Use this:

DriWater, Inc. The Natural Plant, Shrub and Tree Watering Solution - Waters Plants from 30 to 90 Days - Indoor and Outdoor.

I left some for about 3 weeks with this DriWater jelly stuff, and they loved it. When I got back they were healthier than when I left.

Then about 2 weeks later they all got chewed by rabbits.... Go figure.