It works immediately after application to produce beautiful results. What's On the Page. But you don’t necessarily need to get a special fertilizer for your roses. Let us look at some of the best fertilizes for roses in the market today. Espoma RT4 is another best organic fertilizer for climbing roses. This rose plant food is excellent in promoting lush foliage and beautiful blooms. Miracle-gro water-soluble rose food. 1 Best Fertilizer for Knockout Roses- Comparisons; 2 Best Fertilizer for Knockout Roses- Reviews. This is specially formulated for the rose plants in the USA. This fertilizer contains almost 15 essential nutrients including 4% nitrogen, 3% phosphorus,2% potash,3% calcium,0.5% magnesium.The NPK ratio is enough for thriving and blooming of your climbing roses successfully. Roses which are not fed properly will produce less spectacular flowers and are prone to a wide range of diseases. You can also use a general complete fertilizer with a high phosphorus ratio, such as 5-10-5, 4-8-4, or 4-12-4. 2.1 1.Jobe’s Organics 9425W Knock-Out Rose Organic Granular Fertilizer; 2.2 2.Miracle-Gro Water Soluble Rose Plant Food; 2.3 3.Osmocote 274850 Smart-Release Plant Food; 2.4 4.Bayer Advanced 701110A All in One Rose and Flower Care Granules; 2.5 5.Espoma RT4 4-Pound Rose … Inorganic Rose Fertilizers . A Review. Our top pick for the best fertilizer for roses is the Osmocote 274850 Plus Outdoor and Indoor Smart-Release Plant Food.. Fortified with up to 11 different nutrients, this product by Osmocote is such a great way to feed your roses with nutrients. There are special rose plant foods that are tailored to the higher phosphorus needs of roses, with an N-P-K ratio such as 18-24-16. What is the Best Fertilizer For Roses? 1. Rose Fertilizer Guide: Choosing the Best Rose Fertilizer When caring for rose bushes, it is very important to keep proper fertilization in mind to ensure that the plants stay as healthy as possible.