Which Scented Geraniums Repel Mosquitoes?

Which scented geraniums are effective for warding off mosquitoes?

The Citrosa geranium has been promoted as a mosquito repellent for many years. It has oils in the leaves that mosquitoes dislike. The problem however is that these oils need to be released to the air in order to be effective. Without actually rubbing leaves, which breaks open the tiny glands on the leaves where the oils accumulate, the oils will not volatilize to the air. So, just having plants in the garden or in planters won't have much effect. A study at the University of Guelph verified that Citrosa had little effect on mosquitoes in garden settings.
The same study, however, found that lemon thyme was more effective. Because lemon thyme is lower – many varieties have a mat-forming growth habit – it is suited for walking on. Walking on the foliage releases the lemon-scented oils in great quantities and helps to discourage mosquitoes. Lemon thyme won't get rid of your mosquito problem completely, but it will help reduce numbers as long as someone walks on the plants.