Fertilizing your cedar trees is an effective way to help heal an ailing tree. Causes of cedar distress can be lack of nutrients in the soil, poor drainage or disease. Before applying fertilizer, it is a good idea to first find the cause of the distress and correct it. Lightening heavy clay soils with peat moss will improve drainage. Diseases, such as the common cedar apple rust, can be treated with an application of fungicide. Problems such as these should be treated prior to the application of fertilizer.
Fertilize only after a cedar tree has had a minimum of one full growing season to establish itself.
Look for signs of distress on the cedar tree. Changes in color of the needles, from green to a red or rust color, or slow growth are signs the cedar needs fertilizer.
Apply fertilizer during the cooler months. Mid-October to early April is best.
Choose a nitrogen-rich fertilizer. A fertilizer with a nitrogen, phosphorus and potassium mix of 10-8-6, respectively, is a suitable choice.
Calculate the square footage of area the cedar tree covers. Use a tape measure to measure the width of the cedar tree, in feet. Multiply that number by itself to arrive at the total square footage. Measure out and mark this area on the ground with wooden stakes, from the cedar tree outward.
Test the soil using a soil test kit. Cedars prefer acidic soil. If the soil tests alkaline, with a pH of 7 or higher, spread a 1- to 2-inch layer of sphagnum peat over the measured area and mix well with the existing soil, using a rototiller.
Saturate the marked area with water. This makes drilling the holes for the fertilizer easier.
Make holes with a metal rod or soil auger. In the marked and saturated area, dig holes 8 to 12 inches deep, by 2 inches in diameter. The holes should be 2 inches apart, starting 1 1/2 feet from the base of the cedar, for young trees, and 3 feet away for larger, more mature cedar trees.
Pour 2 1/2 tbsp. of fertilizer into each hole. Water the area thoroughly.