A hot greenhouse can be a difficult place for a plant – there are plenty that would prefer life outdoors, even in a traditional British summer. But if you'd like to squeeze a few treats in with the tomatoes and cucumbers, then there are some tropical herbs that thrive in hot and humid conditions.
Culantro, Eryngium foetidum, is also known as spiny coriander, and is used as a coriander substitute in places such as the West Indies (and parts of Latin America and Asia) where Coriandrum sativum wilts in the heat. A perennial with a strong coriander-like flavour, culantro leaves can be used fresh, or dried. They can even withstand a little bit of cooking. The plants grow to around 20 cm and like it hot and steamy. They are less inclined to flower in partial shade (letting you harvest leaves for longer), so they'll be happy behind the tomatoes. Sow seeds indoors in spring, and protect plants from frost.
Lemongrass, Cymbopogon citratus, is a stalwart of Thai cuisine, and also makes a lovely herbal tea. Evergreen in the tropics, it likes to be hot and wet during the summer, and warm and dry during the winter – keep it above 8°C if you want it to survive. Lemongrass can grow up to 1.5 metres tall, and 1 m wide, and likes to be planted in soil, or soil-based compost (it's happy growing in containers). Seeds sown in spring germinate in 15 – 25 days, but you could also buy a plant or try rooting stems from the supermarket in a glass of water.
Cardamom, Elettara cardamomum. Cardamom pods will be familiar to anyone who likes Indian cuisine, but unfortunately this tropical evergreen perennial is unlikely to flower and set seed in our climate. However, the leaves can also be used to infuse food with flavour, and are often used to line steamer baskets. A mature clump can grow to 3 metres tall, and will spread sideways, but needs to be kept above 10°C all year round. Cardamom likes rich, moist soil, and is another plant that thrives in partial shade. Usually hard to find, plants are available from Sutton Seed's Homegrown Revolution range this year.
Curry leaf, Murraya koengii, is a tree that can grow to 6 metres tall, with a 5 metre spread – but it's a slow grower, and mature trees can be pruned in winter, so it can be kept small. It needs a minimum temperature of 13°C, and young plants need very regular watering in their first three years of growth. The curry leaf tree can be grown in a container, and pot-grown plants should be given a regular liquid feed. Leaves have a curry flavour, and are usually used like bay leaves – added to food during cooking, and removed before serving. You can use the leaves fresh, but they have a short shelf-life; dry leaves to store them. This plant can be tricky to grow from seed, and as it's slow-growing to boot the best bet is to source a plant.
Vietnamese coriander, Persicaria odorata, has leaves that have a mild coriander flavour when they're young (with a lemony kick). Mature leaves develop a pungent heat. Plants need a minimum temperature of 7°C, and grow to a height of 45 cm. Clumps spread, and in a heated greenhouse are best grown in pots to prevent them becoming invasive (in colder areas unprotected plants are killed by winter frosts, so this plant's weediness is not a big concern). Vietnamese coriander is another herb that prefers a nice, hot environment but partial shade. Seeds aren't available, so look for a plant supplier. As you can guess from the name, this herb is mainly used to flavour Vietnamese dishes.
None of these lovely plants will survive the winter in an unheated greenhouse, but if heat is not an option then you could always keep them in pots and bring them indoors during the cold months. Most would be happy on the kitchen windowsill, and cardamom in particular makes a very exotic houseplant.