Uhhhm, the protocol is not meant to hot-plug. I never done this and think it's only possible by detecting if the all values are all 255 and if they are run the initial sequence again - ie. the init/setup function.

Yea me too. I used these cheap moisture sensors you can get from China for $1 first and had really weird results. These sensors were totally instable and almost every sensor was getting different values. I then tried tinned nails in a similar setup and had very good results. But it took only 2-3 days and they were corroded - so no long term solution.

Have you considered watering the plants only if they are dry, like using a moisture sensor in an open loop?