In order for the Roundup to be effective, it has to be sprayed onto the foliage of plants. By your description, I'm not sure if you just sprayed the Roundup on the soil after you removed the Loosestrife. Spraying it on the ground will have no effect. If you did apply to the leaves, that would be sufficient to kill it although some plants need a 2nd treatment to kill it entirely. You should see the effects of the herbicide within a few days as the plants will turn yellow and die, which should be evident by now. If that's the case, you can go in and plant.