Oxalis in my organic garden
I have a huge mat of oxalis stricta/wood sorrel in my garden and I want to know if it will die in the winter due to the cold, or if I need to try to kill the plants now with something (I was thinking vinegar spray, or chelated iron (Iron-X on gardensalive.com). I know that any seeds that have already been spread will be viable next year, but are the tubers? Or another way to ask this is whether or not the oxalis (which is a perennial I think) essentially becomes an annual in MN because of the winter kill? Thanks!
Washington County Minnesota
There are two yellow woodsorrels that are closely related. Oxalis stricta spreads by rhizomes (underground) and Oxalis corniculata spreads by stolons (above ground) and both types of oxalis spread by seed, also. Oxalis stricta is classed as an annual but can overwinter as a simple perennial (meaning a single plant won't spread but it could stay alive) ; Oxalis corniculata is classed as a creeping perennial (meaning a single plant can spread). How to treat a weed depends on whether it's an annual or perennial. Here are some ID photos of the two weeds:
There are a few ways to attack a weed. One is to try to prevent it from re-seeding. If you are mowing over the plants when they have the capsules full of seeds and you are using a mower without a bag, then the seeds are staying on the lawn to germinate for a new crop. So even though it's good to use a mulching mower to keep the clippings on the lawn, at this time it would be better to use the bag and get those seeds off the lawn. That will help prevent more but it won't get rid of what you've already got.
Oxalis stricta, usually an annual, is very late germinating so if you use a broadleaf weed control to get rid of weeds, the oxalis will take advantage of that to grow in the empty spaces where there used to be weeds - so maintaining a good stand of thick turf is the best defense. These sites list chemicals you can use to control oxalis. It lists both pre- and post-emergent alternatives. The one most commonly used in the "weed and feed" products is 2,4-D but the others are available, too, often sold separately. Whatever you use as a post-emergent should specifically list oxalis as one of the weeds it controls.
In general, you want to use the pre-emergents on annuals before they germinate and you want to use the post-emergents on weeds when they are actively growing so they will be taken up by the plant. You may need to use them more than once, too. An OSU fact sheet on post-emergents (http://ohioline.osu.edu/agf-fact/0402.html) says "This means that spring applications should be made from mid-April through early June, and fall applications should be made during the months of September and October. "
And lastly, here's a good guide about over-seeding times: