Feeding Your Hungry Roses

by Nanette Londeree, Master Rosarian, Marin Rose Society

This is a 2005 AOM article

ABOVE: Healthy roses really perform with a balanced diet - the right amount of the right nutrients at the right time.

Photo credit Rita Perwich.

You know when you get that rumble in your stomach that it is time to eat or it is time to feed your pets when they bark, meow, or otherwise try to get your attention. Hungry roses aren’t quite so noisy. Like other flowering plants, roses need food to grow and bloom, to keep them healthy and strong so they can withstand stress conditions from weather or attacks by pests and disease. Roses have the undeserved reputation of being demanding plants that need lots of care to flourish and flower. It’s my experience that they’re really not; they need to have a balanced diet just like we do - the right amount of the right nutrients at the right time. Providing that balanced diet to your roses is what fertilizing is all about.

What a fertilizer is and what it does

Webster defines a fertilizer as “a substance (as manure or chemical substance) used to make soil more fertile.” Fertile is defined as “capable of sustaining abundant plant growth.” A refined definition of fertilizer could be – a substance added to soil to sustain abundant plant growth. The “substance” can have an organic origin or a chemical one; it may be in a liquid, powder, granular or natural form, and may be incorporated into the soil, added to the soil surface, or applied directly to the leaves of the plant.

Plants require sixteen different chemical elements for healthy growth, but most of these elements are already in the soil or the air and don’t need to be added regularly. These sixteen elements are broken down into four general groups - the essential elements, the macronutrients, the secondary nutrients, and the micronutrients. The elements essential to all forms of life, carbon (C), hydrogen (H) and oxygen (O), are derived primarily from the air and water. There are three macronutrients – nitrogen (N), phosphorus (P) and potassium (K); they are the most common fertilizer ingredients. The three secondary nutrients are calcium (Ca), magnesium (Mg) and sulfur (S), and the remaining seven micronutrients, also known as trace elements, are boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo) and zinc (Zn).

The macronutrients are the elements most people are familiar with, and next to the essential elements, play the most important role in the overall vigor of your roses. Nitrogen in most fertilizers is generally in the nitrate form because it is available to the rose without going through any chemical change in the soil. Nitrogen is converted into amino acids that make up the genetic materials DNA and RNA and enzymes that control the plants functions such as photosynthesis, metabolism, and respiration. It stimulates dark green, healthy foliage growth and because a plant’s energy to make flowers is manufactured in its leaves, healthy leaves mean more flowers. Nitrogen is present in all young, tender parts of the plant tissues, chiefly as protein. As new cells form, much of this protein may move from the older cells (the mature leaves) to the newer ones. If the available nitrogen begins to fall short of the plant’s needs, the nitrogen that is available is used by the new shoots at the expense of the older leaves. This is the cause of the characteristic general yellowing of leaves in nitrogen-deficient plants. Overuse of nitrogen can discourage blossoming and can reduce the number of flowers if used excessively during the blooming period.

Phosphorus (P) is a naturally occurring element in the soil and makes up only 0.5% of the plant’s weight. It is the one element that is greatly affected by environmental factors that can limit uptake by the plant. Only about 10 – 20% of the applied phosphorus is used by the plant within the year it is applied. It doesn’t move easily through the soil like nitrogen and potassium – it gets “tied up” and has to stay put (it moves approximately one inch from its original placement, unless physically moved by cultivation). Phosphorus encourages development of strong roots and blossoming, adds strength to the stems, and overall plant maturity and is attributed to an increase in disease-resistance. If the supply is too low, like nitrogen, existing phosphorus moves from the older tissues to the younger ones.

Potassium (K), or potash, is also a naturally occurring element in the soil. The K stands for the Latin word “kalium” and this element makes up about 1% of the plant’s total weight. It is absorbed by the plant as the elemental ion, K+1; it does not enter directly into the green growth or flowering effects as do nitrogen and phosphorus but is an essential part of the plants healthy diet. It encourages strong roots and plant structure, develops general overall vigor in the plant and promotes good bloom color. Potassium is readily leached from the soil so that regular applications should be made.

Calcium, magnesium, and sulfur are generally found in sufficient quantities in most neutral to alkaline soils. They are major constituents of soil additives like gypsum, dolomite, and Epsom salts. Magnesium sulfate is a key component in photosynthesis and is often added to soil (Epsom salts) to stimulate new basal breaks in roses. Care should be taken to not add too much – resulting in buildup of salts in the soil. Sulfur is just as essential as nitrogen in the making of new plant cells. It is generally available in adequate amounts in soil and as a result of the breakdown of organic material. It is also in the atmosphere in the form of sulfur dioxide. Too high a concentration of sulfur can severely damage or kill plant tissue.

The amount of the micronutrients that a plant needs is relatively small, though they too are as essential for plant growth as the macronutrients. The trace element that is the most obvious when insufficient is iron, producing the telltale signs of chlorosis (yellowing of the leaves while the veins remain green). Chlorosis may be a result of the form of iron being unavailable to the plant. Adding micronutrients in a chelated form can remedy the condition. Chelating agents are synthetic organic substances that have the property of maintaining copper, manganese, zinc, and iron in a non-ionized, water-soluble form so that the micronutrients can be readily absorbed by plants.

Types of available fertilizers

If you are shopping at a nursery or home improvement center for fertilizers, you can be faced with a bewildering array of different forms and formulas. Before you purchase, you should have an idea of what you are trying to achieve with the fertilizer. Are you trying to improve the soil, correct a deficiency, or simply provide general nutrition? Deciding what you want the fertilizer to do will help make your selection easier. When you purchase fertilizer, you need to look at what elements it contains, and their respective proportions. The relative amount of each macronutrient is listed as a percentage of the total on the fertilizer package label, always in the same sequence – nitrogen, phosphorus, and potassium. A fertilizer that is 10-20-10 contains 10% nitrogen, 20% phosphorus and 10% potassium, or a total of 40% active food value (10+20+10), with the remaining 60% of the mixture made up of inert, inactive material that is generally of little or no practical value from the fertilizer standpoint. The ideal N-P-K proportion for roses is 1:2:1. The law requires that a manufacturer guarantee what is claimed on the label, so in some cases, a fertilizer will contain secondary nutrients or micronutrients not listed because the manufacturer does not want to guarantee their exact amounts.

Fertilizers can be of a natural origin or synthetic. Natural does not equate to organic, and synthetic does not mean that the fertilizer doesn’t have natural elements. Chemical elements like potassium, sulfur and iron are all found in the natural environment in their elemental form. Synthetic fertilizers are the result of creating one or more of the macronutrients through chemical manipulations or synthesis; they come in a wide variety of concentrations and formulations, are generally convenient to use, readily available, provide rapid results and are the cheapest sources for the three macronutrients. On the flip side, the macronutrients can be rapidly leached during periods of heavy rain or irrigation, they can result in a buildup of chemical salts in the soil following heavy applications over long periods of time and many formulations contain little or no trace elements thus requiring addition of supplemental materials.

Organic materials are derived from once-living animal and plant products. Most of the elements in organic fertilizers are found in low proportions and they release their nutrients slowly as they are broken down by microbes in the soil. Organic materials serve as both fertilizers and soil conditioners – they feed both soils and plants. As nutrient release is slow, they are much less likely to burn roots than more concentrated chemicals. Bone meal, kelp meal, fish emulsion and manure are examples of organic fertilizers. The advantages of using organic fertilizer include a minimum loss of nutrients through leaching, improvement of soil structure, building the populations of beneficial soil organisms and a minimum accumulation of chemical salts. The difficulty in obtaining some of the materials and their potentially objectionable odor may be disadvantages for this type of fertilizer

Available Forms of Fertilizers

Once you have decided what elements you want to apply to your roses, you should consider the different forms that each may come in. Whichever material and method you use to fertilize, liquid or dry, simple, or multi-nutrient, chemical or organic – don’t try to second guess the manufacturer and follow the directions. Too much of a good thing, even manure, can be dangerous.

Liquids or solids – liquids deliver nutrients to roots immediately and are easy to use. Solid fertilizers are sold as powders, granules, or pellets; they can be broadcast, scratched, or dug into the soil, or used when planting.

Simple or single nutrient fertilizers like ammonium sulfate (21-0-1), urea (46-0-0) or superphosphate (0-20-0), are relatively inexpensive and generally very concentrated; they take up little storage space, but they may burn due to high concentration.

Slow-release fertilizers are sold as spikes, tablets or bead-like granules that release nutrients gradually over a fairly long period, 3 – 9 months if the soil receives regular moisture. These types of products like are very convenient to use as you only have to apply once in a season. They may not provide sufficient amounts of the macronutrients and require supplements and cost more than other alternatives.

Multipurpose – a combination of fertilizer with an insecticide or fungicide is appropriate if you need the extra ingredient every time you fertilize, otherwise it is more economical to use fertilizer alone.

Soluble complete fertilizers contain macro, secondary and micronutrients and get to the roots quickly. They are very concentrated so that a little goes a long way. You need to dilute them according to the manufacturer’s instructions.

Natural organic fertilizers like fowl manure, fish emulsion or blood meal add valuable organic matter to the garden, act slowly with less chance of burning, are beneficial to microbes though results are less dramatic.

When to fertilize

When developing your fertilizing routine, you may want to consider using a combination of all of the above – for example, you may want to add a single nutrient (like superphosphate) and / or a slow-released fertilizer when planting new roses to ensure that they get a good, steady supply of the essential elements, then supplement with fish emulsion a few times during the season. Immediately after the first big bloom, you may want to add a liquid or granular all-purpose fertilizer to give the plant a quick boost – especially if applying it to the leaves. Or you can take the easy path that I follow – lots of good compost (usually turkey) early in the season, then regular applications of inexpensive all-purpose granular fertilizer (16-16-16) three to four times during the growing season. It is an easy plan, relatively inexpensive and produces great soil and happy plants! Just remember – there is no one regime that is “the right one” – experiment and see how your plants respond.

Timing is important to maximize the benefit of your fertilizer so that the nutrients are available to the plant when it needs it most – during the active growing and blooming stage. It generally starts when the weather has warmed the soil and slows down as the soil temperature drops (in our climate generally April through late September).

How to apply fertilizer

How you apply fertilizer will depend on both what you are applying and when. Most fertilizing is done by applying the material to the soil, either when planting or during the growing season. Adding fertilizer directly to the soil is the fastest means of application since the route that water and nutrients travel is through the soil and into the plant root hairs. For liquid products, add around the drip line of the plant.

You can also feed your plants through their leaves by spraying them with a dilute solution of fertilizer directly on the leaves. Foliar feeding can be used when a quick growth response is wanted or micronutrients such as iron or zinc are locked in the soil. When using the foliar feeding approach, use a surfactant such as a mild soap (1/4 teaspoon/gallon of spray) to ensure better coverage of the leaves, otherwise the spray may bead up on the foliage. Any sprayer or mister will work, from hand trigger units, hose end sprayer to knapsack sprayer. Spray as fine a spray as possible. Never us a sprayer that has been used to apply herbicides. The best time to spray is early morning and early evening when the liquids will be absorbed most quickly and won’t burn foliage. Spray until the liquid drips off the leaves, be sure to spray on the underside of the leaves where pores are more likely to be open.

One of the most important steps in fertilizing is to make sure the plant is fully hydrated BEFORE and AFTER fertilizing. Water plants the day before and the day after fertilizing (the soil should be moist). A plant stressed from lack of water is more likely to be burned by fertilizers. Watering after fertilizing helps move nutrients into the root zone.

While roses are hungry plants and will reward you with lush foliage and lots of blooms if you feed them, remember that too much of a good thing can actually have negative results. If you apply too much fertilizer or feed too often you can burn a plant – the edges or the entire leaf turn brown and scorched looking, they may die and fall off. And if that doesn’t happen, the overly lush growth is a favorite of many insect pests, especially aphids.

The key things to remember about fertilizing is that your roses, like you, will do their best with a balanced diet; there is no prescribed “right” diet, you can experiment with the bounty of available materials and find something that works for you and your plants. Just do it in moderation, and water well before and after feeding. Then sit back and relax, and watch your roses perform!