Thankfully, Mandragora was a fairly hearty plant. Probably not even he could kill the little buggers. Still, it would probably a good idea for him to simply fill the pots he'd found with sod, and let Lily actually transfer the plants. That way they could be sure the Mandrakes would be trying to move into one another's pots before long at all. The randy little things.
At times like these, Severus sincerely wished he had been better at Herbology. He knew the theory back to front and sideways, even, but that had never really helped him much when confronted with an actual plant. They hadn't been too impressed that he practically had One Thousand Magical Herbs and Fungi memorized, had they? No, they had still determinedly shriveled and died, no matter what he did. Ah, well. He just ... wouldn't touch the Mandrakes.
To that end, as he waited for Lily to arrive, Severus filled the bottoms of the pots with sod, leaving a sizable hole in the center of each for the Mandrake itself. It was filthy work, and most certainly not the sort of thing he would have preferred to be doing, but even as his nose crinkled in distaste, he knew it was necessary. He could only hope their captors didn't intend to set a basilisk on them.
But Mandrake was used in a number of antidotes, however, and not just something as potent as the Mandrake Restorative Draught. Hopefully this was simply so that they would have the plant on hand, with the added possibility of knocking out a few unsuspecting people, for their amusement. Hopefully.