Ok, I've looked at pictures online of all of your suggestions. 1. The leaves of Climbing Buckwheat do look the same as my vine. The flowers do not, however, at least from the pictures I've seen online. The flowers I've seen online are open, but the flowers on my plants always stay very compact and "closed." 2. I do believe this is a potato. The question is how it got there. 3. This could be horseweed. The pictures I've found online do look similar, especially the flowers. I'm not sure about the foliage, though.

Same as what? Perhaps a seedling hellebore?

I'm not good with specific names of weeds, but I do recognize number 3 as a dandelion. The last one I have in my yard. Those yellow flowers will turn into puff balls of seed that fly everywhere. You don't have time to spray those away. Get a shovel and cut them off at the base before the flowers become puff balls. The first two I'm going to classify as wildflowers. It's great that you seeded your lawn in the fall. We talk for what seems like hours about the problems with spring seeding, one of those being that in the spring you also germinate the summer weed seeds. Well when you seed in the fall you are also germinating the spring wildflower seeds. Those plants take root but only barely. They remain dormant until spring when they bolt and become a flowering plant. They send up flower spikes quickly, flower, and then die away. The seed remains on the surface waiting for perfect circumstances in the fall when they will germinate again. The good news is that you can almost always control the circumstances so that they will not germinate. One way you could have changed this outcome would have been to RoundUp once last fall, then water daily as if you were sprouting grass seed. This frequent watering would have sprouted these spring wildflower plants. Then a week later you would spray a second time with RoundUp to kill all the newly sprouted weeds. Then you could clear the surface, sow the seed, roll it down, and begin watering your new seed. But here we are in the spring and you have weeds before you have grass. This happens to everyone every year, so don't get panicky. Weed-B-Gone spray should take care of these weeds and leave the grass alone. Don't listen to the TV when deciding when to fertilize in the spring. Your grass will awaken from dormancy and shoot out of the ground like crazy grass. What you need to do then is hold your horses until the grass growth slows back down to normal. There is no need to fertilize early but there is a need to fertilize later, after the growth slows. Usually Memorial Day is a good time to do your spring fertilizer. By then everyone's grass should have slowed down. If you fertilize early you force the plant to use resources which were supposed to last it much longer. Then the grass falls into poor health until next spring. If you want to do something for your grass and soil now (or any time) you could apply an organic fertilizer. Organic fertilizers act differently from synthetic fertilizers. They are a biological product which feeds the soil microbes. Once those microbes are well fed, they improve the overall health of your soil. Feeding the plants is more like a byproduct of improving your soil's health. Since the two types of fertilizer work differently, you can apply both on the same day without any fear of interaction.

it's about 25 yds away from the dead elm that was cut down two years ago, but there may have been another I didn't know about. In any case, for propagation purposes isn't a sucker as good as a self-sown seedling?