Any additional additions you can add to soil to keep it moist for a longer period of time? even slightly would be ok.
pine needles on top or that stuff (forget the name) they put on top of soil to keep the moisture up. someone's gotta help with the name of it
A simple and effective way to retain moisture is to place a bunch of small (roughly 1in. in diameter) stones around the base of each plant like a mulch. I wouldn't use pine needles due to the chance of the Ph becoming too acidic. Not 100% on that, just my opinion.