Gary, I know the type of Oxalis you are having issues with. It has yellow flowers and long tap roots, not little bulbs....and it spreads like crazy by seed. Hint: when you use a bit of Round-up, don't use it as a liquid. You WILL lose a drop onto the roots of your orchid and that will be death to the plant. First of all, repot and pull those tap roots as much as you can. Then, when the Oxalis begins to come back, Mix a tiny amount of the Round-up concentrate with some thick, heavy yogurt or warm, smooth peanut butter, etc. You want to make a nice, smooth "non-drip" paste. That way, you will not have any accidental drops getting onto your orchids as you work. Don't forget that you don't need to paint every leaf, just one or two on a plant with suckers and runners, will do the job. The poison travels down the stem to the tap root and kills it. Repeat applications to the same pots, as more Oxalis grows, will be necessary until all the seeds that lie dormant on the surface of your pots has germinated and been treated.
