Plum trees are stone fruits. All stone fruit trees, as the fruit ripens, require adequate moisture to produce quality fruit. According to the Texas A&M Extension service, water is essential for producing large fruits and the overall health of the tree. Irrigation methods range from garden hoses and sprinkler systems to through-drip irrigation systems.
Remove all weeds from the base of the plum tree using the garden hoe. Exercise caution as some of the roots may be just below the surface, at 1 inch or deeper. Clean the ground of all debris and expose the bare soil from the main trunk to the outer drip line of the upper limbs.
Apply a 3-inch to 4-inch thick organic mulch around the tree. Keep the mulch 5 inches from the trunk and extend the thick ground cover to the outer area of the upper limb drip line. The mulch will retard weed growth and retain water to the plum tree.
Water the plum tree in early spring, during fruit production, so it receives adequate moisture every three weeks. Irrigate through a basic garden hose or through a drip-irrigation system. Since most plum varieties ripen between May and late June, irrigation to the plum tree may not be necessary. A good rule of thumb is to check the moisture condition under the mulch layer. If the soil under the mulch is dry, add approximately 1 inch of water around the base of the tree.
Irrigate the plum tree every week after harvest during the heat of summer. Heat stress and lack of moisture during the summer may cause damage to the plum tree, which can induce irregular fruit sets for the following year. Typically plum trees will not require any watering during late fall to early spring, unless drought conditions are present, while the tree is dormant. If drought conditions are present, water the dormant plum tree so the soil under the mulch is kept moist.