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Question of the Week © 2017 Mike McGrath
Note: Hundreds of Mike's informative articles are available (in alphabetical order!) right here at the Gardens Alive website. To find Mike's answers to your specific garden problem, Click here and find your topic (like Weeds, Worms, Rhododendrons...) in our complete alphabetical archive of Questions of the Week.
Chasing Creeping Charlie
Q. I unknowingly made a huge mistake by mowing over a patch of creeping Charlie last fall and it has now taken over almost ALL of our backyard. We have small children so we wanted to solarize the entire yard with clear plastic sheeting instead of spraying poison. Is it too late for that this year? If it is too late, when should we try? Are there any other methods that are "safe"? We were hoping to have a nice lawn by next Spring but now I think that might be impossible.
----Holly in Silver Spring, Maryland
Q. My lawn is two-thirds covered with Creeping Charlie. My lawn guy recommended applying weed killer this Fall, but I'm balking as I hate chemicals—and he says my lawn would be all muddy in the Spring. So—If I use the 20 Mule Team Borax home-made herbicide solution as described in your A to Z article on creeping Charlie, when would be the best time to apply it? Or would you just leave Charlie alone to finish killing the lawn and be happy ever after? (Not counting my having to periodically weed the flower beds.)
---Susana in Minneapolis
A. Well first, don't beat yourself up too much about the mowing, Holly. You may have spread some seed around, but this weed (also known as 'ground ivy, 'lawn ivy' and 'Gill Over the Ground') mostly spreads via underground rhizomes, often overtaking lawns that are cut too short or that have to tolerate a lot of shade.
Now, soil solarization? Yes, it should work well in Holly's near-to-DC region, but like her Nationals (and the Dodgers, and the Phillies, and the Mets, and…), you'll definitely have to 'wait until next year'.
As we explain in detail in a (very popular!) previous question of the week, solarizing can destroy the absolute worst weed seeds, disease spores and invasive rhizomes lurking in soil, but it has to be done precisely and correctly. The area must be tilled up—or at the very least, scalped down to bare soil (we want to see dirt blowing out the back of the mower!). Then it has to be leveled, absolutely saturated with water and covered tightly with clear plastic (1 or 2 mil thick is ideal) for a good long time.
Here in the mid-Atlantic, it takes an entire summer of sun to achieve the desired result of cooking every bad thing in the soil—grubs, disease spores, weed seeds and the invasive rhizomes specifically in question here —to absolute death; making this a technique much better suited to reviving a single bed than an entire yard. AND effective solarization requires full sun or close to it. If the area is shady, solarization is not a viable option. (And 'creeping Charlie' is a weed of shady places. It also does well in the sun, but its specialty is to thrive in the kind of shade that makes lawn grass struggle.)
Borax: Yes, researchers did find a very specific concentration of the element Boron—in the form of the classic laundry additive 20 Mule Team Borax—to be more effective on creeping Charlie than chemical herbicides. You'll find the exact recipe in our previous article on creeping Charlie.
But that cure—made famous by Iowa State Turfgrass Professor Dr. Nick Christians (who also discovered the pre-emergent properties of corn gluten meal)—was popularized many years ago, before the new Iron-based broadleaf herbicides hit the market. And from what we hear from listeners, iron might be a better bet here. (Although if the area is big enough, you could try a sample of each non-toxic herbicide and see which one works best.)
Now: herbicides are going to work best on most weeds when the weather is hot and dry, but that's not the case here. Most sources specify that creeping Charlie is much more vulnerable to this kind of attack right now—in the fall, when the plant is storing energy—than in the summer. So if you are going to go after Charlie with Iron or the Mule Team, don't delay. Do a first run ASAP, and then repeat it a few weeks later.
And be warned that if the 'lawns' in question are almost all creeping Charlie, it will turn out just like the lawn guy said—the area will be all muddy in the Spring because you killed the only viable plant out there.
So first, let us make the pitch for leaving the bad thing alone; because while creeping Charlie is invasive, it's also a very useful ground cover. That's how it got its start here in the US—as an attractive groundcover sold to thrive in the shady spots where lawn grasses struggle. It prevents erosion, needs no mowing and produces beautiful purple flowers in the Spring that attract pollinators like 'the hairy-footed flower bee' (which like Charlie, emigrated to the US from Europe) and several species in the Osmia genus—the prime pollinators of blueberries and orchard fruits. Very good bees to have around—and gentle!
So one option is to consider making peace with Charlie. Install strong edging to keep it out of flower beds and neighboring yards and then enjoy the aroma (it's actually a member of the mint family), the beautiful flowers and the pollinators it attracts. (Plant blueberries!)
BUT: It is difficult to keep this plant contained. And while I would love to say it makes an ideal lawn alternative, many people suffer allergic-like skin reactions to the plant, making it a potentially poor choice for an area with lots of child activity. (But if it's in a low-to-no traffic area and you're diligent about keeping it in bounds, it does look very nice.)
Back to getting rid of it. A couple of direct assaults now should weaken it, but there will probably be survivors in the Spring. (Once established, this is a tough plant to control using any kinds of tactics.) So repeat the iron and/or borax in the early spring, before the plant really gets growing. Then a round of pulling (wearing gloves so you don't get a rash) could be very helpful if you're careful to go slow and low and get it out roots and all.
Then how do we establish the new lawn?
If our questioners were in warmer climes, they could cover the previously-weeded area with a nice load of compost and/or topsoil and install a warm season lawn in the Spring. (In years past, you had to install warm season grasses 'vegetatively' via springs, plugs or sod, but these days you can do it by seed.) Go with an aggressive, spreading grass that will have the ability to out-compete any missed rhizomes.
But in the mid-Atlantic and North, you probably want a cool-season grass. If the area is smallish (or if you just have the money) laying sod in the Spring is your first shot at a fast cure. Sod is much more expensive than seed, but high quality cool-season sod is widely available in the Spring and provides an almost-instant lawn for children and/or dogs to play right away. And the installers would remove all or most of the existing Charlie rhizomes as they prepare the soil for installation. And then the thick sod and its roots should prevail. But sod is expensive. And you'd probably want to hire someone to do the (hard) work of installation.
To get the best results from seeding a cool-season grass, you have to wait until August to install the seed; but you'll probably need the time in between to prepare. I'm not sure if soil solarization is viable up in "Minnie-Snow-Ta" (the season might be too short) but if Holly in Silver Spring wants to pursue this path, she now understands the commitment—that area would be covered in clear plastic and unusable over the summer.
Either way, do what you can to knock Charlie back now and in the Spring; and DON'T TILL! Tilling would essentially cut up and replant the rhizomes and make the problem worse. Maybe hit it a couple of times with non-toxic herbicides, scalp the area down to bare dirt, and then cover it a few inches deep with arborist wood chips for the summer. (Don't use nasty dyed wood mulch!) That would give the kids some non-mud to play on and could well smother most if not all of what's left of the Charlie.
Then in August, clear away the mulch, apply an inch of topsoil or compost, sow your new seed and keep everyone off of it for a month while it establishes. After that, it is critically important to care for the lawn correctly:
Never cut the grass lower than three inches; three and a half inches for areas in the shade. Feed cool season grasses in the spring and fall. Use a mulching mower to return the clippings to the lawn. And most of all, be vigilant! If Charlie starts to creeps back in, pull and/or spray right away. And if you see any of those distinctive flowers, burn them with a flame weeder before they can set seed.