Lavender oil, from the bottle to the diffuser

In short

• Lavender oil is steam-distilled from Lavandula angustifolia and smells floral, herbal and slightly sweet.
• In a water diffuser, 3 to 5 drops per 100 ml is the usual guideline; on skin it is always diluted, and it is never swallowed.
• A real essential oil carries the Latin name, plant part and method on the label; a fragrance oil says parfum and belongs in burners, reeds and cold-air machines.

What lavender oil actually is

Lavender oil is made by passing steam through freshly cut lavender flowers. The steam carries the volatile oil out of the plant, the vapour is cooled, and the oil separates from the water and is drawn off.

The Latin name on the bottle tells you which plant you are buying. Lavandula angustifolia is the classic, softer oil. Lavandin, Lavandula x intermedia, is a hybrid that yields more oil and smells sharper and more camphorous, which is why it is cheaper and ends up in soaps and detergents.

What it smells like, and what it pairs with

Fresh lavender oil is floral first, then herbal and a little green, with a soft sweetness underneath and none of the flat, soapy note of a bathroom cleaner. It sits in the middle of a scent, lasting longer than citrus and fading sooner than wood.

• Sweet orange or bergamot, for a lighter, less floral result in a living room.
• Cedarwood or sandalwood, when you want the evening scent to linger.
• Eucalyptus or peppermint, in a hallway or bathroom where clean matters more than soft.

The oil only goes in a machine built for it. If you are still weighing a water diffuser against a cold-air one, our comparison of diffusers for essential and fragrance oils sets the two side by side, with reach, noise and what a fill costs.

How much goes in the diffuser

In an ultrasonic diffuser the usual manufacturer guideline is 3 to 5 drops of essential oil per 100 ml of water. Start at three. Lavender is not a shy oil, and a room that already smells of it does not need another drop; your nose simply stops registering it.

Run a session of 20 to 60 minutes rather than the whole evening, and use the low setting in a bedroom. Pour the water away and dry the tank afterwards, because oil left in stale water gives the next fill a musty edge. On a bedside table the noise matters as much as the output, and our guide to water and cold-air diffusers looks at noise alongside reach and running cost.

How to store it so it stays good

Light, heat and air are what spoil an essential oil. Keep lavender in the dark glass bottle it came in, cap screwed tight, in a cool cupboard rather than on a sunny windowsill or above the radiator. Oxygen does the slow damage, so a half-empty bottle ages faster than a full one.

Lavender keeps reasonably well next to citrus oils, which lose their brightness sooner. An oil that has turned smells flat, sharp or faintly of turpentine, and may have thickened. Old oil goes in the bin, not in the diffuser and not on skin.

Real lavender oil or a fragrance oil

The two sit on neighbouring shelves, often in the same amber bottle. A fragrance oil is a blended scent, usually made to smell like lavender rather than from it, and it belongs in a burner, on reeds or in a cold-air diffuser. It never goes in a bath, on skin or in a water diffuser. A real essential oil shows most of the following on its label.

• The Latin name, Lavandula angustifolia, rather than only the word lavender.
• The plant part and the method: flowering tops, steam distilled.
• A country of origin, and often a batch number.
• The words essential oil on their own, not aroma oil or lavender scented oil.
• A modest bottle; pure oil in a large, cheap bottle is a warning sign.

A fragrance oil label says parfum, aroma oil or for oil burners, and its ingredients list may run to several lines. One household trick: a drop of essential oil on plain paper evaporates and leaves little behind, while a fragrance oil on a carrier leaves a greasy ring. It is not proof, but it is a start.

Neither type is wrong; they are made for different machines. Once you know which oil you want to run, the guide to which diffuser suits which oil walks through what each machine costs to keep going and how much noise it makes at night.