I noticed these plants growing along the footpath between my home and town when I got back from holiday. Here is one example of the plant; it looks like a grass or grain of some sort. This is the tallest of them, about knee height. A couple of closer views of the seeds I took off a few of the spikes of seeds and photographed them on a table, using extension tubes (I'm starting to get the hang of using these). Then I used the little "USB microscope" I got a couple of months ago... Each seed looks to be about 3mm long. These are growing outside an apartment building; I wonder if it might be millet or similar, from mixed seed that the people living in the building put out on their balconies for wild birds.