Plum trees flower most profusely when they reach maturity at four to six years of age. When a mature plum fails to flower year after year, cultural (cultivation) practices are out of balance, buds are being damaged or the tree is under drought stress or some other form of stress. Providing good cultural support, temperature regulation during budding, ample but not too much moisture and proper fertilization will all aid a plum tree in producing abundant spring flowers.
Fertilize your plum tree once a year in the spring after the last hard frost with a granular fertilizer formulation for fruit trees that has either a balanced formulation such as a 10-10-10 or a low nitrogen formulation such as a 20-50-30. Balanced or low nitrogen formulas will help prevent nitrogen overload, which discourages bloom and fruit development. Apply no more than 8 oz. of fertilizer for each year in age of the plum tree. Apply the granules at the drip line (the line around the tree parallel to the ends of the branches) in a wide doughnut shape starting at least 6 inches out from the trunk to just past the widest diameter of the tree canopy. Drip line feeding will get the nutrients to the roots while not overloading the immediate area around the trunk, which can cause damage.
Keep the soil evenly moist around the roots of your plum throughout the year while not allowing the roots to sit in soaking wet soil. Drought-stressed or drowning roots can affect bloom.
Mulch with an organic material such as shredded bark, straw, wood shavings or compost, laying down a blanket 2 to 3 inches thick. Place the mulch from 2 inches outside the trunk to 2 feet past the drip line of the tree. Mulch will help regulate moisture loss and feed the nutrient value of the soil.
Prevent the killing off of flower buds by frost by raising the ambient temperature around your plum tree during periods of frost in early spring. Wrap the tree branches in small Christmas lights and leave lit during the night or wrap the tree with a wind break fabric to insulate the tree.