We have had this problem for years and years. It would be too expensive to put preemergent all over the yard. I only use roundup on the real stubborn ones, like the ones growing in between the cracks in sidewalk, I cannot dig them up.
Everytime I walk around yard I pull up at least ten saplings. That is the only way to keep up with it. The only ones I am concerned about are those in the flower beds. The ones in the yard get mowed.
