Bower, I don't put them directly into solo cups. I use small 2 ounce plastic Dixie cups with seed starting soil. I pot them up to Solo Cups with potting soil instead of seed starting soil when they have at least 2 sets of true leaves. Like I said, what works for one, might not work for anther. If you start hundreds of seeds for resale, doing individual cups might be out of the question. Also if you already have seed trays and they work for you why change?