Thai cuisine utilises three main type of basil: Thai basil, holy basil, and lemon basil. A fourth less common basil is tree basil. The latter two are the most difficult to find outside of Thailand and the first in the list the most commonly available. Thais treat them as separate entities and so these basils are generally not interchangeable with each other. The fresher the leaves, the more you'll get out of them; the flavours will dull over time.
Thai Basil (Bai Horapha)
Thai basil looks similar to your everyday Italian basil but with purple stems, and the flavour is of bold aniseed and liquorice. If you can't find this, you can substitute with Italian basil, although the flavour will be markedly different.
Some Thai dishes that utilise Thai basil
- Kaeng keow waan (green curry)
- Kaeng phet pet yaang (red curry with roast duck – easily veganised)
- Kaeng panaeng (Panang curry)
- Fresh spring rolls
- Nam prik (Thai relishes), as an accompaniment with other vegetables and herbs
Holy Basil (Bai Kaprow)
This is a milder variety with jagged green leaves and has a peppery flavour with a grassy clove like undertone. This is a key ingredient in pad kaprow, which translates literally to fried holy basil. Always enquire at Thai restaurants when ordering this dish to see that they don't use Thai basil – this is a good way to judge the reliability of the rest of a menu!
This basil may also be spelled krapow, gaprao, gaprow, among others.
Some Thai dishes that utilise holy basil
- Pad kaprow (holy basil stir fry)
- Kaeng pa (jungle curry)
- Pad cha (spicy stir fry with fresh green peppercorns)
- Plaa dook grop pad ped (easily veganised crispy fried fish dish)
Lemon Basil (Bai Maenglak)
This is the most delicate and least utilised (outside of Thailand) of the three, and needs to be used fairly quickly as the aroma will dull after a couple of days. In the dishes where it's used (e.g. kanom jin nam yaa) it plays an important role in the overall flavour. I've found a worthy substitute to be Vietnamese lemon balm (cockscomb mint), which you can find in some Vietnamese shops in East London (try Star Night, among others). If you can't find either, omit it from the recipe without replacement.
Some Thai dishes that utilise lemon basil
Tree Basil (Bai Yee Raa)
Also known as African basil or clove basil, this unique leaf is difficult to find in the UK. If anyone sees it for sale anywhere, please let me know. The scent is a cross between cloves and bubble gum. In Thai cuisine this is used in curries, stir-fries, and as a herb to eat alongside meals.
Some Thai dishes that utilise tree basil
- Pad pet bai yee rai (spicy stir fry with tree basil)
- Different types of red curry
In London Thai basil is easily accessible in most well stocked East Asian supermarkets. You are likely to find it in Chinatown. Holy basil is a little trickier to find, but if you seek out a Thai supermarket and ring ahead, then you might have luck. Sometimes online retailers Thai Food Direct and Thai Food Shop have lemon basil in stock. They usually have Thai and holy basil as well. I have never seen tree basil for sale in the UK (I grow my own).