Oleanders are fast-growing shrubs gaining up to 2 feet per season until they reach 8 to 20 feet, except for shorter dwarf varieties. They also produce large flowers and are resistant to heat and drought. Despite their vigor, oleanders don’t require regular fertilizing to thrive. Feed your shrub when it shows signs of nutrient deficiency, which include light-green foliage, small leaves -- healthy leaves are 4 to 6 inches long -- and few flowers.
Measure the area the roots of your oleander shrub occupy with a measuring tape to calculate how much fertilizer they need. Mature oleander roots can be 10 feet long all around. Another option is to use the drip line, the point the oleander’s outermost branch reaches, as a reference. Draw a square around the oleander 2 feet beyond the drip line. Calculate the area inside the square to find out how much fertilizer to use.
Determine the percentage of nitrogen in your fertilizer. For instance, a synthetic ammonium nitrate fertilizer would be about 33 percent nitrogen, while an organic urea fertilizer would be approximately 45 percent nitrogen. Measure out the number of pounds of fertilizer necessary to apply the recommended 1 lb. of nitrogen for every 1,000 square feet taken by the oleander roots.
Add the fertilizer to a broadcaster and spread it from the base of the shrub to 2 feet beyond its drip line. Walk in a circle as you also move away from the plant to evenly distribute the nutrients around the oleander. Fertilize it in early spring and in early fall.
Water the fertilizer in by irrigating your oleander shrub with 1 inch of water.