There are herbicides that target oxalis. If you must resort to chemical control, ask your nurseryman for a recommendation. If you have St. Augustine, point this out, as some herbicides will damage this turf. We have two magnolias that are more than 20 feet tall. At certain times of the year, they shed leaves.
Magnolia leaves yellow and drop when they age.
And this time of year magnolias are shedding heavily in order to put on new leaves. Magnolias suffered during recent droughts, and some have died. Keep your trees watered during dry spells so they won't be forced to drop even more leaves. A good layer of mulch helps maintain soil moisture. Mail garden questions to Kathy Huber, garden editor, Houston Chronicle, P.O. Box 4260, Houston, TX 77210.