This came up in my garden possibly from something in my compost. The seed leaves looked like zuchini or squash both of which could have been dug in last fall from compost. But now that it is in flower I am stumped. Your thoghts please.
Are you absolute, positively sure that the flowers are coming from the same plant as the vining leaves with tendrils?? Your vine appears to be a cucurbit (squash family) but the flowers appear to be a brassica (cabbage family). I ask because it's very common for two plants to grow on top of one another and it's easy to assume the flowers are from the same plant.
There is no other plant involved. The other side of fence is grass only. I took your question seriously and checked. There are four plants here all the same. Nothing behind - Raspberry to left
Echinocystis looks right but there is no sign of the seed pod. Line drawing copied from: http://plants.usda.gov/java/profile?symbol=ECLO
Yes the flower is similar to wild clematis which is common around here, usually taking over another low tree or bush. However the vines are soft, definitly not woody.
Clematis don't have tendrils and the leaves tend to be opposite. You won't see fruit on this cucumber until the flowers are done!