Double Knockout roses have double flowers, providing a fuller bloom more reminiscent of a classic rose. Like all the Knockout rose cultivars, doubles require only minimal maintenance in order to flower profusely and to remain healthy in the garden. These disease-resistant roses do require regular fertilization, just like any garden rose. Maintaining the proper nutrient level in the soil helps ensure consistent blooming and continued healthy growth each summer.
Apply a 2-inch layer of compost over the roots of the Knockout rose shrub in spring once the soil begins to warm. The compost acts as mulch and also leeches nutrients into the soil as it breaks down.
Mix up a soluble rose fertilizer or other high-phosphorus fertilizer once the first leaves begin to unfurl on the shrub in spring. Mix the amount of fertilizer indicated on the label with the exact amount of water recommended, then place the mixture in a watering can.
Pour the fertilizer solution onto the soil over the root zone of the rose shrub. Avoid direct contact between the plant and the fertilizer, because it may cause burning.
Water the Knockouts after fertilizing. Spray the lower foliage and stem of the shrub so any fertilizer splashes are rinsed away and do not damage the plant.
Fertilize the double Knockouts a second time after the complete their first flowering cycle, using the same fertilizer solution used in spring. Repeat the fertilization after each flush of blooming, or approximately every six to eight weeks.