Lonely flower

This is the first clematis flower on this plant. Spent more time clearing out ground elder, think I have dug out the lot just need to go over ground again (again,again,again). Have found that next door has a patch and it got through concrete base of fence, so will apply weedkiller in that area. Now, of course, have bare ground, how soon before the weeds find out, before Iv'e finished typing this I expect.