FACT SHEETS > SPECIFIC CROPS > GERANIUMS

Growing Quality Geraniums

Geraniums often take a back seat to many of the other species of spring bedding plants. In the last twenty five years many hybrid seed geraniums have been introduced for the commercial market. Propagators and growers are reacting to changing consumer purchase patterns by offering a wide selection of geranium flower and foliage colors. Producing quality geraniums does not happen without attention to detail. The following information will explain a few of the common problems that occur in growing geraniums.

Growing Medium
In selecting a growing medium, physical and chemical properties of the soil are important. Growers often ignored these factors when using soilless mixes. Mineral soils and soilless mixes share many chemical and physical properties. In both cases these soils can be engineered and made desirable for geranium production. The fact remains, soilless mixes offer more flexibility and give the grower a greater margin of error in controlling both the physical and chemical properties of the medium. A very important physical property of the growing media -- porosity -- influences gas exchange and water availability. Good porosity eases water management and increases the flexibility needed to control moisture levels while extending the schedule of fertilizer applications.

If the roots are to develop and take up water and nutrients, they must take in oxygen while giving off carbon dioxide and respiration products. These gases must diffuse in and out of the soil. Gas diffusion is much faster through air than through water. For greater air-filled pore space, coarse particles must be present in the soil. Porosity is influenced by the growth medium's particle size and distribution.

In general, soilless mixes have proven excellent for growing geraniums. Before switching to a new mix, you should experiment on a small scale before making a complete change. Different mixes do require different management. It takes most people some time to learn how to handle watering the different soilless mixes. The light weight and rapid drying of some soilless mixes can be a problem with geraniums and depends on growing conditions in the greenhouse.

Fertilization
To maximize growth, the fertilization of geraniums during production is extremely important, and an understanding of complete nutrient management is essential if fertilizer problems are to be avoided.

Many growers use a constant fertilization with a water soluble fertilizer. Although this is an excellent fertilization method, it is very important that adequate water be applied so that a sufficient amount of water drains or drips through the medium at each watering. A buildup of fertilizer salts will occur in the medium when watering is not thorough. In many instances the total fertilizer content (soluble salts) of the medium will become so high that plant growth will be severely checked.

The recommendation for constant feed fertilizing of geraniums is generally 200 to 250 ppm of nitrogen. Experience suggests nutrient problems are minimized when a constant fertilizer program is used.

Although many growers use a constant feed when using soilless mix, liquid fertilizer can be applied at weekly intervals. When this approach is used, the rate of application is generally in the range of 450 to 500 ppm of N.

pH
The pH scale indicates acid and basic values in the growing medium. The pH level of 7, midpoint in the scale from 0-14, is neutral and the values below this point are acidic and those above are basic. Geraniums are influenced by the growing medium's pH. It is believed that many hybrid geraniums will not flower well at a pH below 5.5, and that leaves will develop brown spots. At a pH below 5.8, geraniums are susceptible to iron and manganese toxicity. Every effort should be made to maintain a pH in the range of 5.8 to 6.2.

Temperature
The effect of temperature on the growth and development of geraniums can be dramatic. Temperature influences the rate of photosynthesis and respiration, processes of floral initiation and development, the length of time to maturity, final plant quality, and ultimate post production life. While geraniums can be produced over a wide range of temperatures (45 to 80 degrees F), they respond best when treated as a warm temperature crop. For the production of geraniums using the "Fast Cropping" technique, night temperatures of 60 to 65 degrees F and day temperatures of 70 to 75 degrees F are best for optimum plant growth. When the temperature is lowered below 60 degrees F, growth begins to slow. At a night temperature of 55 degrees F growth is slow and plants will flower later. At 50 degrees F, growth almost stops and maturity can be delayed substantially. If plants are exposed to temperatures of 50 degrees F or lower for more than 12 hours, they will often develop reddish color in the older leaves. Mean daily temperatures above 80 degrees F or day temperatures exceeding 85 degrees F for 12 hours or more can cause a loss of chlorophyll (heat stress) in the youngest leaves and cause a sharp decrease in plant growth and development. There is considerable variation in the response of geranium cultivars to low and high temperatures. While some cultivars may exhibit heat or low temperature stress, other cultivars grown under the same conditions in the same greenhouse may show no signs of stress.

Irrigation
Watering greenhouse floriculture crops remains the most difficult task to perfect. Many factors such as type of crop, pot size, temperature, soil mix, bench design, and type of heat can effect watering practices.

The decision relating to when to water is still an "art" rather than a "science". Criteria such as touching or looking at the medium and foliage color (shiny - no need to water; dull - need to water) have been used for years. Unfortunately, most floriculture crops are watered when the grower notices the leaves are wilting. When the leaves of a plant wilt, the plant has already undergone a water stress which results in a decrease of the growth rate. This is especially true with geraniums which are slow to wilt, even when the plants are under a water stress. Subjecting geraniums to water stress is sometimes used as a growth regulator. However, it is not generally recommended to regulate geranium growth in this way. Additionally, allowing a soilless mix to dry out makes it difficult to rewet unless a wetting agent is added to the medium or irrigation water. Every time a medium is irrigated it is strongly recommended that a sufficient amount of water be applied to avoid water stress, allow adequate moisture for plant growth, and allow for some leaching to occur.

Increased Branching of Flowering Plants
Some cultivars are not as free-branching as others, so they will not produce as many flowers stalks. The use of ethephon (Florel) to increase branching is a common practice. Florel applied at a rate of 350 to 500 ppm, 4 to 6 weeks prior to sale, is recommended. When Florel is applied late in the production cycle, flower buds will continue to abort after the plants are sold. Note: treated plants will have smaller leaves, and internodes will be shorter.

References
Geraniums IV, John White, Ball Publishing, 1993.

Tips On Growing Zonal Geraniums, Ohio Cooperative Extension Service, The Ohio State University, 1988.

Prepared by:
Paul Lopes
Extension Specialist
East Wareham, MA

Links to Further Resources on the Web

Geranium Culture for Home Gardeners Although designed for home gardeners, this factsheet from the Extension Service at North Carolina State University gives some of the most complete information on this topic that we have found online.

Fertility Management for Geraniums -- North Carolina Cooperative Extension Service

Bacterial Diseases on Geranium -- University of Connecticut Integrated Pest Management Program

Disease control for Geranium (Pelargonium spp.) Statewide Integrated IPM Project, University of California

Leaf Spot and Stem Rot (Bacterial Blight) of Geranium (Pelargonium spp.) -- North Dakota State University NDSU Extension Service

Ohio State University Extension Fact Sheet -- Black Root Rot of Greenhouse Floral Crops -- Ohio State University Extension Fact Sheet

Detection and Partial Characterization of Pelargonium Viruses with emphasis on Pelargonium Flower Break Virus -- Dr. Stephen T. Nameth Ohio State University

University of Nebraska Cooperative Extension -- Geranium Culture -- Indoor and outdoor care of geraniums, as well as propagation, are covered here.