Plants must be actively photosynthetic, that is to say actively growing and not under stress for maximum control with roundup. You can spray Roundup and replant as soon as 2 days after treatment and there will be no adverse effects from the treatment. I would wait until you get soil temps that have the plants actively growing, make your rup apps and sow your plots a couple of days later. Dont worry that you dont see symptoms on treated plants at this point.. if you applied the roundup at the correct rates, anything that you sprayed post-emergent will DIE.