Shrubs require less fertilizer than vegetables or flowers. Roses are an exception, since they prefer extra fertilizer through spring and early summer. Other shrubs should only be fertilized if they truly need it, such as when they suffer from poor growth, feature small and pale leaves or when disease or insect attacks stress the plant. A healthy shrub that develops 3 to 6 inches of new growth each spring needs no extra nutrients.
Buy a fertilizer recommended for the soil in your area. Some parts of the country have soils deficient in phosphorous and potassium, others have soils with adequate amounts of these nutrients. Your county extension service will be able to guide you. For roses, a standard rose fertilizer should be adequate for most soils.
Estimate the area of your shrub's root system, about one and half times the spread of the branches. For instance, if the branches extend out from the trunk 2 feet in each direction, that's a circle with a radius of 2 feet. To find the area, square that figure (4) and multiply by pi, approximately 3.14. Because you just need a rough estimation, multiply by 3, giving you 12. Add half of that, giving you an area of 18 square feet for a circle extending 3 feet from the trunk.
Consult the directions on the fertilizer package for the exact amount to be applied and measure that into a can or other container.
Sprinkle fertilizer evenly over the root system and rake it into the ground, mixing it with the top 1/2-inch of soil. The best time to do this is early spring, as the leaves are starting to unfold. Fertilizing too late in the year can force tender growth that will not have time to harden before winter.
Do not to spread more than the recommended amount of fertilizer on the soil. Too much fertilizer will burn the leaves, causing brown patches at the edges.
Water well, soaking the soil to a depth of at least 6 inches.