The death of the turf grass and take over by oxalis is nature's way of telling you turf grass just isn't likely going to grow there anymore. You could try to find a shade tolerant grass seed and see how it goes. I don't know of any turf grasses that thrive in the shade though, just some that are more tolerant than others. I think you have to make a choice at this point. Work with nature or against nature. Either is fine, but working against nature will always involve more money, time and labor than working with it. You might get grass to grow and suppress the oxalis, but only with a shade tolerant variety and if the roots of the tree extend into the area then with increased water and fertilization as well. A personal recommendation would be replanting the area with shade tolerant wild flowers adaptable to your cold hardiness zone. Get a mix with a lot of varieties because some likely won't be adaptable to the area. You can pretty much forget about killing the oxalis as it will readily regrow from it's underground parts where it stores energy. Even synthetic herbicides may be of limited use with it. You likely have to outcompete it by choosing plants adaptable to the current conditions that can shade it out by virtue of being taller.

@nancyd ... it was definitely a lot of work to do in the first couple of days, until I figured out I could just dilute it and spray it on. this way I get to avoid the commercial fungicides, which usually contain copper, and bother me (i have some chemical sensitivities) ... so getting rid of them organically was the priority and after the first few days it was less work... i just kept the sprayer bottle around (a used windex bottle i keep around for this sort of thing) and would run out and mist them before dusk. ill do just about anything to avoid real pesticides :) @flying_wahini the first couple days I painted it on, cuz i honestly didn't even know it it would work.... i didn't have to cut the foliage down ... most of the little orange dots just turned brown and went inactive, i started experimenting with the spray bottle because it was a lot faster than dabbing it on with a napkin... so i just diluted it down to the point that it would get through the sprayer nozzle. i just used the cornmeal mixture the one time. i use cracked corn from a local feed store, and usually the little corn husks start to turn green with mold, which means that it should be working... i think as long as there is mold present it attracts the parasitic molds that can feed on a variety of species...so i usually only do a single treatment with cornmeal after a few days of letting the cornmeal sit in and spraying with the kefir there was a lot of new growth popping out (fast grower) and the leaves that had been infected just looked kind of burnt where the rust had been, but otherwise healthy.... every few days or so a leaf would pop out with a rust spot, and i would sometimes cut just that piece of the leaf off to speed up the process... i think that even after you get rid of the visible rust, the spores are still hanging out in the soil, so you are basically just waiting for them to mature and pop out back on the leaves, and spraying it seemed to seemed to keep that part in check. you might be able to speed things up by cutting off the really persistent spots occasionally one seemed to take more than a single spray to make it inactive. also don't forget to spray around the base and on the area around it.

Seysonn: the cinnamon remedy is a deterrent only. Sounds like Rey needs a bit more help than keeping just a few away. It does work though-- learned that one from habjolokia(Thanks Mark). The chamomile tea remedy is for damping off and other fungi. Recipe is: steep a bag normally, then take that bag and steep in another cup of fresh hot water, then take diluted tea and add it to a quart of fresh tepid water... water plants-- drink original cup of tea and relax. Not sure if the the tea actually kills the gnat eggs like the peroxide or just the fungus. Maybe Jean001a will pop in. Kevin

They do love Cassia and ours is with the mail box so I have to really keep after them there. I also notice them with my sea grapes.... damn these ants I have so many different kinds in the small yard we have, from tiny tiny that love my sunflowers, regular black ants and these fat old bull ants... ugh. Rooted some Hibiscus branches the other month, just stuck them in my sandy soil and then added some black Kow and mulch on top and they seem to be doing ok. Lol as you can tell I did not really care if they rooted, but a month later and they still have the little leaves and look plumper than the day I cut them so I am guessing they have rooted and are starting to grow. I think I have 5 or 6 right now do I did not root any of these ones.