Answer: Nitrogen, phosphorous and potassium are the 3 major nutrients for all plants. (They correspond to the 3 numbers on fertilizer packages, such as 15-30-15.) Nitrogen promotes growth of green leaves. Phosphorous is essential for blooms. The second and third numbers on the package should be higher than the first. Or, roses benefit from applications bone meal, which contains phosphorus, and greensand, which contains potassium. Both of these nutrients are important for flowering. The thing to avoid is feeding them with high-nitrogen fertilizer, such as 21-0-0, that will encourage foliage growth at the expense of flowers. Fertilize with a slow-release fertilizer every 6 weeks starting after pruning in Jan or Feb, continuing until June. Some rosarians continue feeding at half-strength through the summer, others stop, thinking it is too stressful for the roses. Begin feeding again in September, with the last application in mid-October. Don't feed during the cold winter months, as fertilizer promotes tender new growth which is susceptible to frost damage. It's a good idea to layer several inches of organic mulch around the base to maintain soil moisture and cool soil temperatures. Good luck!
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