Although they are often called geraniums, the familiar annual flowers with red, pink, purple, or white blooms and thick, pleated leaves are not geraniums at all, but rather members of the Pelargonium genus. True members of the geranium genus are often known as cranesbill or hardy geraniums. Originally, both types of plants were part of the Geranium genus, before Pelargonium was designated in 1789. The geranium name, however, persists to this day as the common label for many species of Pelargonium.
The Pelargonium species most often go by the common name annual geraniums or zonal geraniums. These tropical perennials from South Africa are usually grown as annuals, though it is possible to overwinter them in very warm climates. Annual geraniums are favorites for container plantings and hanging baskets, and they also work well as bedding plants.
Annual geraniums are normally planted from potted seedlings. Give them a sunny spot with soil that is rich but well-drained. Most annual geraniums start blooming mid-spring and will repeat bloom until the first frost. It is recommended that you deadhead the entire flower stalk after the flower fades to encourage more blooms.
In climates with cold winters, bedding plants should be pulled up and discarded as soon as frost kills them. Container plants can be brought indoors if you wish. You can grow them as houseplants in a window with bright, direct light. Or, you can overwinter your geraniums in their dormant state—a common strategy for geraniums grown in hanging baskets. Some people take cuttings at the end of the growing season, rooting them indoors, then planting them in potting soil several weeks before outdoor planting time arrives again.
These plants will grow in full sun or partial shade, but they bloom best in full sun punctuated with some shady relief in the hottest hours of the afternoon. Plants that stop blooming in hot weather may just need a little shade for the hottest time of the day.
Annual geraniums prefer rich but well-drained soil that is slightly acidic—5.8 to 6.5. When growing in containers, use a well-draining general-purpose potting soil—not garden soil.
Stressing the plants slightly by watering only after the soil has been sitting completely dry for a day or two seems to encourage more profuse blooming. These are relatively drought-tolerant plants, which is why they are so popular for cemetery urns that are tended only sporadically. However, do not leave them dry for so long that they start dropping leaves and declining.
Temperature & Humidity
These are warm-region plants that dislike cold temperatures. They thrive at the same temperatures preferred by humans—55 to 80 degrees Fahrenheit. Prolonged temperatures below 55 degrees Fahrenheit will stunt these plants, and temperatures at the freezing point usually kills them. It may be possible to overwinter annual geraniums as far north as zone 7 if you cover them heavily in the winter, but it is generally best to grow them as annuals.
Zonal geraniums are not heavy feeders, but since they are usually grown in containers, a light feeding with your favorite fertilizer, every two to four weeks, will keep them vigorous.
Is Annual Geranium Toxic?
Many types of Pelargonium are toxic to humans and animals, including dogs, cats, and horses. The toxic parts of the plants are the leaves, which can also cause contact dermatitis. Symptoms related to ingestion include vomiting, depression, anorexia, and gastrointestinal upset. Ingesting large quantities of leaves can cause hypothermia and muscle weakness, primarily in cats. However, the good news is that they’re also toxic to Japanese beetles, so if you’re inundated with them on your roses, try interplanting Pelargonium with your rose shrubs.
Information courtesy of TheSpruce.com