The first leaves have a 6.5 cm (2.5 inch) spread. Does anyone recognize it?

I'm not very good at comparing photos but I wonder if it could be a Rumex seedling?

Images of Rumex seedlings: https://www.google.com/search?...

Also the last photo on this page: http://keyserver.lucidcentral....

Thank you Lin! The first leaves on my mystery plant are distinctly butterfly-shaped and i don't see that same shape on any of the seedling photos on your links, so i think not. I appreciate your efforts though

Could be some species of ipomoea

Yes, snarfie, i do believe it is an Ipomoea lobata...first two leaves are practically identical. Thank you so much for heading me in the right direction. Nice vine, but not so much for a garden catering to the nectar lovers and host plants for butterflies. I have a butterfly garden by design which turns out to be a hummingbird and bee garden by default Unfortunately the robust and hopeful little vine is history. Still there are Ipomoeas everywhere here and although i feel badly for having pulled it up, i think i'll get over it...after a little while

So glad you got an ID, I hope you will add a few photos to the database: Firecracker Vine (Ipomoea lobata)

Ooooh Lin, i'll not be growing this plant and already it is on the compost pile because i have limited garden space and all my plants are either nectar rich for the bflies and hummers and bees and/or host plants for the bflies and most of the Ipomoeas are not sufficiently interesting in this regard. As for adding the photo of the seedling, i am not certain that it is in fact I. lobata. There could be several Ipomoeas that may look like this photo that are not lobatas. I didn't research that thoroughly because if it's I. lobata or something similar, i am not interested in growing it. Also, i am very careful about images i add to the database. I won't add anything that i am not 99% (i always like to leave room for possible error) certain of. As far as i am concerned, the thread isn't so much "solved" as it is "finished" in that i have enough information to serve my purpose. I don't know how the thread could be solved unless i grew the plant to see what it actually is and i don't have enough space to do that.

On the bright side, i am growing another volunteer that showed up in my garden. It is a Lupin and i am hoping it will be a native and that i will be able to identify the species so i can add it to the database. I am really excited about this one.

That's not Spanish Flag - Ipomoea lobata has very distinct trilobed leaves.

I arrived late to this party and the plant is already on the compost heap. I respect vitrsna for making a quick decision.

My suggestion would have been the Smallflower Morningglory - Jacquemontia tamnifolia.

Well @greene...HA! It would serve me right!!! Jacquemontia tamnifolia! I've been trying to get my mitts on a Jacquemontia for 2 or 3 years now without success and have recently given up. I fell in love with this vine after seeing photos of it growing in my friend's garden in India. I did look on line at some images and it very well could have been a Jacquemontia. Both the first leaves and true leaves fit. This will teach me a lesson!!! Historically, when a volunteer has appeared in the garden, it has always proved to be a gift worth nurturing...yet another example of silly human error. Well, lesson learned anyway. Of course, i don't know it was a Jacq, but the evidence is compelling and it is the closest fit yet.

If it was truly a Jacq. there should be more popping up soon. Hope you find one.

Jacquemontia tamnifolia was my first impression also.

Jacquemontia tamnifolia does look very similar, but the venation on the cotyledons is very simple by comparison to the seedling in question. Nor do the cotyledons have lobed bases like the one in question.

http://oak.ppws.vt.edu/scott/w...

I took a snip blown to 250% ..

Thank you so much Janet for the vocabulary and for this wonderful link. I still had doubts about the Jacquemontia ID because the photos i found of the seedling online did not show the venation of the cotyledons which your blow-up shows to be different from the mystery seedling. Your input is very much appreciated.

You're welcome Beverly.

It is similar to Ipomoea purpurea, maybe it's a cross with it. Seedlings of some species do vary quite a lot, do you have any Ipomoea purpurea?

http://www.forestryimages.org/...

Janet, no i have not grown Ipomoea purpurea, but many species of Ipomoea grow wild here...it is everywhere. I have grown I. quamoclit and a large white night blooming Ipomoea (forgot the species) neither one was a big hit with the nectar lovers and so they have been replaced. If one of these volunteers should show up again, i will grow it at least until it can be identified. The problem when i do this is that i become fond of the plants and want to keep them. The solution is my neighbor who has a rancho on the edge of town that he has turned into an eco zone of sorts that attracts all kinds of bugs, birds, and critters. I give the plants that won't fit in my garden to him and he happily grows them at the rancho.

Moonflower, Ipomoea alba. Could be that! I've only found two seedling photos, one has the cotyledons still crinkled but they look to have similar venation.

I alba has heart shaped leaves.

I noticed the heart shaped leaves, but wonder if the first true leaves differ?

I think we can say it's an Ipomoea at least. I think seedlings of Ipomoeas are generally variable?

I grow tons of Ipomoea and I don't recall the first true leaves ever changing all that drastically.