> The first time we pumped water into our planter, the tube directed a small stream into one spot, completely soaking that area but leaving the rest of the soil dry.

I use irrigation tubing with drip line emitters in my garden. That might be a solution for you. The cool think about the emitters is that they control the flow of water and are easily positioned. I think they start at .5ga/hour on up to 10ga/hour.