In addition to weeds, unwanted or random flowers can show up from seeds spread by wind, birds, and wildlife. Here in So. Calif. I have had Queen Anne's lace, columbine, and sunflowers sprout from volunteers. In Ukraine, you probably have a different selection. Some common ones for your area might be:
orange hawkweed, marigold, common broom, thistle, mugwort, bindweed, yellow flowered lamium, and Sosnowsky's hogweed which is a serious skin irritant.