True cedars (Cedrus spp.) are native to the Old World. Slow-growing, they have evergreen foliage, bold outlines and eventually impressive size, up to 100 feet tall for Cedar of Lebanon (Cedrus libani). They root deeply into the ground and need little care or maintenance. Eastern red cedar (Juniperus virginiana), which grows in U.S. Department of Agriculture plant hardiness zones 2 through 9, is native to the United States and is also slow-growing to 50 feet tall. All these cedars can benefit from periodic fertilizer applications.
Three true cedars are used in landscaping. Atlas cedar (Cedrus atlantica), which grows in USDA zones 6 through 9, has short, silvery-blue needles. Cedar of Lebanon is also hardy in USDA zones 6 through 9. Deodar cedar (Cedrus deodara), which grows in USDA zones 7 through 9, has drooping branch tips and a pyramidal shape. True cedars and eastern red cedar all are efficient at harvesting the nutrients they need from the soil they're planted in because of their efficient, far-reaching root systems. Because they do well on their own, horticulturists often recommend not fertilizing them unless growth is suboptimal. Cedars all respond to fertilizing, but you need a suitable formulation and application rate.
For established cedars, select a complete fertilizer such as 10-8-6 and apply it in spring before new growth starts. Use 2 pounds of fertilizer for each inch of trunk diameter, measuring the trunk at waist height. Scatter the granules evenly over the soil surface around the tree, extending it slightly beyond the drip line where the branches end. Water it in thoroughly.
You can also use evergreen tree spikes with a formulation of 13-3-4. Evenly space them in a circle around the tree's drip line, using two spikes for a 1- to 2-inch tree trunk diameter, adding another spike for each additional inch of diameter. Drive the spike so it is 2 inches below the soil surface. Apply them in spring and fall. Keep them at least 30 inches away from the tree trunk.
Learn to read your cedar tree. If the tree is growing normally and has foliage and color typical for its species and size, fertilizing probably isn't needed. If trees have slow growth, sparse foliage, an abnormal color or short needles, fertilize them. Don't confuse stress from dehydration with lack of fertilizer. True cedars need occasional deep watering in hot summer months to thrive, and fertilizing heat-stressed cedars would only make matters worse. On the other hand, fertilize cedars regularly if you want faster growth on young trees, if plants are growing in poor soils or if they need to repair damage.