Knockout roses are beloved for their long blooming cycles, hardiness, and low maintenance. However, to support continuous flowering, strong stems, and lush foliage, they benefit from the right fertilizer applied at the right times.

The best fertilizers for Knockout roses provide a balanced mix of nitrogen for growth, phosphorus for blooms, and potassium for disease resistance. Micronutrients like iron, magnesium, and calcium are also valuable for supporting healthy rose production.

In this guide, we review the 10 best fertilizers for Knockout roses, considering NPK ratio, release type, ease of use, and whether they are organic or synthetic. Use this list to choose a fertilizer that maximizes bloom output and keeps your roses vigorous.

10 Best Fertilizers For Knockout Roses

1. Espoma Rose-Tone (4‑3‑2)

Espoma Rose‑Tone is an organic fertilizer tailored for roses and other flowering plants. Its formula includes beneficial microbes and a balanced nutrient profile ideal for sustained growth.

Apply around the base of the rose bush in early spring and again mid-season, then water in well. It supports strong roots and abundant blooms.

This is a go-to organic option for gardeners seeking soil health and long-term rose vitality.

2. Miracle‑Gro Water Soluble Rose Food (18‑24‑16)

This water-soluble fertilizer delivers a bloom-boosting ratio, with a high phosphorus content to encourage flowering in roses.

Dilute per label instructions and apply every 4–6 weeks during the growing season. Use as a root drench or foliar feed.

It’s ideal when you want fast-acting nutrients for real-time bloom production.

3. Jobe’s Fertilizer Spikes for Roses & Flowers (2‑7‑7)

These fertilizer spikes are easy to insert and deliver slow-release nutrition over time. The 2‑7‑7 NPK ratio supports both flowers and root health.

Insert spikes around the drip line of the Knockout rose bush and water them in. Replace as directed, usually every 8–10 weeks.

Perfect for gardeners who want a low-effort, long-term feeding solution.

4. Dr. Earth Total Advantage Rose & Flower Fertilizer (4‑3‑2)

This organic pellet fertilizer includes beneficial soil microbes and nutrients formulated specifically for roses and flowering plants.

Work it into the soil around your roses at planting and then side-dress twice per season. Water well after application.

It supports strong root growth, flower production, and overall soil health.

5. Espoma Bio‑Tone Starter Plus

Bio‑Tone Starter Plus combines a balanced fertilizer with beneficial microbes and mycorrhizae to help establish roses and improve root systems.

Mix into the planting hole for new Knockout roses or apply lightly when transplanting. Water after planting.

This product is especially useful for new rose bushes or when rehabbing existing plants.

6. Osmocote Smart-Release Flower & Vegetable (14‑14‑14)

Osmocote controlled-release granules gradually release nutrients based on soil temperature, providing steady food for your roses for months.

Sprinkle granules around the rose bush, cover lightly with soil or mulch, and water well to activate.

This ensures your roses receive a consistent supply of nutrients without frequent reapplication.

7. Schultz All Purpose Fertilizer Plus (10‑15‑10)

This water-soluble formula delivers phosphorus-rich nutrition to support flowering. Essential micronutrients like magnesium and iron are also included.

Mix in water and apply every 4–6 weeks to the soil base of your roses.

This feed helps encourage bigger, more vibrant blooms.

8. Milorganite Slow-Release Nitrogen Fertilizer (6‑4‑0)

Milorganite is an organic, biosolid fertilizer that provides slow-release nitrogen with very low salt content, making it safe for roses.

Broadcast around the rose bush in early spring and again mid-season, then water into the soil.

It improves soil structure, delivers gentle feeding, and supports long-term plant health.

9. Alfalfa Meal (2.5‑0.5‑2)

Alfalfa meal is a natural organic amendment that provides nitrogen, trace minerals, and a natural growth stimulant called triacontanol.

Work alfalfa meal into the topsoil or apply as a side-dress around your roses in early spring and after pruning.

This is a great choice for self-reliant gardeners who prefer natural, organic nutrients and improved soil life.

10. Bone Meal (3‑15‑0)

Bone meal is a natural source of phosphorus and calcium, both of which support strong root development and healthy flowering.

Apply bone meal around the base of the rose bush when planting or in spring. Mix into the soil and water in.

Use it in combination with a nitrogen-rich fertilizer to ensure balanced nutrition.

7 Buyer’s Tips for Choosing Fertilizer for Knockout Roses

• Know your soil: Perform a soil test to check pH, nitrogen, and phosphorus levels. Roses prefer slightly acidic soil (pH ~6.0–6.5).
• Use a rose-specific or flowering formula: Fertilizers with higher phosphorus help promote blooms.
• Choose the right release type: Slow‑release granules or spikes give long-term feeding; water-soluble feed boosts quickly.
• Feed at the right times: Start in spring when new growth appears, then feed every 4–6 weeks through summer, taper off in fall.
• Supplement with organic matter: Use compost or alfalfa meal to improve soil life and structure.
• Don’t over-fertilize: Too much nitrogen can produce lush leaves but fewer blooms and weaker stems.
• Water after fertilizing: Always water in your fertilizer to help it reach the roots and prevent salt buildup.

7 Frequently Asked Questions

1. How often should I fertilize Knockout roses?

Fertilize every 4–6 weeks during the active growing and blooming season. Reduce or stop feeding in late fall to prepare the plant for dormancy.

2. Can I use regular garden fertilizer on Knockout roses?

You can, but a fertilizer formulated for roses or flowering plants is better because it typically contains the right balance of nutrients, especially phosphorus for blooms.

3. Is organic fertilizer better for roses?

Organic fertilizers improve soil structure and feed beneficial microbes, but synthetic fertilizers can deliver nutrients more quickly. Many gardeners use a mix of both.

4. Should I apply fertilizer when I prune my roses?

Yes — after pruning in early spring is a great time to apply a balanced fertilizer to support new growth.

5. What if my Knockout roses have yellow leaves?

Yellow leaves can signal nutrient deficiencies, such as nitrogen or iron. Use a fertilizer with these nutrients or supplement with a micronutrient feed.

6. Will too much fertilizer harm my Knockout roses?

Yes — too much fertilizer can burn roots, encourage weak growth, or reduce flowering. Follow recommended rates and water in well.

7. Can I use compost instead of chemical fertilizer for roses?

You can apply compost to improve soil health, but compost alone may not supply all the nutrients roses need. Use compost alongside a balanced fertilizer for best results.

Conclusion

Feeding your Knockout roses with the right fertilizer can dramatically improve bloom production, plant vigor, and overall health. Choose formulas that provide balanced nutrition with emphasis on phosphorus for flowers, while also feeding the roots.

Whether you prefer organic slow-release, water-soluble, or a hybrid approach, applying fertilizer at key growth stages will give your roses the boost they need.

With consistent care, proper fertilization, and timely application, your Knockout roses will reward you with vibrant, continuous blooms season after season.