The silk tree (Albizia julibrissin) is native the Asia and Africa, making it ideal for warmer climates. It is grown widely in the southern United States because the humid air is inviting to the needs of the silk tree. It is a small to medium tree, growing broad and flat on the top as it ages, according to the University of New Mexico. The pink clusters of blooms arrive in spring and summer, leaving their litter underfoot. Fertilizing the tree is essential in the first three years to promote continued growth for many years to come. Fertilizing should begin in late spring, following an early spring transplant.
Apply mulch around the base of the tree, ensuring it does not touch the trunk of the tree directly. Apply a starter fertilizer with a nutrient ratio of 12-4-8, four to six weeks after planting the tree. A water-soluble fertilizer will absorb and spread to the roots more evenly than granules. Mix the fertilizer with water, following the package directions. Spray the fertilizer around the base of the tree, on top of the mulch. Continue to spray throughout the area under the tree to a circumference of one and one-half times that of the tree.
Apply a fertilizer with a slow release nitrogen base in the late summer or early fall. Choose a fertilizer with a nutrient ratio of 16-4-8 to provide adequate amount of nitrogen. Mix the fertilizer according to the package directions, and apply in the same manner as the spring feeding. A lower nitrogen fertilizer can be applied, if nitrogen is added secondary to the soil.
Calculate the area under the tree to be fertilized, measuring away from the trunk one and one-half times the circumference of the tree coverage. Apply 1 lb. of nitrogen for every 1,000 square feet of coverage area. Spread the nitrogen according to the package direction and follow with a 15-5-15 fertilizer.