GROWING AND CARING FOR PELARGONIUMS
Wayne Handlos, Ph.D.
The common bedding geraniums (Pelargonium x hortorum) are so readily available it hardly seems necessary to talk about their growth requirements. Almost anybody can grow them. But for the sake of success for everyone, the following information may be useful.
The basic requirements for the major types of geraniums (zonals, ivies, regals and angels) are pretty much the same. They all need light to produce food to grow and flower. Most will grow well in full sun or with at least six hours of sunlight a day. Fancy-leaved zonal geraniums do not do their best in full sunlight where the leaves may develop burn spots. The recommendation for them is a half day of sunlight, preferably in the morning. The rule of thumb for them is: morning sun, afternoon shade. All the rest are happy with as much sunlight as you can give them. Don’t plant them on the north side of buildings or walls. They will not do well.
Most geraniums like the same temperatures that humans do. During the day, 70 degrees are ideal for them. Good night sleeping temperatures around 50 degrees suit them as well. Most can withstand and grow in higher daytime temperatures but may suffer as temperatures get above 80 degrees. Temperatures below 40 degrees at night will slow their growth significantly. Most common types can tolerate periods of 32 degrees without damage. Freezing of most plant tissue does not occur until air temperatures drop to 27 degrees or lower. For short periods of time (a few hours), even this low temperature will not kill all Pelargoniums; many of the Uniques and regals will sprout again from the base or from root buds.
The common geraniums are not fussy about humidity. High humidity becomes a problem because it may encourage the growth of disease organisms like bacteria and fungi. Very low humidity encourages the development of spider mites which may attack some geraniums.
The water requirements of most geraniums are similar to most other house and garden plants. They should never be allowed to stand in water nor should they be allowed to become desert-dry. Feel the soil, if it is dry give the plant a good soaking; if the soil is moist wait until the next day and test again. When the soil is dry, water the plants; if the soil is wet, wait. Excess water provides conditions favorable to root rotting fungi and should be avoided. Continuously dry soil slows the growth of plants and (except for special requirements by some of the succulent types of Pelargonium species or those with dormant periods) should also be avoided.
Soil. The key to success in growing geraniums is to have an open, freely draining soil with a modest amount of nutrients available. If plants are to be grown in the garden, heavy clay soil should be lightened up with sand and well decomposed compost. Very sandy soil will benefit from the application of well decomposed compost. In the absence of compost, peat moss or commercial compost will be beneficial in sandy soil but may hold too much water in clay soils.
If plants are grown in pots or other containers a special soil or potting mix is a must. Soil from the garden is generally unsuitable. It tends to be too heavy and the drainage may be poor. There are many commercial potting mixes available. They contain a variety of components which may vary regionally. (Common components including peat or peat moss, decomposed wood or forest products [bark] and coir [coconut fiber] are most frequently found.) Most mixes are a bit too heavy or dense for ideal growth of geraniums. The addition of pumice (Perlite) will lighten the soil and increase the porosity and draining qualities of the soil. Many of our members use a commercial mix like Supersoil ™ and add additional perlite to it. I do not recommend the use of vermiculite (expanded mica) because it tends to compact over time. Some growers like to add sand or lava rock to the mix for weight and drainage.
Other soil amendments and additions include coir (coconut fiber) which has become popular recently. It is good organic matter that does not break down rapidly. Some people add kelp meal. It will help hold water and is thought to help make the plants more resistant to white fly attack. Worm castings are favored by some gardeners for its nutrients but may make the mix too dense.
If you plan to grow the succulent types of Pelargonium, you may use a commercial mix designed for cacti. Excellent drainage is the key here.
Nutrition. Plants in the well composted garden probably will not need any additional nutrients. The decomposing compost will release nutrients and should be sufficient for many of the plants commonly grown in the garden. If you think that your soil is deficient in nutrients, you may want to add quantities of cottonseed meal (a slow release form of nitrogen), or Milorganite™ (Milwaukee sewage sludge with many micronutrients). Always follow instructions on the packages.
For plants in pots, the soil volume is limited and the lack of nutrients will become an issue. There are many points of view on what kinds of fertilizers to use. Organic fertilizers like fish emulsion are favored by many. Inorganic fertilizers come in an almost endless variety of types and are sold under many brand names. An easy solution is the addition of slow release fertilizers like Osmocote ™. These granules release nutrients slowly and will not burn the roots of plants. They release fertilizer for a few months but then must be renewed. Chemical fertilizers are used by most commercial growers and are dissolved in water and applied to the plant at watering time. There are many such products available to the home gardener. Most are labeled with the three major components – nitrogen (N), phosphorus (P) and potassium (K). The numbers are always listed in the same sequence, N first, P second and K third. It has been suggested that regals and uniques do better with elevated potassium, so buy a fertilizer with the last number bigger than the other two. On the other hand, zonals and ivies do better with more nitrogen and phosphorus, so buy a fertilizer with the first two numbers bigger than the last number. If you use these chemical fertilizers, be sure to buy ones which also contain micronutrients. These will be listed on the label. Dilute these fertilizers according to the directions on the package and apply as directed.