Calendulas Reseeded All Over

Last year I planted a new bed of Calendulas (a mix of Oktoberfest and Flashback). Unfortunately, as detailed in this post, they ended up with some kind of disease. Over the winter I discovered that it probably was insect damage from the aphids, as well as thrips - because the markings on the leaves looked exactly like the ones in slide 31 of this pdf.

So I had no idea what was going to happen to them this year - but it turns out they rampantly self-seeded! I took this picture of the bed on April 5th. All of the little green things are Calendula seedlings.

Yesterday I finally thinned them. I'm sorry that I didn't take a picture before I did that, because I knew I had this one. But I didn't realize until afterwards how much they'd grown in the past month! There were at least 3 times as many seedlings yesterday before I thinned them than the April picture. Below you can see a picture of the result - with the huge pile of thinned plants in the background:
I really hope they stay healthy this year. I found some information online about beneficial nematodes that should help. These are microscopic creatures that naturally live in the soil. The type I bought are considered an organic method and don't harm anything except insects that are bad for the garden, including thrips. So I applied them as directed a couple weeks ago.

Unfortunately, it wasn't until after I placed the order I realized it probably technically isn't veganic, since they do eat the unwanted insects. Granted, this is a pretty small thing to worry about considering insects are harmed in one way or another no matter what you do. Also, I rationalize that they're normally in the soil anyway, and there was probably an imbalance in my soil because the previous owners applied pesticides, which kill beneficials as well as unwanted insects. So hopefully I just restored these creatures to their natural habitat, and it will work to improve the balance of insects in my yard. I hope that's what happens anyway, so I don't have to apply them again. Guess we'll see.