Weed control these last months – oxalis

I’ve spent a fair bit of time the last four to six months on major weed control. I’m a big believer in weedscaping, and letting weeds do my work for me as much as possible. They condition my soil, protect it, attract predators and helpful insects, protect and assist new seedlings, and sink carbon into much-needed organic matter. However, I have three weeds amongst the many here that are actually problematic: oxalis (soursob), couch grass, and brome grass. These, I have to do work to either remove completely or at least contain to the areas I wish them to be in. This time of year is the time to deal with oxalis.

Oxalis doesn’t really benefit me or the soil much. It is pretty, and so I’m keeping it in certain areas. But it chokes out a lot of the other weeds I’d like to have, stops my chosen annuals volunteering to return, and conflicts with the root systems of some of my perennials. To stop it though I only need to stop it for one year – it doesn’t set seed, only corms (and hundreds of them) underground, and the corms don’t last more than one year. Keep it from making new corms, and the next year you’re oxalis-free. So when I set up the garden for summer, in some places I used weed mat with mulch on top in an effort to keep the oxalis from shooting this year. There are areas I did different things to last year during its growing season too. It’s now oxalis reshoot time and I can see what worked.

The cheaper weed mat? Waste of time, the oxalis pushed right through it. Those corms store up a lot of energy and strength. The better weedmat with macerated barley on top? Much more effective, though not perfect. It came in narrower strips, and any seam is a weakness for the oxalis to work its way around. The better weedmat with a heavy layer of woodchip mulch on top? Better, less of the oxalis pushes the mat up enough to work its way through the seams. What’s most interesting though is the areas where I got no reshoot – where last year’s efforts worked. The strip where I had mulch over cardboard last year for the entire winter season has had no regrowth, except along its edges where some oxalis shoots managed to get out from under cover. The area where I had a plastic cover down that my straw was being stored on top of – no regrowth of anything, it stayed dry all winter. Now completely bare. I also had a tarp down under a pile of branches I was storing for chipping into mulch, that got moved half way through winter. Where it was for the first half: last year the oxalis did some regrowing after I moved it, and this year it’s as thick as ever. Where it was for the second half: already had some oxalis growing and setting corms when I moved it into place, and this year is now completely bare. Both areas managed to set corms, but one lot didn’t make it through to the new growing season. So, any area I can prevent growth in or water-penetration in during June, July, August and September will be oxalis free next year. With this in mind, I’ve cardboard-and-mulched several areas that will become paths.That’s preventing growth though I’m having to do a little weeding in places where the seams between cardboard pieces is allowing some oxalis through.

I’ve also done a lot of physical removal in beds that I want to become oxalis free – going around with a trowel (to ensure I get the corm and lower roots) and weeding out any plants I can find. Hearsay says this method works – as long as you get the plants out in The Window, a magical period approximately three weeks long, occurring sometime in May or June. The trick is knowing when it is. It’s the time when last year’s corms have exhausted their reserves to make the new plants, but the plants haven’t started making enough new energy to make the new corms for next year. You only know for sure when the window ends: once the flowers come out, it’s too late. I don’t know how successful I’ve been with that this year. I removed a lot of plants after they germinated, but more have come up in the same spots. They’re usually smaller and much weaker, so I don’t know if they’re weak corms from last year that were being suppressed by the bigger plants, or new tiny cormlets that were forming when I weeded their parents and have taken separation as the cue to attempt plant growth. I’m weeding them too, and crossing my fingers I get enough of them. I saw the first flower stalks forming yesterday though so my window for physical oxalis removal is just about over.

So, I’ll see how well this has all worked next year. The one area that will be a problem is around the lemonade lemon tree. I need to clear the oxalis away from the citrus rootzone, but I failed in that this season. I even tried glyphosate at one point, as an experiment, but it’s not particularly effective – the oxalis doesn’t really take off until we’re starting to get heavy dewfalls almost every day (if not actual rain), which is enough to wash off the glyphosate before it can really act. And the oxalis dies back when the moisture does, so it’s never going to be suitable for spray treatments. I already know from my first year’s work that simply digging the soil out and sifting it is a lot of work for very little effect. Digging it out and removing it completely to a depth of about a foot, as I did around the lemonade lemon initially, doesn’t get all the corms either. They get too deep down in this sandy soil. The window treatment I’ve used on other weeds – laying an old window on top and letting the sun’s heat cook the soil underneath – hasn’t worked for the same reason, many of the corms are too deep for the heat to reach them. The only thing that is consistently working is blocking moisture/shooting. However, I don’t really want to do that in my lemon tree’s active root zone, which next year will be most of its bed. I was hoping the mulch and weedmat would block it sufficiently this year, but that was where I’d used the cheap weedmat so no go. I have a high grade broad-width weedmat to try with mulch for next season, and I think that will be my best option.

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