I would agree Crataegus is a distinct possibility. A tree/shrub anyway. Where is this roof garden?

Thanks for your feedback!

I'm in Seattle, which seems to only have the Douglas Hawthorn in its natural range. My roof is 4 levels up -- could Crataegus seeds have arrived by wind or birds? Or via potting soil bought at stores? Also, the size of the seeds I've seen in images online would suggest a much larger sprout than the tiny seedlings shown in my pictures. The dicots are much smaller than those from my lupine and salvia splendens seedlings. And they don't look much like the Douglas Hawthorn seedlings shown in the following link: http://www.fs.fed.us/wildflowers/Rare_Plants/conservation/success/crataegus_douglasii_restoration.shtml

I guess I'm just hoping that it's something else that I can keep, since my container pots cannot accommodate shrubs (and certainly not a dozen or more of them). I'll post more pictures as they grow, and I'll include a coin in the images for a size comparison. I look forward to working with this community to figure this out!

Hawthorns - Crataegus - produce large crops of berries much beloved by birds. And ornamental varieties of hawthorn abound in the greater Seattle area. If these are crataegus seedlings, the seeds were no doubt deposited by a visiting bird. I have a young hawthorn growing up through the middle of a large forsythia shrub that was "planted" in this exact manner :-)

One also sees random starts of laurel, English holly and cotoneaster - all berry producing shrubs - and English ivy (a berry producing groundcover) all over this area, generously distributed by birds.

Crataegus have often been planted as ornamentals so hawthorns outside of your native range may be possibilities as birds happily drop the seed here, there, and everywhere. While hawthorn is a possibility - and I've seen lots of newly sprouted C. phaenopyrum, I think the plant is rather too tiny to say for sure. Let another pair of leaves emerge or see them get a little larger for a better ID.

FataMorgana

I'll follow up soon with more pictures :) Thanks everyone!

Here are a couple seedlings after a few more weeks of growth:

Yep. It's even got some leaf spot starting :-)

I know it's a little late to join the discussion but these also look a whole lot like Betula seedlings. Shown is Betula nigra. Do you happen to have one of those in your area? The seeds can be blown easily to the 4th floor.

I have thousands of these things in the back yard...

@macranthos, yes there are quite a few river birch in the neighborhood, some within a block. Thanks for pointing that out, I didn't know what kinds of trees those were before your comment. And it probably explains this other volunteer I have (picture below), which seems likely to be Betula nigra. Interestingly it has the same red stem as the smaller seedlings I originally asked about, which look a great deal like the photo you posted. And yes, I also have them all over the place in my back yard and deck container pots.

That second guy isn't a birch... Prunus maybe.

I am now thoroughly confused :) How long would these need to keep growing before we could be certain?

Well we can be certain immediately that the last picture isn't a Birch. And pretty sure it is a Prunus.

And I still vote for Crataegus. If I get a chance, I'll have to find some of the seedlings around here and take a picture. Immature plants/foliage always make for a difficult ID.

FataMorgana