The Knock Out family of shrub roses were bred by the Conrad Pyle Co. to be disease resistant, low maintenance and tolerant of weather extremes most rose plants would not be. They do, however, still benefit from regular applications of rose fertilizer to boost the health of the plants, compensate for deficiencies in the planting soil and boost bloom capacity.
Clear the soil below your Knock Out rose shrubs of any fallen leaves, petals, cane cuttings, weeds and debris so that you have a clean soil surface, free from disease breeding materials, to apply the fertilizer over.
Choose a good quality slow release synthetic fertilizer or an organic rose fertilizer with a guaranteed analysis of 5-10-5, 4-8-4, or 4-12-4 or a similar ratio of macro-nutrients. Look for fish emulsion, blood meal, bone meal and greensand in organic formulations.
Scatter the label recommended dose of organic or slow release rose fertilizer around the entire root zone staring a few inches out from the mail trunk and extending to past the drip line of the shrub. Nestle the fertilizer into the top inch or two of soil but do not dig it in as you may disturb the more shallow roots. Repeat applications throughout the year from spring through early fall as recommended for your product and climate.
Boost the soil acidity around your Knock Out roses to withing their preferred range of 5.5 and 6.5, if needed. Make a single annual application of ground limestone in the spring or summer. Apply two Tablespoons of the soil amendment for each shrub, each year. Cast the amendment around the root zone of the rose keeping clear of the trunk and extending to the drip line. Nestle the lime into the top inch or two of soil and water in well until the soil is drenched several inches down.