Basil Thai Siam Queen

Ocimum basilicum var. thyrsiflora 'Siam Queen' Award-winning Thai basil — bold anise flavour and showy purple flowers

The aristocrat of Thai basils. 'Siam Queen' is an improved, award-winning cultivar of Thai sweet basil — an All-America Selections winner, no less — bred to take everything that's good about traditional Thai basil and make it bigger, bolder, and more beautiful. It carries the unmistakable anise-and-licourice flavour of Southeast Asian cooking, but with extra intensity and a hint of clove and citrus, on a plant that's as ornamental in the border as it is indispensable in the kitchen.

What sets Siam Queen apart from ordinary Thai basil is breeding. The leaves are notably larger — close to twice the size of older Thai basil strains — which makes for easier picking and a more generous harvest, while the plants are more uniform, more vigorous, and more resistant to bolting in the heat. The flavour is correspondingly bigger: a powerful, sweet-spicy fragrance of anise, clove, and mint that defines a proper green curry or a bowl of pho. It's a genuine improvement on the traditional types, which is exactly why it earned its award.

It is also strikingly handsome. Glossy deep-green leaves sit on purple-flushed stems, and as the season goes on the plant throws up showy spikes of deep purple-lavender flowers that stand proud above the foliage — beautiful in a herb bed or container, lovely in a cut arrangement, and a genuine magnet for bees and butterflies. Few culinary herbs earn their place on looks alone, but Siam Queen comes close.

Like all Thai basil, it's far more heat-stable than Italian sweet basil, holding its aroma through cooking rather than fading the moment it meets a hot pan — the quality that makes it essential to curries and stir-fries. It's a tender annual in the UK, easy to grow for a gardener of any level, and equally happy in a greenhouse, a sunny border, or a pot on a warm patio or windowsill.

A note on growing

Sow indoors from March to May. As with all basils the seed needs light to germinate, so scatter it across the surface of moist seed compost and cover with only the finest dusting of vermiculite, or nothing at all. Keep it warm at 20–25°C on a sunny windowsill or in a heated propagator; germination takes around 7–14 days, sometimes a little longer. Basil loves warmth and won't grow away strongly until both day and night temperatures have risen, so there's no advantage to sowing too early in a cold spring.

When the seedlings have their first three sets of leaves, pinch out the growing tip to encourage a bushy, well-branched plant, and keep pinching through the season. Prick out and pot on into good compost. As a warmth-loving plant, Siam Queen should be kept under cover until the nights are reliably warm in summer; it can then go to the sunniest, most sheltered spot outdoors, though in much of the UK it crops best in a greenhouse, conservatory, or on a bright windowsill. It's a naturally sturdy, upright grower, reaching around 45–75cm.

Water in the morning, at the base, keeping the foliage dry to avoid mildew, and keep the compost evenly moist. Feed occasionally through the season. Harvest leaves regularly from summer into autumn — ideally first thing in the morning, when the aromatic oils are at their most concentrated — and pinch out most flower spikes to keep new leaves coming. Siam Queen is more bolt-resistant than older Thai basils, so it stays productive well into the season, but it's still worth letting a few of those gorgeous purple flower spikes develop for the bees and for the kitchen.

Where it shines

In the kitchen, Siam Queen is the basil for authentic Thai and Vietnamese cooking: green and red curries, pad-krapow-style stir-fries, pho and other noodle soups, fresh salads, and spring rolls. Because it holds its flavour under heat, it can be cooked into a dish as well as scattered over it — add a handful early to build the sauce and another at the end for a bright aromatic lift. It also makes a wonderful infused oil or vinegar, and the leaves dry and freeze well for use out of season.

In the garden, it's one of the few herbs you'd happily grow for its looks alone. The deep purple flower spikes are genuinely ornamental, excellent in containers and herb beds, lovely in a cut arrangement, and alive with pollinators through the warm months.

At a glance

• Type: award-winning Thai sweet basil (var. thyrsiflora), a tender annual
• Award: All-America Selections winner — a proven, improved cultivar
• Flavour: bold, sweet-spicy anise and licourice with clove and citrus notes
• Plant: sturdy and upright, 45–75cm, large leaves, showy purple flower spikes
• Cooks well: heat-stable, holds its flavour in curries and stir-fries
• Sow: March to May, indoors — surface sow, needs light to germinate
• Germination: 7–14 days at 20–25°C
• Bonus: bolt-resistant, highly ornamental, and a pollinator magnet

Plant alongside

Siam Queen shares its love of warmth and sun with chillies, peppers, and tomatoes, making it a natural greenhouse companion and a culinary partner too — chilli and Thai basil belong together on the plate. Its showy purple flowers make it more ornamental than most herbs, so it earns a place near fruiting crops that benefit from the bees and butterflies it draws in, and it looks particularly handsome grown among other flowering herbs in a sunny container collection.