Vegetables through the Thai year

Many vegetables grow most easily in the cool season, from about mid October to mid February. The rainy season, from about mid May to mid October, brings the most trouble, mainly fungal disease and dull skies. In those months growers lean on quick, hardy crops, and tender salad greens can end up much smaller. Most vegetables can still be grown in every month, so the change through the year is in effort and size, not in whether vegetables exist at all.

Three seasons, five planting windows

The Thai Meteorological Department divides the year into three seasons. The hot season runs from about mid February to mid May. The rainy season runs from about mid May to mid October. The cool season runs from about mid October to mid February.

Vegetable growing follows these seasons closely. Kasetsart University's agricultural information service describes five planting windows. These are months for sowing, so the harvest comes some weeks later.

Hot season, February to April: coriander, spring onion, water spinach, cucumber, radish, yard-long bean, Chinese kale, choy sum and bitter gourd.
Early rains, May to July: Chinese kale, Chinese chives, angled luffa, shallot, baby corn and choy sum.
Late rains, August to October: dill, amaranth, Chinese chives, choy sum, Chinese cabbage, sweet potato, eggplant and chilli peppers.
Cool season, November to January: garlic, celery, cabbage, cauliflower, tomato and Chinese chives.
All year: choy sum, lettuce, coriander, yard-long bean and bitter gourd.

Why the cool months suit many crops

The Department of Agricultural Extension gives October to January as the cool-season window. It names salad greens, cabbage, cauliflower, Chinese chives, celery, carrot, garlic, shallot, tomato, peas and sweet pepper as vegetables that grow well then. Its reasoning is simple: most vegetables thrive in the pleasantly cool weather, so large volumes reach the market at this time of year.

The same page also reminds readers that vegetables can be grown all year round. The cool months are the easy months, not the only ones.

Why the rains are harder

The Department of Agricultural Extension notes that heavy rain and high humidity can damage vegetable crops if they are not managed well. For the rainy season it recommends short-cycle leafy vegetables such as choy sum, water spinach, lettuce and members of the cabbage family. It also lists climbing crops, including yard-long bean and bitter gourd, and plants such as chilli and eggplant.

The Highland Research and Development Institute, a Thai public organisation, describes two specific problems for organic vegetable growers.

Fungal diseases. Root rot and collar rot are common in the rains, and leaf spot affects salad crops such as lettuce.
Overcast skies. When the sky stays dull, plants make less food through photosynthesis, and weaker plants are attacked more easily by disease and insects.

The Institute gives one example. A cos lettuce that reaches about 150 to 200 g per plant in the cool season may reach only 20 to 50 g in the rains. It adds that growers with a greenhouse have an advantage, because they can choose from more crops without worrying about rain.

Smaller plants and more disease mean fewer good heads of tender greens in these months. That is why some leafy vegetables look scarcer, or smaller, at certain times of year, even though the farm calendar still has them on it.

In short

Thailand has three seasons: hot, rainy and cool, with the cool season from about mid October to mid February.
Salad greens, cabbage and cauliflower grow most easily in the cool months, according to the Department of Agricultural Extension.
Rains bring fungal disease and dull skies, and a lettuce can end up a fraction of its cool-season weight.
Many vegetables can be grown all year, but the effort, size and choice change with the season.