Hot new method of weed control?

There’s a new report out from University of Copenhagen on killing weeds between paving stones. What they recommend is burning or steaming the weeds lightly and repeatedly. Boiling water, steam, even flamers can be used to wilt the leaves over the course of several treatments (six was recommended). This process damages the leaves beyond repair, slowly starving the roots to death.

I’m not sure exactly how I feel about this study (which is getting a lot of attention on the internet). On one hand, it is a chemical-free way to kill weeds…but on the other hand, it’s pretty labor intensive and requires energy inputs for generating heat. Moreover, what does one do once those weeds are gone? Those bare patches of sterilized soil are just going to be recolonized by new weeds.

Several years ago I had a Master’s student look at different methods of killing English ivy. She also tried the steam treatment.  Ivy laughs at steam. Aggressive perennial weeds like English ivy or blackberry or Japanese knotweed are unlikely to be much affected by blanching, and certainly not by half a dozen treatments.

But most of us probably don’t have big, woody-rooted weeds amongst our paving stones.  In my own garden, it’s a mixture of species that fill these gaps and some of them – like mosses and some smaller ferns – I actually enjoy.  So I pull out the things I don’t like, leaving the desirable species to fill in the gaps.  It’s simple and requires no special equipment.

Am I missing something here, or is this really much ado about nothing?

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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 an Associate 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 - "The Garden Professors" Facebook group - Books:

17 thoughts on “Hot new method of weed control?”

  1. In agreement about pulling out what I don’t like, and encouraging what I do like. I had a beautiful patio at my last house, with Elfin thyme filling the gaps. My only use for a flamer is on my decomposed granite paths. After the first rains, when the weeds have just sprouted (one or two pairs of leaves) a quick pass cleans up the whole path. Yes, some energy input, but very little if the weeds are very small. Many people don’t know that you don’t have to burn the plants to a crisp. A quick pass is all it takes – the cells burst, but the plants may not look dead until the next day.

  2. Pulling weeds is fairly easy after a rain or irrigation and it’s a satisfying way to get some good exercise. If done regularly it also does essentially the same thing as the heat treatment, robbing the plant of it’s top growth which ultimately starves the roots. Of course, if the area is a large one, it can become quite a chore. Gravel driveways may be the major exception. Pulling weeds there is backbreaking and hard on the hands as well. (One friend firmly believes the best way to have grass is to sow gravel!)

  3. I would rather have a line of green between my flagstones than bare dirt or (worse) empty space. I don’t have those tightly interlocked pavers. So a weed whacker is all I need, ever. No need to kill.

  4. I have paths made both of pavers (which have a small amount of soil/sand between them) and stone (which have larger open areas).

    I quite like moss growing between pavers/stone steps. What I don’t like is the grass, dandelions, plantain, etc.

    I use a mix of approaches to keeping out the undesirables. Sometimes I weed, though we have a big area to cover. Sometimes I use a weed wacker. I must confess that once or twice per year I might use Roundup. When I do this I make myself skip desert after supper as punishment.

    Finally, I’m trying to introduce plants that in theory would create a mat between pavers/stones. Just recently I planted some Australian violets in a shady path. We’ll see how they do.

  5. It’s not uncommon to do that here in the American southwest. Propane tanks with hoses and igniters, sometimes strapped to moving dollies. Very energy intensive.

  6. I can understand the need for a different type of weed control in gravel, decomposed granite, etc. In fact, I like the idea of using a flamer over gravel rather than pouring on pesticides. I think this study was limited to solid rock surfaces, however. In any case, I couldn’t find more detailed information (it’s from a recently defended PhD thesis and not yet published for wider distribution.

  7. Ironically, I just recommended boiling water to manage grass coming up between a small area of pavers to a client earlier today! Meanwhile, the term for all of these “weed” management practices is “thermal vegetation management.” Do a search and you’ll uncover a number of formal reports (dozens to hundreds of pages in length) from departments of transportation, etc. assessing their efficacy.

  8. We have a large paver patio and two paver walkways. I’ve used the flame torch but this spring I bought this handy tool from Garret Wade- a narrow wire brush on a handle. I keep a pretty tight rein on the weeds to begin with but this tool works great if the weeds are small and fairly random. Otherwise a apply a commercial pre-emergent or the long acting round-up in the spring.

  9. Doesn’t even slow down horsetail – which was on my property when we moved in and will be here when we move out!

  10. I didn’t understand the part about this being a new study, maybe it’s the steam method as I have seen the weed torch for several years now. Over here in Sweden they do many pathway with pavers and there is no getting around the weed popups. Favourite method of removal is the propane torch or weedwacker with fishing line. (I know it’s not really fishing line) I use the weeder or grass edger. The main target is not mosses or lichens but dandeloins and grasses, even some occasional Sow Thistle. Sure it comes back but then so do weeds after folks kill things with Monsanto products.

  11. Isn’t the current mindset the issue here? Outside of the necessary passages to our houses from, say, the driveway, much of paving is ornamentation. But once the ornamentation is in place, it becomes an extension of our home, and many people want the outer “rooms” to be as clean as the hardwoods or countertops.

    Gardening is nothing if not control. I just think that the controlling part gets out of balance sometimes, trending from the art of WORKING with nature to simple OWNERSHIP of an always-photogenic piece of property that probably has an immaculate patio or entryway. I’m not saying that it’s not important to keep things like Campsis from sprouting next to a foundation or poison ivy away from kids, but here in Atlanta “landscapers” use an abominable amount of gasoline on making the outside look as clean as the inside. (I recently spoke with the leader of a mow-and-blow crew. He told me that he uses a tank of gas for his box truck every day and that they go to fifteen accounts every day, too. That’s four guys with small engines. They also take all of their clippings to the dump, and because recent rain has made their local landfill impassable, they have about 500 bags waiting at their headquarters to be buried.)

    While garden magazines and television shows can be informative, a lot of folks buy them or watch them for the pretty pictures, in which everything is very clean, nearly perfect. If they offer the examples that the majority of people aspire to have, then the expectation of a completely weedless patio or path seems like a predictable outcome.

    I think that slowing the process down and changing expectations is the way to go. I think that too many of us (myself included) strive for a measure of beauty, control, or both that exceeds what we need to make ourselves simply happy.

  12. Unfortunately for me my tongue-in-cheek post title didn’t come across well. This really is not a new treatment for weeds (as I alluded to later with our own study on steam and ivy control). Nor do I think it’s the best way of managing weeds between pavers and other solid surfaces (decomposed granite and gravel are another story). I’ll try to do a future posting with pictures of ground covers that work well between pavers.

  13. Linda, please include Dwarf Mondo grass in your assessment. It was planted when my flagstone patio was put in & has been very slow to fill in. Zoyzia & clover have aggressively attached to the mondo risomes & took over in many places. I’d be forever grateful if you have any suggestions to fix that problem.

    Moms aka Eve

  14. Paving is great to prevent muddy grass paths. Weeds and grasses between the pavers are so invasive here (midwest) that there would be no path if something isn’t done to get rid of them. If pretty little flowers and mosses came up of their own, that would be great, but that’s not what happens here–a neighbor doing a landscaping project just dug out a buried 2 and half foot wide (!) brick path that the lawn had simply grown over. Similarly, I’ve dug out paving blocks from 2-3 inches down in my own yard. So, it’s either hand weeding, weed whacking, propane-burning or Round up. If there were a steam generator which could do the job, and if it were more efficient than a propane torch, I’d totally be interested. Different conditions require different responses, I guess.

  15. I have used boiling water on my driveway cracks to take care of persistent weeds. Much better than using herbicides (which were NOT effective at all, the weeds were back within days). But I wouldn’t do it for a big patch of land or on paving stones.

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