Geraniums are one of the most commonly produced floriculture crops this time of year, but recently Michigan State University Extension specialists have received several inquiries from growers about their geranium crop. The most common symptom reported is a purpling of the lowest leaves, while in other cases marginal chlorosis or necrosis is reported.
The two most common causes of purpling of the lower leaves are excessively low growing temperature and phosphorus deficiency. In some cases, the leaves can even turn a bright magenta color. Crops grown cool, or less than 55 degrees Fahrenheit, for a period of time can develop such symptoms. Little or no phosphorus can also cause the purpling of foliage.
If you’ve been using a fertilizer with little or no phosphorus, especially if the media EC is low or less than 1.0, there’s a good chance you need to provide more fertilizer. With the extremely cold weather experienced this winter, it’s also possible the plants have been kept too cold for too long. If the growth rate is slow, increase the temperature to 60 to 65 F°.
Marginal Leaf Chlorosis Or Necrosis
There are several potential causes of this symptom which is the yellowing or browning of the edge of leaves. They include drought stress, excessively high salts where EC is greater than 2.5, potassium deficiency and nitrogen deficiency. A nitrogen deficiency can also cause purpling of the leaf veins. Leaf yellowing can also be caused by insufficient micronutrient fertility, such as low sulfur, iron or zinc.
When a nutritional problem is suspected, test the pH and EC of the growing media. Recommended pH values are 6.0 to 6.4 for zonal geraniums and 5.5 to 6.0 for ivy and regal geraniums. The substrate EC should generally be between 1.0 and 2.0. If outside that range, adjust your fertility program or acid injection to get within those targets. Also, collect and send a media sample for nutritional analysis, including macro- and micro-nutrients. You may also want to contact an MSU Extension educator or MSU Diagnostic Services for advice and possible additional testing.
There are two good online resources about geranium nutrition and deficiency symptoms. These resources can certainly help you identify the problems, although a media test is still encouraged.
- Geranium nutrient deficiencies: A visual primer for grower diagnosis and correction: An article in the OFA Bulletin by Jonathan Frantz and colleagues at the USDA-ARS. This article includes a diagnostic key for identifying nutritional deficiencies, as well as good photos of individual nutrient deficiencies.
- Fertility management for geraniums: An online resource from Brian Whipker of North Carolina State University. This webpage provides desirable substrate nutrient levels for geraniums, as well as leaf tissue analysis standards.