I was wondering if you get a new caste what happens to all your plants in your old one? and if they just stay there then is there any way you can put the seeds back in your inventory without losing the seeds forever?
You can own multiple houses and you can "equip" them in your backpack similar to how you switch mounts. I don't think you can take back the seed after it has been planted. You should continue tending to the garden until the plants reach elder, and then build a new garden in the new house.
I was wondering if you get a new caste what happens to all your plants in your old one? and if they just stay there then is there any way you can put the seeds back in your inventory without losing the seeds forever?
Your plants stay in your other castle. There is no way to remove the seeds back from the plant. You can try to get the plant elder and hopefully get another seed to plant in your new house/castle, but you cannot extract the seed after it is planted.
You're plants stay where you planted them for eternity. What you can do is go to the Wizard 101 wiki, go to housing, then gardening, seeds, and look at the articles on the plants you have and see the chance of that plant giving a seed and at what harvest. Then, just garden till that point, harvest, and make an identical garden at the new castle!