Mosquito repelling plants

Every summer, garden centres promote pots of citronella grass, lavender, mint and scented geraniums as natural mosquito repellents. The idea is appealing. Arrange a few fragrant plants around the deck and mosquitoes will supposedly stay away.

Unfortunately, plants do not work that way. Some contain chemicals that can repel mosquitoes when extracted, concentrated and properly formulated. A living plant quietly growing in a pot generally releases far too little of those chemicals to protect the people sitting nearby.

That does not make mosquito repellent plants uninteresting. Their chemistry, and the claims made about them, offer plenty to explore.

1. A mosquito repellent plant does not create a protective force field

There is little evidence that the aromatic plants commonly sold for patios, including the scented geranium marketed as the mosquito plant, provide meaningful protection from mosquito bites simply by growing nearby.

The mosquito repellent aromatic compounds of lavender, mint, basil or citronella grass remain largely inside the plant’s oil glands. Unless the foliage is damaged, heated or processed, relatively little is released into the surrounding air.

Even when leaves are rubbed, the effect is local and brief. It does not create a reliable mosquito-free zone around a patio.

2. Citronella is a plant, an oil and a product ingredient

The word citronella can refer to several different things. True citronella oil is distilled mainly from tropical grasses such as Java citronella, Cymbopogon winterianus, and Ceylon citronella, Cymbopogon nardus.

The oil contains compounds including citronellal, citronellol and geraniol. These compounds can affect mosquito behaviour, which is why citronella oil is used in some registered insect repellent products.

A pot of citronella grass, however, is not equivalent to applying a tested product containing a measured concentration of citronella oil.

3. The “mosquito plant” is usually a scented geranium

Plants sold under names such as mosquito plant, citronella plant or citrosa are usually scented pelargoniums (annual geraniums), not true citronella grass.

Their leaves may smell lemony when touched, but a pleasant citronella like scent is not proof that the plant repels mosquitoes. The marketing name appears to have become much more convincing than the plant itself.

It can still be grown as an attractive, fragrant annual in temperate climates; in warmer climates, it is a perennial. It simply should not be relied upon as mosquito protection.

4. Essential oils can repel mosquitoes, but often evaporate rapidly

Many essential oils can discourage mosquitoes during laboratory tests. Citronella, peppermint, clove, thyme and other oils have all shown some repellent activity.

The difficulty is that these chemicals are volatile. They evaporate rapidly from skin and surfaces. A product may work initially but lose effectiveness much sooner than DEET or icaridin. A systematic review found that citronella products generally provided a shorter duration of protection than DEET products.

This is one reason a plant extract that looks promising in a laboratory does not automatically become a useful backyard repellent.

5. More essential oil is not necessarily better

Tests sometimes obtain the longest protection from highly concentrated or even undiluted essential oils. That does not mean people should apply undiluted oils to their skin.

Some concentrated essential oils can irritate skin and eyes and may cause allergic reactions. Their composition can also vary according to the plant variety, growing conditions, harvest time and extraction method. A homemade mixture does not come with the safety testing, standardized concentration or application directions of a registered product.

“Natural” describes where a chemical came from. It does not tell us whether a particular dose is safe.

6. Oil of lemon eucalyptus is not ordinary eucalyptus oil

Oil of lemon eucalyptus is one of the few plant-derived ingredients recognized as an effective personal mosquito repellent. Its effectiveness is associated largely with a compound called “para menthane 3,8 diol”, usually abbreviated as PMD.

This is not the same as ordinary eucalyptus essential oil from a bottle, nor is it the same as growing a eucalyptus plant on the patio. Public health authorities recommend formulated and registered oil of lemon eucalyptus or PMD products, not unformulated essential oil.

Products containing these ingredients should be used according to their Canadian labels and age restrictions.

7. Catnip really does contain an insect repellent chemical

Catnip, Nepeta cataria, contains nepetalactone, the chemical responsible for much of its effect on cats. It can also interfere with mosquito behaviour.

That makes catnip chemically interesting, but it does not mean a border of catnip will stop mosquitoes from entering the garden. As with citronella, the presence of an active chemical inside the plant is not the same as distributing an effective dose around a person.

Mosquitoes may be unimpressed, while neighbourhood cats could show considerably more interest.

8. Crushing leaves is not a good substitute for repellent

Rubbing mint, lavender, basil or geranium leaves on the skin may release aromatic oils, but it provides no dependable information about dose, coverage or length of protection. Don’t use it as a substitute for a tested repellent when mosquito-borne disease is a concern.

Some mosquitoes can transmit West Nile virus and other illnesses. Canadian public health guidance recommends registered repellents containing ingredients such as DEET or icaridin rather than relying on crushed plants or homemade preparations.

9. Citronella candles offer limited protection

Citronella candles may reduce mosquito activity immediately around the candle, but field research has found only partial protection. Their effect is limited outdoors because the repellent compounds disperse quickly and are affected by wind and distance.

One candle in the middle of a table should not be expected to protect everyone sitting around it. A fan may be more useful because moving air can interfere with mosquito flight and disperse some of the carbon dioxide, heat and body odours mosquitoes use to find people.

10. The best mosquito control may begin without a plant

The most useful garden-based mosquito control is usually to remove opportunities for mosquitoes to reproduce. Containers, clogged eavestroughs, toys, tarps, wheelbarrows and plant saucers can all hold water.

Water features do not need to be eliminated, but water should circulate or be managed so mosquito larvae cannot develop. Bird baths should be emptied and refilled with fresh water at least twice a week. Screens on water barrels and mosquito dunks in ponds are effective. For people, loose fitting long clothing and registered skin repellents provide more dependable personal protection. Health Canada identifies DEET and icaridin products as effective options when used according to their labels.

Lavender, catnip, basil, mint and scented geraniums are still worth growing. They offer fragrance, flowers, culinary uses and interest in the garden. Those are good reasons to plant them.

Just do not expect them to stand guard over the patio.