Vegetables in the Thai hot season: mulch depth, shade cloth % and watering
The Thai hot season wilts vegetables by afternoon. What to mulch with and how deep, which shade cloth percentage, and a watering schedule that uses less water.
- Organic mulch such as rice straw, dry leaves or dry grass keeps the topsoil from drying out and cuts the water you need a lot. Typical depth is about 5-10 cm depending on the material.
- For vegetables, about 30% shade cloth is the usual choice; heat-sensitive leafy greens can take up to about 50%. Denser than that and plants don't get enough light.
- A vegetable bed needs about 25 litres per square metre per week as a starting point. Water deeply in the early morning rather than a little every afternoon.
- Push a finger 5-7 cm into the soil. If it's still moist there, don't water yet.
Thailand’s hot season runs from about mid-February to mid-May, peaking in April. Vegetable beds struggle now for two reasons: the soil dries out fast and the afternoon sun is too strong. Three things work together: mulch so the topsoil stays cool and moist longer, shade cloth for leafy crops, and deep early-morning watering instead of frequent sprinkles.
This is lesson 13 of the “Grow your own vegetables” path, building on sun, soil and water from starting a pesticide-free garden.
How hot is the Thai hot season?
According to the Thai Meteorological Department, the national average maximum temperature for the hot season over 1981-2010 was about 34.8 °C, against about 31.4 °C in the cool season. Those are averages; plenty of April days run much hotter. The actual start date shifts each year, and the department declares it once winds and temperatures have changed.
In the bed you’ll see leafy crops such as lettuce wilt in the afternoon even when the soil is still damp, bare soil dry and crack quickly, and small pots dry out within a day. Fix soil moisture and afternoon sun, and most vegetables get through the hot season.
What to mulch with, and how deep
Mulch is any material spread over the soil around your plants. Thailand’s Department of Agricultural Extension recommends using leftover material you already have, such as grass, rice straw, leaves or coconut fronds: it keeps the topsoil from drying out, holds moisture longer, suppresses weeds and turns into organic matter as it breaks down. Colorado State University Extension says mulching cuts irrigation needs by around 50%.
Sources give slightly different depths, so here they are as ranges:
| Material | Approximate depth | Watch out for |
|---|---|---|
| Rice straw, dry grass | 7-10 cm (straw settles when wet) | often carries weed or grain seed |
| Dry leaves | 5-10 cm | shred them first so they don’t mat |
| Finished compost | 2.5-5 cm | works well as a thin layer |
| Fresh grass clippings | no more than about 0.5 cm per layer | let each dry first; never from a recently sprayed lawn |
Spread it evenly and leave a small gap around each stem rather than piling it against the plant. A stem base that stays wet rots easily, and seedlings need a wider gap than older plants. If you make your own compost, a thin layer underneath topped with straw works well.
Mulch thick enough that you can’t see bare soil, but don’t pile it against the stems. That is the most common mistake.
Which shade cloth percentage?
Shade cloth is sold by the percentage of light it blocks, from about 20% to 90%. University of Delaware Extension recommends 20-40% for vegetables, with about 30% the most common choice. Another extension sheet gives 30-50% as a general rule for vegetables. Go denser and plants don’t get enough light to photosynthesise well.
| Crop group | Shade cloth | Why |
|---|---|---|
| Heat-sensitive leafy greens such as lettuce | 30-50% | helps slow bolting, less heat load on leaves |
| Fruiting crops such as chilli and tomato in hot spells | about 30% | they still need sun to flower and set fruit |
| Sun-loving crops | none, or afternoon only | all-day shade makes them leggy and less productive |
Hang the cloth high enough above the plants for air to move, not draped down to the soil. If you can, put it up from late morning to late afternoon and leave the mornings open for gentle sun.
How much water? A 1 x 2 m bed, worked through
NC State Extension uses a starting figure of about 1 inch of water per week for a vegetable garden, roughly 25 mm. In litres, step by step:
- 25 mm of water over 1 m² is 0.025 m³, or 25 litres.
- A 1 x 2 m bed is 2 m², so it needs about 2 x 25 = 50 litres a week with no rain.
- Split into 3 waterings a week, that’s about 17 litres each, a little under two 10-litre watering cans.
- With good mulch, the water you need can drop by up to about half, to around 25 litres a week.
- Mulch 5 cm deep over 2 m² takes about 2 x 0.05 = 0.1 m³, or about 100 litres of material.
Treat these as starting numbers. Extreme heat, wind or sandy soil can push the need higher, and small pots dry far faster than beds, so check them every day.
The simplest check is a finger 5-7 cm into the soil: still moist means no water yet. To find out how fast your hose or sprinkler delivers, stand a straight-sided can in the bed and time how long it takes to fill 2.5 cm.
When and how to water
- Early morning is best. Water soaks in before the sun gets strong. Evening is fine if it isn’t too late. Avoid the hot afternoon, when much of the water evaporates.
- Deep and less often beats a little every day. Keep the soil evenly moist, not soggy.
- Water the base, not the leaves. Drip lines or soaker hoses put water at the roots and lose less than sprinklers.
- Check the soil every time. If it’s still moist 5-7 cm down, don’t water, even if the leaves look soft in the afternoon.
Water by the soil, not by the clock. Soil that stays soggy starves roots of air, just as soil that’s too dry starves them of water.
Common mistakes
- Watering every afternoon because the leaves droop. The soil gets soggy, roots lack air, and much of the water evaporates.
- Piling mulch against the stems. The base stays wet and rots easily.
- Shade cloth that’s too dense. Plants stretch, leaves thin out and fruiting crops flower less.
- Straw or grass of unknown origin. It may carry weed seed or herbicide residue that damages vegetables.
- Letting the soil dry out completely, then soaking it. Fruiting crops that go short of water and then get a lot at once can split their fruit.
The next lesson covers the rainy season, where the problem flips: too much water instead of too little.
Sources
- DOAE promotes mulching to keep topsoil moist in the dry season (Thai) — Thai Post
- Country Report Thailand: seasons and seasonal temperatures — Thai Meteorological Department / WMO
- Shadecloth for Hot Season Vegetable and Fruit Production — University of Delaware Cooperative Extension
- Mulches for the Vegetable Garden — Colorado State University Extension
- Mulching Vegetables — University of Georgia Extension
- Watering Wisely: How Much Your Summer Vegetable Garden Really Needs — NC State Extension
FAQ
Can I mulch with fresh lawn clippings?
Yes, but thinly: layers no thicker than about half a centimetre, letting each dry before adding the next. A thick pile mats down, smells and blocks water and air. Never use clippings from a lawn treated with herbicide or other pesticides in the past month.
Black or white shade cloth?
Black and green are the most common. White or aluminised cloth reflects more light, so it may keep things slightly cooler underneath. The shade percentage matters more than the colour.
Is evening watering OK?
Yes, as long as it isn't so late that leaves stay wet overnight. Early morning is still best. What to avoid is watering in the hot afternoon: much of the water evaporates, and wet leaves in strong sun can scorch.
General information, not medical advice. If you are pregnant, take regular medication, or have a liver or kidney condition, ask a doctor or pharmacist before taking herbs beyond normal food amounts.