Well, let's be generous and answer again anyway - is this plum tree a baby in a pot, or a mature flowering plum tree in the ground?
For a potted plum, a soluble fertilizer might be the best way to go but a large tree in the ground should get any balanced, pelleted time-release fertilizer.
Something like Osmocote 14-14-14 would be fine. Depending where you are it may or may not last the whole growing season for you. Even though the package reads "lasts for 3 to 4 months" down here in the south it only lasts about 8 weeks because of heat, high humidity and heavy rain. So what fertilizer you use and how often you use it depends upon where you live.
Please would you complete your personal profile to let us know where (approx.) you live?
