I learned the hard way with tiny Kalanchoe fedtschinchoi keikis...had a bunch in a pot to root and they got caught out in a downpour. After the rain, I thought I had lost most of them, then they began to sprout up all over the place.
Now, when I have a lot of little plants like that, I use a "collar" of window screening. I cut a strip about 6 inches wide and a couple inches longer than the inside distance around the container. As I fill the container with soil, I slip the ring of screening into the container and leave about 3 inches sticking out above the top of the container. The rain may cause the plants to float out of the soil, but they do stay in their container.
