Does a potted citronella or mosquito plant keep mosquitoes away?

A citronella plant or “mosquito plant” sitting in a pot does almost nothing to keep mosquitoes off, because an untouched plant releases very little oil, and many plants sold as mosquito plants are scented geraniums, not citronella at all. If you want fewer mosquitoes at home, the thing that works is not letting water stand for more than a week.

This lesson separates what has evidence from what is folklore, and where your time is better spent.

An untouched plant gives off very little

Iowa State University’s extension service puts it plainly: plants sold as mosquito repellents, including citronella grass, citrosa geranium and lemon thyme, release little or no oil while they just stand there. The leaves have to be crushed to release it, and even then the effect is only a fraction of a product such as DEET.

Citronella grass really is the source of the oil used in citronella candles. But “has oil in its leaves” and “puts enough oil into the air to repel mosquitoes” are different claims. Most of the oil stays inside the leaf cells until the leaf is crushed or distilled.

Even citronella candles help only a little. Iowa State reports that standing directly in the smoke cut bites by about 40%, but once the smoke dispersed there was no benefit.

The shop mosquito plant is usually a geranium

The plant sold as citrosa or “mosquito plant” has lobed, geranium-like leaves that smell lemony when crushed. It was marketed as releasing citronella oil continuously and protecting an area of about 0.93 square metres around it.

A University of Guelph team examined it in 1996 and found:

1. Its shape and oil match the Pelargonium group of the commercial rose geranium. No trait of the Cymbopogon grasses that yield citronella oil was found.
2. Total oil in the fresh plant was only about 0.2% of its weight, mostly geraniol and citronellol.
3. In a field test with real mosquitoes, people next to the plant were bitten no less than people without it. The group wearing 75% DEET had bites cut by more than 90% for up to 8 hours.

A lemony smell and a name are not the same as the same chemistry, and neither one means the plant repels mosquitoes.

Home mosquito methods compared

Method	Reported effect	Note
Growing citronella or citrosa	No reduction in bites found	The plant barely releases oil
Citronella candle	About 40% fewer bites, only when standing in the smoke	No effect once smoke disperses
Citronella oil on skin	About 10.5 minutes of full protection in one study	Needs very frequent reapplying
Registered repellent such as DEET	Several hours	Read the label, especially for children
Fans, nets, window screens	Mosquitoes are weak fliers and cannot reach you	Works every day, nothing on the skin
Removing standing water every 7 days	Breaks the cycle before larvae become adults	Works at the source

The figures of 10.5 minutes for citronella oil and about 6 hours for DEET come from studies summarised by the University of Maryland’s Maryland Grows. The same summary notes that repellents made only of essential oils gave full protection for under 20 minutes.

Larvae need about 7-10 days, and that is where you cut in

Iowa State notes that after flooding, standing water has to stay for about 7 to 10 days before mosquitoes develop. Thailand’s Department of Disease Control builds its campaign “3 keeps, prevent 3 diseases” on this, covering dengue, chikungunya and Zika:

1. Keep the house tidy: no dark, cluttered corners where mosquitoes rest; wash and turn over containers, and change the water in pots and vases every week.
2. Keep rubbish cleared: broken containers, cans and old tyres must not collect rain.
3. Keep water covered: lid every jar and container that holds water.

Example: one week on a balcony garden

Say a balcony has 6 vegetable pots, each with a saucer, and one rain barrel.

1. It rains on Monday, and all 6 saucers hold water.
2. Left alone, eggs laid on Monday could become adult mosquitoes somewhere between the next Monday and the next Thursday, using the 7-10 day window.
3. If every Sunday you tip out the saucers, scrub the bottoms so eggs stuck to the rim come off, and close the barrel lid tightly, each round always comes before day 7.
4. One round takes about 5 minutes, compared with a mosquito plant that interrupts nothing.

If a saucer is too much trouble to empty, take it away and stand the pot on bricks so water drains out. Saucers and watering are covered in Watering vegetables right.

Grow citronella if you like it, knowing what you get

Citronella grass is easy to grow and smells pleasant, so grow it for that. Just do not expect it to replace repellent. Kitchen lemongrass is a different plant, so do not put citronella grass in tom yum. Telling them apart and growing the kitchen kind are in Growing Thai kitchen herbs in pots.

Plants do not drive mosquitoes away. Water that never stands is what keeps them from being born.

Common mistakes

• Relying on a mosquito plant and skipping repellent for children. During a dengue outbreak this is more dangerous. Use nets, screens and a registered repellent as labelled.
• Watering the mosquito plant until its saucer becomes a larva pool. The plant does not help, but the water really does add mosquitoes.
• Rubbing concentrated oil on a child’s skin. Concentrated essential oils can irritate skin and protect only briefly.
• Changing vase water without scrubbing the vase. Eggs stick to the sides, so scrub as well.
• Missing hidden water. Gutters, the drip tray under the fridge, old tyres, crumpled plastic bags.

Sources

• ODPC 11 dengue warning and the “3 keeps” measures — Department of Disease Control, Thailand
• Mosquito Control — Iowa State University Extension and Outreach
• Essential oil analysis and field evaluation of the citrosa plant as a repellent against Aedes mosquitoes — PubMed
• Mosquito Repellent Myths — University of Maryland Extension, Maryland Grows
• Preventing Mosquito Bites — CDC