How to Fertilize Your Roses

By The National Gardening Association, Bob Beckstrom, Karan Davis Cutler, Kathleen Fisher, Phillip Giroux, Judy Glattstein, Mike MacCaskey, Bill Marken, Charlie Nardozzi, Sally Roth, Marcia Tatroe, Lance Walheim, and Ann Whitman from Gardening All-in-One For Dummies

To keep roses blooming again and again, you should fertilize them about every four to six weeks, although the type of fertilizer you use may alter this rule a bit. Always follow label instructions when determining how much fertilizer to use. You don’t need to fertilize roses that bloom only once in spring as often as repeat bloomers. Fertilizing once in early spring may be enough, but increase the number of applications if your plants aren’t green and healthy-looking or aren’t blooming up to your expectations.

Here are some general fertilizing guidelines:

Water before and after fertilizing: A plant stressed from lack of water is more likely to be burned by nitrogen fertilizers, so make sure that the soil around the plant is wet before you add fertilizer. Watering after fertilizing helps to move nutrients into the root zone.
Start fertilizing in early spring and stop in late summer or fall: Make your first application about four to six weeks before growth begins in spring or, in areas where winters are cold, about the time you take off your winter protection. Continue through summer until about six weeks before the average date of your first frost. Employees at your nursery can tell you exactly when that date is, but for most cold-winter climates, it’s sometime in late August or September. Later fertilization may encourage growth that will be damaged by frosts and can result in roses that aren’t fully cold resistant.

No fertilizer on earth will help your roses if the pH of your soil is too high or too low. When the pH is off, important nutrients already in the soil are unavailable to plants.

As long as you apply it often enough, you can use any type of fertilizer. The granular form is easy to use and doesn’t need mixing. Water soluble fertilizers get to roots quickly and are easy to use on container plants, but you usually have to apply them more often. Timed-release fertilizers are convenient, but alone they often don’t supply enough nutrients to keep roses growing well over a long time; you usually have to supplement with granular fertilizers.

The following list explains the major and minor nutrients your rose plants may need:

Nitrogen: Nitrogen fuels a rosebush’s growth, and you must add it to the soil regularly. This element stimulates dark green, healthy foliage growth; because a plant’s energy to make flowers is manufactured in its leaves, healthy leaves mean more flowers. Most rose foods have several times more nitrogen than phosphorous and potassium. Don’t worry about the numbers too much. Just don’t buy one of those “bloom” foods that has no nitrogen at all.
Phosphorus and potassium: Phosphorus and potassium are called macronutrients because roses need them in larger supplies than other nutrients. Some soils already contain enough phosphorus and potassium for healthy rose growth; adding more to them does little good. If your soil is short on phosphorus, add some directly to the planting hole when you put in your roses, so that it gets where it needs to go.

Only a soil test can tell you for sure whether your soil needs either of these nutrients. But if you use a complete fertilizer — one with a lot of nitrogen and a little phosphorous and potassium — on a regular basis, you should be okay.
Iron: In areas where the soil is on the alkaline side, a rose plant may need applications of fertilizers containing iron. You know your roses need iron when their leaves turn yellow with green veins.
Magnesium: Many rose growers swear by magnesium applications, but only when the soil is deficient in magnesium. Magnesium sulphate — called Epsom salts in drugstores — is the form that’s usually applied. This chemical helps intensify flower color and increases production of new flowering canes. Water in 1⁄4 to 1⁄2 cup per plant once or twice a year.