Marigolds do NOT repel mosquitoes.
More Info: According to the experts at the Alabama Cooperative Extension, contrary to popular belief, marigolds do not repel insects, mosquitoes or otherwise. The experts actually discourage their use in the garden because they attract spider mites and rob nutrients from the soil away from the very plants that the intended use is attempting to protect. [“Dispelling Marigold Myths.” Alabama Cooperative Extension System]
What about Citronella Plants?
Citrosa plants are specifically marketed and sold as mosquito repellents, but do they work? According to Arthur Tucker, Ph.D.., plant fragrance specialist at Delaware State College in Dover, the answer is likely, no. The plants contain less than 1% citronellal, the fragrance that is touted as a mosquito repellent. The plant does have a minimal repellant effect when the leaves are broken up and crushed.
Plants That Do Repel Mosquitoes
If you absolutely do not want to use bug repellant, there are a few plants that are touted to have some effect, if nominal. No plant sitting in its pot is going to repel mosquitoes to any extent. The leaves will need to be crushed to extract the oils and their fragrance.
Lemon thyme has nearly 62% of the mosquito repellent power as bug sprays containing DEET. The Extension at Iowa State University suggests that they may even be helpful as a direct skin mosquito repellant as long as you are careful to do a patch test to ensure that you are not allergic to the plant.
Chamomile and elder leaves in infusions can be used on the skin as a repellent. Colorado State University Extension specialists suggest making the infusion by placing 50% chamomile and 50% elder leaves in a bowl and pouring hot water over them, allowing them to steep for 15 minutes.
Plant Essential Oils to Repel Mosquitoes
Essential oils that are extracted from plants may be useful in repelling mosquitoes as well. Using diluted lavender, tea tree oil, and citronella may help to naturally repel mosquitoes.
“Dispelling Marigold Myths.” Alabama Cooperative Extension System – ACES.edu. N.p., n.d. Web. 18 May 2012. http://www.aces.edu/dept/extcomm/specialty/marigolds.html
“Mosquito Repellent Plants .” IPM at ISU | Integrated Pest Management at Iowa State University. N.p., n.d. Web. 18 May 2012. <http://www.ipm.iastate.edu/ipm/hortnews/1993/5-26-1993/plant.html>.
“Plants Help Keep Mosquitos Away.” Colorado State University – Fort Collins. N.p., n.d. Web. 18 May 2012. <http://www.colostate.edu/Depts/CoopExt/4DMG/Pests/mosquit.htm>