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Fertilizing Roses & Rose Bushes

Sometimes it seems there are as many kinds of rose fertilizers as there are roses! Choosing between them can be a daunting task, especially for those who are new to rose gardening. Ask for advice, and you’ll likely be overwhelmed with information; everyone has a favorite fertilizer that they swear by. “Home-brewed” fertilizers read more like a recipe, and can include intriguing ingredients like vinegar or molasses. How does one decide what to fertilize with, and how often?

First, be aware that roses in general are heavy feeders. Roses love being fed, and will reward you with more blooms if they are fertilized. Healthy roses are also better equipped to ward off pathogens, so fertilizing is beneficial from a disease prevention standpoint as well. Can roses survive without being fertilized? The answer is yes, but you probably won’t like the results. There are some exceptions to this rule: species or near-species roses are used to growing in the wild and are adapted to neglect, so to speak. These are selections like Rosa Mundi, Rosa glauca, or the Hybrid Rugosas; larger ramblers like ‘Darlow’s Enigma’ and ‘Paul’s Himalayan Musk’ can also usually fend for themselves. These varieties tend to be once-blooming, but are good choices for rose gardeners that don’t have the time or inclination to fertilize. But anyone trying to grow repeat-blooming roses like Hybrid Teas and Floribundas should fertilize regularly during the growing season.

In order to choose the right fertilizer, it helps to understand the basic nutritional building blocks that all plants need. Most important are the Big Three: Nitrogen (N), Phosphorus (P) and Potassium (K). These are the three numbers you see on all fertilizer packages, and are also referred to as the N-P-K ratio. Without getting too scientific, if you can remember “Up-Down-All Around” you will have a good idea of how these nutrients operate. Nitrogen helps shoots (above ground), Phosphorus helps roots (below ground) and Potassium is used by the whole plant (kind of like a vitamin). More specifically:

Nitrogen (N) promotes healthy vegetative, green growth. Nitrogen is a component of all proteins and because water washes it away from the root zone, roses require a consistent supply of it. It is needed to build chlorophyll and allows the plant to use light to turn water and carbon dioxide into sugars to feed itself. Too much and you produce lush plants with few or no blooms. Too little, and the rose will have yellow leaves, no new growth, and small pale roses.
Phosphorus (P) makes for strong roots and abundant flower production. Too little will cause dull foliage, falling leaves, weak flower stems and buds which will not open.
Potassium (K) also known as potash; encourages vigorous growth and makes sure all is in good working order. It is like an immune system booster that helps the plant through stressed times such as disease/insect damage, drought and cold temperatures. Lack of potassium will produce weak steams, poorly developed buds, and yellow edges on the leaves which turn brown.

In addition to the above, there are several other nutrients that roses need. These include: Calcium (Ca), Magnesium (Mg), Sulfur (S), Boron (B), Copper (Cu), Iron (Fe), Manganese (Mn) and Zinc (Zn). Calcium increases the strength of cell walls, so theoretically, a rose with adequate calcium will be able to better ward off sucking insects like aphids.  Magnesium is a particularly crucial nutrient for roses, promoting dark green leaves, intensified flower color, increased flower production, and can also help flush harmful salts through the soil. That is why Epsom Salts (a form of Magnesium Sulfate), is a time-honored secret for rose gardeners (applied at the rate of 1/3 to 1/2 cup per plant at the beginning of the growing season). The most important thing to remember, however, is that ALL of these nutrients are necessary for roses, so look for a balanced, high quality rose fertilizer that includes macronutrients as well as micronutrients.

Now that you know what your roses need, how do you go about giving it to them? Fertilizers are generally categorized as either organic or inorganic. Organic fertilizers include manures, compost or other plant and animal products (alfalfa, bone meal, fish fertilizer, kelp extract, etc.). Their nutrient content is usually low and their cost generally higher than synthetic alternatives; however, organic fertilizers are better for the environment and can help build humus and improve your soil. Fish/Kelp liquid fertilizer, for example, has an N-P-K ratio of 2-1-2; fish provides a nitrogen source and kelp adds necessary trace minerals.  At these low concentrations, organics can and should be used on a continual basis. If you have a ready source of organic material, such as aged manure, kitchen scraps, composted yard debris or lawn clippings, try making your own compost. Compost feeds the soil by adding organic matter and basic nutrients, which in turn improves soil texture and feeds soil micro-organisms. Healthy soil makes for healthy roses.

Inorganic (synthetic or man-made) fertilizers are manufactured and make up the bulk of what you can purchase ready-made at the store. In today’s fast-paced life, they offer ease of convenience, and are usually more concentrated and less expensive than the organics. Inorganic fertilizers are available in a variety of forms, including water soluable or liquid, granular and slow-release. Rose plants make no distinction between the type of fertilizer they receive, as long as the nutrients are available, so the cost and ease of handling can be a determining factor. Just keep in mind that inorganic fertilizers do not help condition your soil, so never have a residual effect.

When should fertilizer be applied? The rule of thumb is every 4-6 weeks during the growing season for granular fertilizer.  The actual weather condition, not any specific date, is what matters. When coming out of dormancy, if you have 4-6 inches of new growth, and can see the first real leaflet with 5 -7 leaves, then it’s time to fertilize, regardless of what the calendar says. Any potential risk of spring frost damage is outweighed by the fact that your roses are hungry. At the end of the season, if you live in a colder winter climate, stop fertilizing 8 weeks before you typically get your first frost. This will allow any tender new growth to harden off, thereby reducing frost damage.

Once you establish your basic fertilizing regimen, you can fine-tune your applications for optimum rose health. Liquid fertilizers, for example, are more immediately available to the plant and can be used as a rescue treatment for plants with serious deficiencies. This includes foliar fertilizers that are quickly absorbed when sprayed on the leaves. Fertilizers with little or no nitrogen content can be applied later into the fall; this includes bone meal or rock phosphate, which helps promote root growth and next year’s blooms.  Additional factors to consider include:

  • Granular fertilizers are generally hard for young plants to process; Heirloom recommends the use of a liquid fertilizer on our younger rose plants during the first growing season to prevent burning of roots.
  • Roses grown in containers should be fertilized with water-soluable or liquid fertilizers on a more frequent basis. (Since containers are watered more often, granular fertilizers are generally washed out before they can act).
  • Compost or mulch can rob the plant of nitrogen as it decomposes; it may be necessary to increase the level of nitrogen to counteract this process if you use compost.
  • Your local soil conditions have a lot to do with what nutrients are available to your roses. In our area, for example, phosphorus and calcium can easily bind with other elements in the soil and become unavailable. They need to be added more frequently or in higher concentrations to ensure they actually get to the plant. In other areas of the country, the soils are more alkaline and may require amendments to adjust the pH of the soil to ensure optimal fertilizer uptake.
  • If you follow these basic fertilizing recommendations, have corrected your pH levels (roses like a pH of 6.0 to 6.5) and still have a nutritional issue, a soil test should help pinpoint the problem.

Finally, remember to watch your roses closely and they will tell you what they need. A rose with iron deficiency will lack chlorophyll in the leaves and will appear yellow with green veins. A deficiency in manganese will also manifest itself in a lack of dark green leaf color. Corrective action should always be taken when individual nutrients are not available to the rose plant. In addition, always remember that fertilizers are essentially salts, and without adequate water can burn your roses. In fact, most of these nutrients cannot be used without water moving into the plant. If your roses are growing in high humidity or excessively dry soil, nutrient uptake will be reduced. So always remember to water your fertilizers in – before and after application.

Useful Links

Homemade Rose Fertilizer recipe: www.dianeseeds.com/gardening/fertilizer.html

Suggested Nutrient Levels for Growing Roses: www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7465.html