Leafy greens in Thai heat: kale, pak choi, choy sum, lettuce, shade and bolting

The leafy greens in this lesson do not handle heat equally. Chinese kale (kana), pak choi and choy sum (pak kwang tung) grow almost year-round in the Thai lowlands if you pick them on time and keep the rain off. Lettuce prefers about 15-20°C and bolts early above 30°C. Getting them through the heat comes down to choosing the right crop, the right timing, and the right amount of shade - not too much.

How heat-tolerant are these four?

Crop	Preferred temperature	Days to harvest	In the heat
Chinese kale	Germinates best at 25-30 °C, grows best at 18-28 °C	Young plants from about 3 weeks; in Thailand about 30-55 days	More heat tolerant than most of its cabbage relatives; grown year-round in the tropics
Pak choi	20-25 °C, germinates in 3-5 days	Usually 30-45 days	Heat- and wet-tolerant, year-round in the lowlands, but not flood-tolerant
Choy sum	15-25 °C gives strong, sweet stalks	30-50 days	Dislikes waterlogged soil; too much rain causes rot
Lettuce	15-20 °C	Loose-leaf types in Thailand about 40-45 days	Bolts early above 30 °C; grow heat-tolerant cultivars

The figures come from the Southeast Asian vegetable handbook (PROSEA), the University of Florida’s choy sum guide, lettuce research, and an FAO report on vegetables in Thailand; see the list at the end.

What bolting is, and why it happens

Bolting is when a plant stops making leaves and sends up a stalk to flower. Leaves become fewer, tougher, and in many crops more bitter. What triggers it depends on the crop.

• Lettuce: heat is the main trigger. In one experiment, lettuce grown at 33°C by day and 25°C by night was compared with 20/13°C. The hot group bolted clearly by about day 32; the cool group did not bolt at all.
• Pak choi and Chinese kale: in the tropical handbooks, cold is what speeds up flowering, which hardly happens in the Thai lowlands. The real problem is leaving them in the bed too long: tropical cultivars start to bolt and lose their market value, and pak choi left too long also turns to soft rot easily.
• Choy sum is different, because you already eat its flower buds. Plants start forming flowers at about 7-8 leaves or roughly 20 cm tall. Harvest when the buds are full but before they open.

In the Thai lowlands, these brassicas rarely bolt from heat - they bolt from being left too long. Lettuce bolts from heat. The fixes are different.

Shade cloth: what percentage?

The percentage is the share of light the cloth blocks. A 30% cloth lets about 70% through.

• A trial in Hawaii grew lettuce in full sun and under 30%, 47%, 63% and 73% shade. In one season, 30% shade gave significantly higher yields than full sun; in the other, yields dropped only slightly as shade rose to 47%. Of the five crops tested, lettuce was the only one that did well under light shade.
• Reading the result: light shade of about 30% is the safe spot. By 47% yields were already starting to slip, so there is no case for denser shade over lettuce.

For Chinese kale and pak choi, the handbook says kale does well in sunny conditions, and pak choi that is crowded or shaded stretches and goes pale. They do not need heavy shade; if you cover them at all, cover them to keep the rain from battering them.

Timing with the Thai seasons

Thailand’s year runs as a hot season from about mid-February to mid-May, a rainy season from mid-May to mid-October, and a cool season from mid-October to mid-February.

Season	Chinese kale, pak choi	Choy sum	Lettuce
Cool	Best	Best	Best
Hot	Fine, water more	Fine	Hard; heat-tolerant cultivars and about 30% shade
Rainy	Fine with rain cover and drainage	Watch for rot	Rain-tolerant cultivars; shelter from pounding rain

For the rainy season, the handbook recommends mulch or a rain shelter so seed is not washed out, and temporary netting or a cover to stop plants from lodging and leaves from being battered. For choy sum, the Florida guide says plants cannot take more than about 300 mm of rain a month or very wet soil without bacterial soft rot. A raised trough under the eaves helps a lot.

Worked example: planning a 1 x 2 m bed for the cool season

Say you have a bed 1 m wide and 2 m long and want Chinese kale and lettuce.

1. Kale on half the bed (1 x 1 m). Final spacing 30 x 30 cm, with a 15 cm margin on every side, leaves a 70 x 70 cm planting area. Rows at 0, 30 and 60 cm give 3 rows of 3 plants: 9 plants.
2. Lettuce on the other half. Assume 20 x 20 cm spacing. In 70 x 70 cm, rows at 0, 20, 40 and 60 cm give 4 x 4 = 16 plants.
3. Harvest dates. Sow kale on 1 November; at 30-55 days that is about 1-26 December. Loose-leaf lettuce at 40-45 days, sown the same day, is ready about 11-16 December.
4. Keep it coming. For a steady lettuce supply, sow a new batch every 2 weeks, but stop around early January: anything sown later grows into mid-February, when the heat returns.

If the bed is in the open and you see daytime temperatures above 30°C several days in a row, put 30% cloth over the lettuce only. Leave the kale in full sun.

The main brassica pest

The most important pest of Chinese kale, pak choi and choy sum is the diamondback moth caterpillar, which is resistant to many insecticides. The handbook notes that in Thailand whole kale fields are covered with netting against it. That works without chemicals in a small home bed too: cover it with fine insect mesh from sowing and open it only to tend the plants. For pest control without pesticides in general, see Start a pesticide-free garden in a small space.

The other disease worth knowing is clubroot. The handbook says pak choi is very susceptible. If you pull a plant and find swollen, misshapen roots, throw out the plant and the soil from that pot, and stop growing cabbage-family crops in that soil.

Common mistakes

• Growing ordinary lettuce in full sun in the hot season. It bolts and turns bitter before you get to eat it.
• Dense 70% shade all day. Plants stretch, leaves thin out, growth slows.
• Leaving pak choi past its harvest date. It bolts and rots.
• Waiting for choy sum to flower yellow. The stems toughen and lose their sweetness.
• Planting too close. No airflow, stretched plants, and rot spreads quickly.

The next lesson is tomatoes in the Thai climate, a crop where both heat and rain hit the harvest hard.

Sources

• Brassica oleracea (Chinese kale group) — PROSEA
• Brassica rapa (Pak Choi group) — PROSEA
• Production Guide for Choy Sum — UF/IFAS Extension
• Lettuce bolting induced by high temperature — PMC
• Productivity under shade in Hawaii of five crops grown as vegetables in the tropics — FAO AGRIS
• The vegetable sector in Thailand: Chinese kale and lettuce — FAO