An important part of caring for roses is regular and proper fertilization because roses are heavy feeders. Roses need the three macronutrients—nitrogen (N) for foliage growth, phosphorus (P) for root growth, and potassium (K) for flower formation—plus various micronutrients, including iron, calcium and magnesium.
There are two basic types of rose fertilizers, organic and inorganic. Most organic fertilizers are naturally released to the soil more slowly and over a longer time period than inorganic fertilizers. The exception are inorganic controlled release fertilizers, which release nutrients over several months depending on soil moisture and temperature.
To have a baseline and take the guesswork out of which nutrients and how much the soil needs, it’s always a good idea to get your soil tested. A soil test will also tell you whether the soil pH is suitable for roses. Roses prefer neutral to slightly acidic soil, a pH of 5.5 to 6.5, so you might have to raise the soil pH by adding limestone.
Organic Rose Fertilizers
Aged or composted manure, worked into the soil about two to three inches deep, is an excellent source of all three macronutrients, N, P, and K. Always make sure the manure is “aged”, i.e. not fresh, otherwise its high nitrogen content will cause fertilizer burn in the plants.
Bone meal adds phosphorus to the soil. Mix it deeply into the soil, not only because it does not enter the soil easily with watering but also to prevent wildlife from being attracted by the smell and digging up the soil. Add one heaping tablespoon per rose plant.
Dried blood meal is also a good source of nitrogen. Apply about one tablespoon around each plant. Too much nitrogen can burn the roots so don’t apply too much.
If you don’t mind the smell, liquid fish fertilizer or fish emulsion, diluted in water according to the product label, is a fast-release organic source of nitrogen.
Other organic fertilizers include:
- Greensand for potassium and micronutrients
- Rock phosphate for phosphorus, calcium, and micronutrients
- Alfalfa meal as a balanced fertilizer
- Seaweed for nitrogen and phosphate
Epsom salt is often praised as an organic fertilizer and pesticide. But the use of Epsom salt as a rose fertilizer to supply the soil with magnesium has not been scientifically proven. Applying Epsom salt can in fact harm the plant.
Inorganic Rose Fertilizers
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. But you don’t necessarily need to get a special fertilizer for your roses. 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.
For the amount, follow directions on the fertilizer label. The general rule of thumb for complete fertilizer is half a cup per plant.
Since inorganic fertilizers are more concentrated than organic fertilizers, don’t pile it up on dry soil, which can cause burn. Instead, spread the fertilizer thinly and evenly around the plant and lightly rake it into the damp or wet soil. The best time to apply the fertilizer is just before it rains or before watering. Keep the fertilizer away from the canes and the graft union, and water the plants well after applying fertilizer.
For controlled release fertilizer, the rule of thumb is about half a cup per plant, unless specified otherwise on the product label.
How and When to Fertilize Roses
Newly planted roses should only be fertilized with phosphorus to encourage the development and establishment of the roots. Wait until the rose has bloomed before adding any other fertilizer.
When and how much to fertilize established roses depends on the type of rose—tea roses, for instance, require more nutrients than shrub roses—and the length of the growing season. Here are some guidelines:
Start fertilizing when the first leaves appear and there is no more danger of severe spring frosts. After that, the fertilization schedule should be based on how long and how often the rose is blooming. A rule of thumb is to fertilize after each bloom cycle but to gradually reduce the amount of fertilizer by half each time. Stop fertilizing six to eight weeks before the first average frost date in your area. The reason behind this is that you don’t want the plant to produce a lot of soft new growth late in the summer that will be damaged by cold weather in the fall and winter.
After the roses have gone dormant, you can fertilize them again. Adding fertilizer will not harm the plants at this point but will get them ready for the next spring.
Container roses may need more frequent fertilizing than those growing in soil. Fertilize them at the intervals described above, and any time the foliage starts to look a bit chlorotic, which indicates nutrient deficiency.
It’s helpful to keep a record of when you fertilize your roses using a calendar or a task reminder app.