Q. What is the difference between seed geraniums and zonal ones? Are there other types?
A. Seed geraniums are a form of zonal geranium that has been grown from seed rather than from tissue cuttings.
Both have zones of color in the leaves (that's how they get the name "zonal"), but seed geraniums are usually more compact and are often the cheapest option. Varieties differ, but they usually have lots of 3- to 4-inch flower heads. Most grow about a foot tall and wide. They are a great option for a mass planting.
Plants labeled "zonal geraniums" can have flower heads up to 6 inches across and, unlike seed geraniums, often have double flowers. But, they usually don't have as many flowers as the seed varieties. The zonal plants grow more upright and can be up to 18 inches tall. As new varieties are developed, you will see fewer differences between the seed- and tissue-propagated zonal varieties.
There are two new kinds of zonals that you may see in garden centers. For something different, look for stellar and fancy-leaf varieties.
Some other types of plants commonly sold as annual geraniums are, ivy-leaf, regal and scented-leaf. Ivy-leaf geraniums have waxy leaves that, well, look like ivy. They trail and are great in hanging baskets.
Regal geraniums, such as Martha Washington, are commonly sold as houseplants. They need cool nights to flower.
Scented leaf varieties flower, but not as impressively as other types. They are grown for the leaves, which can smell like roses, lemons, apples or mints. There is one that smells like citronella and is supposed to repel mosquitoes, but there is no proof that it works.
To confuse things a little more, these plants are called geraniums but are actually pelargoniums. Pelargoniums can be overwintered indoors or grown as a houseplant. True geraniums, sometimes called hardy geraniums or cranesbills, are tough perennials that can be planted outside and, depending on the variety, are hardy down to zone three. Like pelargoniums, they are usually ignored by deer. They don't have big balls of flowers like zonal pelargoniums, but their smaller blossoms and pretty foliage are a great addition to our gardens.
Source: U of M Extension Master Gardeners in St. Louis County