedling shots for the book, I actually got to a point where I could recognize family traits just by looking at the seedling.

Q. Interesting.

A. After awhile, patterns start to appear, and I’m sure your listeners and the gardeners out there would be able to pick up on that soon.

Q. I love that you include all the common names, too, which is often the only one people know if they do know any name for their weeds. I try to learn the Latin names, but some of the common names are hilarious. I have a longtime gardener friend, Marco, who grew up calling bedstraw, or Galium, sticky willy.

A. I’ve heard that.

Q. I was so happy to see that was one of the common names you listed for it.

A. [Laughter.] It’s interesting: Because it was “Weeds of North America,” we tried to get as many common names as we could for each of the plant species, because it’s such a vast area. What somebody on the East Coast might call one plant, the West Coast people would have a completely different name for.

Q. I knew a lot of the name of “my weeds” already, so I looked them up in the book index to learn more about them. What I learned was that they’re not just there making me crazy as weeds—meaning plants in the wrong place—but they also have complex environmental or agricultural impacts. Like they may be a host for a costly disease.

Many common-to-me weeds like Galinsoga ciliata [above] or like Daucus carota—Queen Anne’s lace—they aren’t just taking up space and pushing other plants out.

A. A lot of weeds tend to harbor various fungal pathogens or viral diseases, or they attract insect hosts. In the case of Queen Anne’s lace or wild carrot, it’s closely related to the common garden carrot. If you’ve got the two growing side-by-side it’s easy for the transfer of insects and disease and pathogens from the wild stand of carrot to your garden carrots.

Another one would be common chickweed—we all love to pull chickweed.

Q. [Laughter.] I guess you could say “love,” but I don’t know.

A. It’s a harbor for viral diseases that cause damage to beets, cucumbers, peas, tomatoes, turnips.

Q. I didn’t know that; it’s just astonishing.

A. And another one that came to mind was common barberry, which the farmers planted 100 years ago as a shelter belt.

Q. Is that thunbergii or another species of Berberis?

A. Berberis vulgaris. It turned out to be a host for wheat rust, which caused massive losses of yield for wheat across North America. Just recently, and you just mentioned it, Japanese barberry or B. thunbergii, they have found it’s a host for fungal pathogens for other cereal crops as well. In some places they have started an eradication process for that as well.

Q. So it backfired in more ways than one.

Speaking of strange common names, you are doing work with a weed called the dog-strangling vine. I’m not even sure I can pronounce its Latin name.

A. It’s Vincetoxicum rossicum. In Canada we call it dog-strangling vine; in the Northeast, mostly in New York, they call it pale swallowwort. It’s an escaped ornamental vine that has flesh-colored flowers [above]. One of the problems with it—and I am currently finishing up my PhD on it—is that monarch butterflies mistakenly lay their eggs on it.

The reason for that is that it’s in the same family as common milkweed [the dogbane family, or Apocynaceae]. With dog-strangling vine, the caterpillars that emerge from the eggs feed on the plant and die because it has some toxic compounds in it.

Some of the research that’s taken place in New England states shows that part of the decline of the monarch butterfly is attributed to the dog-strangling vine, or pale swallowwort.

Q. I wasn’t aware that it was an escaped ornamental.

A. It came from the Ukraine in the 1890s. There are kind of two epicenters: the Toronto area, where I live, and another upstate New York.

Q. I am a little farther downstate, but I have friends a bit farther north who have this. It’s not a plant I have ever seen, but again: speaking of impacts that I didn’t know about weeds, besides just the real estate that weeds are taking up, displacing more desirable plants.

A. It’s allelopathic as well, dumping chemicals into the soil, so it has no trouble outcompeting native vegetation. That’s one of the problems.

Q. The book is not about weed control, but I feel like one could draw some inferences reading about the life cycles of the different plants—like for instance that garlic mustard only reproduces by seed, or that ground ivy (Glechoma hederacea) does so by both runners and seed. By learning the life cycles of these plants, with this book, can we infer some control hints?

A. I would say so. When you think of plants, you have annuals, biennials and perennials. Probably the most important hint for all three groups of plants is preventing seed production. If you can start to starve the seed bank in the soil, that’s great.

For annuals, just prevent flowering and seed production, and hopefully in time you can deplete the seed bank.

For biennials, if you miss the seedlings one year, the next year is where they are going to flower and set seed, so that’s when you want to get a hold of those guys. Garlic mustard is in that group, and burdock.

Q. Oh, yes.

A. [Laughter.] We all love burdock.

For perennials, it’s sort of species-specific. With dandelions, for instance, you can dig up the roots. I know it’s a pain, but it’s one way of getting rid of them. But we get back to the field bindweed, with roots up to 30 feet deep—you really can’t do that.

Whatever works best in your area, and for the species. Like you mentioned earlier, getting to know your adversary is the best way for control.

Q. Are there weeds that you are most asked about consistently? Are you like the doctor at the cocktail party, “Oh, I have this ache in my back, Doctor.” Are you asked about dandelions when they hear you’re the weed guy?

A. [Laughter.] Dandelions always, yes—especially at this time of year, when things are starting to green up, and you have this yellow carpet in the park, and you’re downwind from the park and just dreading the seeds coming onto your property.

Another one I get a lot of questions about is giant hogweed [Heracleum mantegazzianum, below].

Q. And that’s kind of a scary plant.

A. It is. It’s a large carrot-like plant that can grow 10 to 20 feet tall. It’s monocarpic, which means it only sets seed once in its lifetime, then it dies. It’s really not a biennial, and really not a perennial—sometimes it takes three, four, five years for it to set seed.

One of the problems with it is the sap in the plant. You basically have to be dressed in your HazMat suit if you want to go out and take care of it, because the sap causes severe blisters and your skin turns black. Your sensitivity to the sun in those areas where you have been burned can last for many years. It’s one that’s gaining a lot of attention because of its human health hazards.

Q. Dandelions sound tamer by comparison, don’t they?

A. Most definitely.

Q. I’m grateful for the book—though I know it sounds silly to say that to you. It’s so thorough, and those photographs and the clarity. Know thy weeds—it makes that possible. It’s on my table all the time.

A. The interactive e-book is coming out later this summer, and it will even help you identify the plant a little easier, so I am looking forward to it.

enter to win a copy of ‘weeds of north america’

I’LL BUY ONE lucky reader a copy of “Weeds of North America,” by Richard Dickinson and France Royer. All you have to do to enter is answer this question in the comments box, all the way down the page after the very last reader comment:

What’s the weed (or weeds) that unhinge you the most, and your tactic of assault?

(I’m a hand-puller or vigorous digger of a bounty including dandelions, garlic mustard, bittersweet, hedge bindweed, spotted spurge, prunella and mugwort, chickweed, ground ivy, galinsoga and commelina, and the list goes on and on and on–but so do I, in my regular rounds to keep things somewhat managed. It comes with the territory in a garden.

No answer, or feeling shy? Just say something like, “Count me in,” and I will, but a reply is even better. I’ll pick a random winner after entries close at midnight Sunday, May 22, 2016. US only; good luck to all.

prefer the podcast version of the show?

MY WEEKLY public-radio show, rated a “top-5 garden podcast” by “The Guardian” newspaper in the UK, began its seventh year in March 2016. In 2016, the show won three silver medals for excellence from the Garden Writers Association. It’s produced at Robin Hood Radio, the smallest NPR station in the nation. Listen locally in the Hudson Valley (NY)-Berkshires (MA)-Litchfield Hills (CT) Mondays at 8:30 AM Eastern, rerun at 8:30 Saturdays. Or play the May 16, 2016 show right here. You can subscribe to all future editions on iTunes or Stitcher (and browse my archive of podcasts here).

(All photos except galinsoga copyright France Royer, from “Weeds of North America.” Used with permission. Leaf-shape illustraftion from Canada department of agriculture. Disclosure: Purchases from Amazon affiliate links yield a small commission.)