lil help...? 4 A ROOKIE

hello and thanks, I have been trying to bonsai cedar ( not sure what kind,wild to Minnesota) and sugar maples for 2 years now last winter there were deaths dew to rodents. so i think i will keep pots off the ground this winter. Also my favorite cedar just died i had it for 2 years now... the needles turned brown ad fell off and when i looked at the stem or trunk there were little white bumps. i watered often mostly every day including rain, fertilized with fish emulsion and have been using a soil blend of small rock, compost, black dirt, and sand. i still have a few cedars going. any advice would be grate. i plan on trying to sprout: horse-chestnut, Japanese wisteria, and cheery. any thing you have to say i will hear thanks.
-Tim
p.s. how do we feel about plastic pots.

I am in Florida so my advice is limited, but I suspect you are overwatering. Plastic pot are frequently used for training.