This guide gives target numbers for the four things every cannabis plant needs: light, water, nutrients and air. For each one, it explains what the number measures and why it matters. It also says what to check first.
Translator’s notes show where conditions in Thailand and Southeast Asia make these targets harder to reach. They also say what to do instead.
Cannabis uses the most common type of photosynthesis, called C3 (the plant fixes carbon through a process called the Calvin cycle). Plants of this type react strongly to changes in light, carbon dioxide (CO₂), temperature and water. A change in any one of these four shows up in growth and yield.
1. Light
What to measure
Plants use light between 400 and 700 nanometres (nm) for photosynthesis. Growers call this band PAR (photosynthetically active radiation). Three measurements describe the light your plants get:
- PPFD (photosynthetic photon flux density): how much usable light reaches the top of the plants each second, in μmol/m²/s. You measure it with a PAR meter held at canopy height.
- DLI (daily light integral): the total usable light a plant receives in one day, in mol/m²/day. The formula is DLI = PPFD × hours of light × 0.0036.
- Spectrum: the colour mix of the light.
Example: a seedling under 300 μmol/m²/s for 18 hours a day gets 300 × 18 × 0.0036 ≈ 19.4 mol/m²/day.
What each colour does
- Blue (400–500 nm) keeps plants short and compact, with short gaps between leaf nodes.
- Red (600–700 nm) supports flowering.
- Far-red (about 700–750 nm, just beyond the classic PAR band) also drives photosynthesis when it is mixed with 400–700 nm light. In studies on 14 crop species, far-red photons worked as well as 400–700 nm photons when they made up to about 30% of the light (Zhen, van Iersel and Bugbee, Frontiers in Plant Science, 2021, 12:693445). This does not mean far-red adds 30% to photosynthesis.
- UV-B (a band of ultraviolet light) is often said to increase trichomes, the resin glands that hold cannabinoids and terpenes. A 2021 study by the same Guelph team as the light study below found no commercially useful gain in flower yield or cannabinoid content from added UV-B. In one of the two cultivars, yield and THC fell (Rodriguez-Morrison, Llewellyn and Zheng, Frontiers in Plant Science, 2021, 12:725078).
Targets by growth stage
The light/dark schedule is written as hours of light / hours of darkness. For example, 18/6 means 18 hours of light and 6 hours of darkness. Cannabis that is not an autoflower keeps growing leaves and stems under 18/6. It switches to flowering under 12/12.
| Stage | PPFD (μmol/m²/s) | DLI (mol/m²/day) | Light/dark hours |
|---|---|---|---|
| Seedling | 200–400 | 13–26 | 18/6 |
| Vegetative (leaf and stem growth) | 400–600 | 26–39 | 18/6 |
| Flowering | 600–900 | 26–39 | 12/12 |
| Flowering with added CO₂ | 900–1,500 | 39–65 | 12/12 |
The DLI column comes straight from the formula. For example, 600 × 12 × 0.0036 ≈ 26, and 900 × 12 × 0.0036 ≈ 39.
A 2021 study: more light kept raising yield
Rodriguez-Morrison, Llewellyn and Zheng published a cannabis lighting study in Frontiers in Plant Science (2021, 12:646020). They grew cannabis indoors at normal CO₂ of 437 ppm, with no CO₂ added. Light levels ranged from 120 to 1,800 μmol/m²/s.
Photosynthesis in a single leaf levelled off well below 1,800 μmol/m²/s. Dry flower yield did not level off. It rose in a straight line across the whole range. At 1,800 μmol/m²/s, the plants produced 4.5 times more flower than at 120.
Potency per gram stayed the same at every light level. The brighter rooms produced more total cannabinoids only because they produced more flower. The flower itself was no stronger.
The study’s authors give a reason why the leaf levelled off and the whole plant did not. A plant has many leaves at different depths. Shaded lower leaves get less light and use it more efficiently, so the whole plant can keep using extra light after the top leaves are saturated.
CO₂ was not needed to keep yield rising, because at normal CO₂ the study found no light level where yield stopped. The study did not test added CO₂. The authors only say that yield may respond to extra CO₂ much as it does to extra light, and that this needs more research. They also estimated that going from 900 to 1,800 μmol/m²/s raised yield by about 70% but cut grams of flower per kWh by about 15%, based on an assumed lamp efficiency.
Translator’s note: In Thailand, every extra watt of light also adds heat to the room. In a room already at 30–35 °C, the air conditioner then has to work harder. You pay twice: once for the lamp and once for the cooling. So the study does not mean “always buy the brightest lamp”. It means light is often what limits yield, and it is worth testing how far your own room can go.
Translator’s note: Outdoors and in greenhouses in Thailand, the natural day stays close to 12 hours all year. It never comes near the 18 hours used for vegetative growth. Photoperiod plants grown under natural light can start flowering while they are still small. The topic why photoperiod plants flower early in Thailand explains this and the four ways growers stop it.
What to do: Measure PPFD at canopy height in the centre and at each corner of your growing area. This shows you where light is weak. Raise the light in steps from one cycle to the next. Each cycle, record dry flower weight and the kWh used for lights and cooling, from a plug-in power meter or a sub-meter. This shows whether the extra light paid for itself in your room.
2. Water
Plants use only a small share of the water they take up for photosynthesis, often given as about 1%. The rest leaves through stomata, tiny pores on the leaf surface, as water vapour. This loss is called transpiration. It cools the leaf and pulls water and dissolved nutrients up from the roots.
How to water
- Check before you water. Push a finger 3–5 cm into the growing medium. If it feels dry, water. If it is still moist, wait. This matters because roots need air as well as water. Watering a medium that is still wet starves the roots of air.
- Water thoroughly. Keep watering until 15–20% of the water drains out of the bottom of the pot. Growers call this drained water “runoff”. For example, if you pour 2 litres, 0.3–0.4 litres should drain out. This wets the whole root zone and washes out leftover fertiliser salts.
- Test the runoff. Measure the pH and EC of the runoff (both are explained in Section 4). If runoff EC climbs well above the EC of the water you pour in, salts are building up in the pot.
Too much water or too little
Both problems make leaves droop, so check the leaves and the medium together.
| Sign | Too much water | Too little water |
|---|---|---|
| Leaves | Droopy, but firm and heavy; medium is wet | Droopy, thin and crispy; medium is dry |
| Leaf colour | Dark green, yellowing from the lower leaves | Yellows quickly, dries from the leaf edges |
| Recovery | Slow (several days) | Fast (a few hours after watering) |
| Main risk | Root rot (Pythium, a water mould that attacks roots) | Stress and lower yield |
Translator’s note: At 30–35 °C, pots dry out much faster than in the cooler climates where most grow guides are written. In the rainy season, outdoor pots can stay wet for days. So do not water on a fixed calendar. Raise outdoor pots off the ground and keep the drainage holes clear, so rain does not sit in the root zone. Warm, waterlogged roots are at high risk of root rot.
What to do: Lift each pot just after watering, and again when the medium is dry at 3–5 cm. Learn the weight difference. After a week, you can judge whether a pot needs water in two seconds, without disturbing the roots.
3. Nutrients
3.1 The three main nutrients: nitrogen, phosphorus, potassium
Fertiliser labels list these three as N-P-K:
- Nitrogen (N) builds proteins and chlorophyll, the green pigment that captures light. Plants need the most nitrogen in the vegetative stage.
- Phosphorus (P) is part of ATP, the molecule that carries energy inside cells. It supports root and flower development. The original article recommends more phosphorus in flowering. The forum’s topic on high-phosphorus feeding tests how far that holds.
- Potassium (K) controls water movement inside the plant and activates enzymes. The need for it stays fairly steady through the whole life cycle.
The figures below are ratios, not label strengths. A 3-1-2 ratio means three parts nitrogen to one part phosphorus and two parts potassium. A product labelled 9-3-6 has the same ratio.
| Stage | N-P-K ratio | What it means |
|---|---|---|
| Seedling | 1-1-1, light | Balanced feed at a quarter of the full dose |
| Vegetative | 3-1-2 | Extra nitrogen for leaves and stems |
| Early flowering | 2-2-2 | Balanced feed during the switch to flowering |
| Mid to late flowering | 1-3-2 | Extra phosphorus for flower development |
| Flush | 0-0-0 | Plain water only for the last 1–2 weeks before harvest |
Translator’s note: Use only fertilisers registered in Thailand. Follow the maker’s own feeding chart for the product you use. If you have questions about a product, ask its official distributor in Thailand. The topic verifying a fertiliser is legally sold in Thailand shows how to check registration on the label.
What to do: Read the N-P-K numbers on every product you feed now. Work out which stage each one fits. This shows whether you are still giving high nitrogen in late flowering. The summary table below lists that as the most common feeding mistake.
3.2 Secondary nutrients and trace elements
- Calcium (Ca) builds cell walls. Shortage: brown spots on new leaves.
- Magnesium (Mg) is the atom at the centre of each chlorophyll molecule. Shortage: yellowing between the veins of old leaves, while the veins stay green.
- Sulphur (S) is part of the amino acids cysteine and methionine, which the plant uses to build proteins. Shortage: even yellowing across new leaves.
- Iron, manganese, zinc, boron, copper and molybdenum (Fe, Mn, Zn, B, Cu, Mo) are needed only in tiny amounts. The plant still cannot grow properly without them.
Whether a symptom starts on new or old leaves is the first clue to which nutrient is short. The guide on reading nutrient deficiencies takes you through the rest.
What to do: When leaves change colour, photograph them and note whether new or old leaves show it first. Check root-zone pH (Section 4) before adding any nutrient. A pH outside the target range can block uptake even when the nutrient is present.
4. Root-zone acidity and nutrient strength
pH shows how acidic or alkaline the water or medium is. 7 is neutral, and lower numbers are more acidic. EC (electrical conductivity) shows how much dissolved fertiliser salt is in the water, in mS/cm. A higher EC means a stronger solution.
| Medium | Target pH | EC seedling | EC vegetative | EC flowering |
|---|---|---|---|---|
| Soil | 6.0–7.0 | 0.4–0.8 | 0.8–1.4 | 1.2–1.8 |
| Coco coir | 5.8–6.3 | 0.4–0.6 | 0.8–1.2 | 1.0–1.6 |
| Hydroponics | 5.5–6.5 | 0.4–0.6 | 1.0–1.6 | 1.4–2.0 |
Let pH move a little within the target range instead of holding it at one number. For soil, for example, it can go from 6.0 to 6.5 and back to 6.2. The usual reason is that each nutrient is taken up best at a slightly different point in the range.
Calibrate your pH meter once a week. pH meters drift over time. A drifting meter can make you push the root zone out of range without knowing it. Use fresh calibration solution and follow the meter maker’s instructions.
Translator’s note: Thai tap water and well water already contain dissolved minerals, and the amount varies by area. Measure the EC and pH of your plain water first. This tells you how much of each EC reading is fertiliser and how much was already in the water. In hydroponic systems, Thai heat also warms the nutrient tank. The topic on reservoirs at 31 °C covers your options.
What to do: This week, measure the pH and EC of three things: your plain water, your mixed feed and your runoff. Write all three down. These readings are your baseline for spotting salt build-up or pH drift later.
5. Air and climate
5.1 How dry the air is: vapour pressure deficit
Vapour pressure deficit (VPD) measures how strongly the air pulls water out of the leaves, in kPa. It depends on temperature and relative humidity (RH) together. When VPD is low (cool, humid air), transpiration slows and the plant takes up nutrients more slowly. When VPD is high (hot, dry air), the plant loses water faster than its roots can replace it.
| Stage | VPD (kPa) | Temperature (°C) | RH (%) |
|---|---|---|---|
| Seedling or fresh clone | 0.4–0.8 | 22–26 | 65–80 |
| Vegetative | 0.8–1.1 | 22–28 | 55–70 |
| Early flowering | 0.9–1.2 | 20–26 | 45–55 |
| Late flowering | 1.1–1.4 | 18–24 | 40–50 |
| With added CO₂ | 1.3–1.6 | 26–30 | 40–55 |
These are common grower targets, not results from a trial. Read the temperature and RH columns as ranges, not as free choices. Not every temperature–humidity pair in a row gives the VPD in that row. Use a VPD chart or calculator to pick a pair that hits the VPD target.
The “with added CO₂” row shows the problem most clearly. If leaf temperature equals air temperature, 26–30 °C at 40–55% RH gives roughly 1.5–2.5 kPa. That is above the 1.3–1.6 kPa target in the same row. To stay at 1.3–1.6 kPa in this temperature range, RH has to be higher: roughly 52–61% at 26 °C and 62–69% at 30 °C.
Translator’s note: Most of Thailand sits at 30–35 °C for much of the year. In the rainy season, humidity stays far above the 40–55% flowering targets. Without air conditioning, the late-flowering targets of 18–24 °C and 40–50% RH cannot be reached.
What you can do:
- Size the air conditioner and dehumidifier for your room. The topic on cooling and dehumidifier sizing works through the numbers.
- Run the light period at night, when outside air is coolest. This cuts the cooling load.
- In late flowering, put humidity control ahead of an exact temperature. Dense buds in humid air are where bud rot (Botrytis) starts.
- Plan for power cuts. During an outage the air conditioner and dehumidifier stop, and humidity in a closed room climbs. If you can afford backup power, give it to the dehumidifier and fans first. After any outage, inspect the densest buds.
What to do: Hang a thermometer–hygrometer with min/max memory at canopy height. Read it every morning. This shows the worst point of the past 24 hours, which is often during lights-off. A spot reading taken while you are in the room will not show it.
5.2 Carbon dioxide (CO₂)
- Outdoor air contains about 427 ppm CO₂ (2025 yearly average at Mauna Loa, Hawaii), and the level rises each year.
- Raising room CO₂ to 1,000–1,500 ppm can increase yield. Published cannabis figures are few, and the gain depends on the setup, so test it in your own room.
- Added CO₂ only pays off when light and nutrients are already right.
- Very high CO₂ is a health risk for people working in the room. See the safety steps below.
What you get: more flower from the same light and space, if the rest of the room is already well set up. Remember the 2021 study above: at normal CO₂, more light alone kept raising yield. So test light first.
How it works in Thailand: Added CO₂ stays in the room only if the room is sealed. Many Thai rooms are cooled by an exhaust fan pulling in outside air, and that fan carries the added CO₂ straight out. Enrichment therefore fits sealed rooms cooled by air conditioning. CO₂ cannot be seen or smelled. Use a controller with a CO₂ sensor, and air out the room before working in it. For equipment and gas supply, ask the official Thai distributor of the equipment or ask the community in the Equipment and Supplies section.
Where to start: Test it in one sealed room. Run one cycle with CO₂ and one without, using the same genetics and the same light. Record dry flower weight, kWh used and the cost of the CO₂. This tells you whether enrichment earns back its cost in your setup.
5.3 Air movement
- Change the room’s air every 1–3 minutes. Use an inline fan (a fan mounted inside the duct) and a carbon filter, which removes smell. This removes heat and humidity and brings in fresh CO₂. Air exchange and room layout are covered in a separate guide.
- Use oscillating fans. Gentle, constant movement makes stems grow thicker and stronger. This response to touch and movement is called thigmomorphogenesis.
- Keep the room at slightly negative pressure. Set the exhaust to remove a little more air than comes in. Air then leaks into the room through gaps, not out of it. Smelly air leaves only through the carbon filter, which keeps the smell under control.
Example: a room 3 × 3 m with a 2.5 m ceiling holds 22.5 m³ of air. One full air change per minute needs 22.5 × 60 = 1,350 m³/h. One change every 3 minutes needs 450 m³/h. Choose your fan within that range.
Translator’s note: In Thai heat, a strong exhaust pulls in hot outside air. In a vented room, the exchange rate is a trade-off between removing humidity and adding heat. In a sealed room with air conditioning and CO₂, the exhaust runs mainly to clear humidity or excess CO₂. In dense Thai housing, negative pressure and a working carbon filter also keep the smell from reaching your neighbours.
What to do: Measure your room and calculate its volume. Compare the result with the airflow printed on your fan. This tells you whether your fan can reach the 1–3 minute target.
Summary table
| Factor | What it does | What to measure | Common mistake |
|---|---|---|---|
| Light | Photosynthesis, flowering | PPFD, DLI | Very high PPFD without added CO₂ (see note below) |
| Water | Carries nutrients, cools leaves | Moisture in the medium, runoff pH and EC | Overwatering |
| Nutrients | Build plant cells | N-P-K, EC, pH | Too much nitrogen in flowering |
| pH | Controls nutrient uptake | Soil 6.0–7.0, hydroponics 5.5–6.5 | Not calibrating the meter |
| Air | Supplies CO₂, drives transpiration | VPD, CO₂ ppm | Poor air movement |
Note on the light row: the original article lists “high PPFD without CO₂” as a mistake. The 2021 study in Section 1 does not support calling it a mistake for yield, because flower weight kept rising at normal CO₂. The original article’s point is that added CO₂ gets more flower from the same electricity. The 2021 study did not test this, so treat it as unproven.
What to do in the next two weeks
- Measure PPFD at the centre and corners of your canopy. This finds weak spots in your light.
- Learn each pot’s wet and dry weight. Then water by need, not by calendar. This prevents overwatering and root rot.
- Record the pH and EC of your plain water, mixed feed and runoff. This gives you a baseline for spotting salt build-up.
- Calibrate your pH meter, and put the date in your notebook. This makes every later reading trustworthy.
- Log daily min/max temperature and humidity at canopy height. This shows whether lights-off humidity is putting your buds at risk.
- Check your fan’s airflow against your room volume. This tells you whether the room can clear its own heat and humidity.
If your numbers do not match these targets, post them below this topic with your room size and equipment. Other growers in similar climates can then help you find what to change first.
Legal note
This article is educational and is meant for lawful growing only. In Thailand, cannabis flower is a controlled herb. This is set out in the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568, in force since 26 June 2025. Flower may be sold only against a PT 33 (ภ.ท.33) prescription, covering at most 30 days. Advertising and online sale of flower are prohibited.
Two ministerial regulations of B.E. 2569 also apply. The Ministerial Regulation on Cannabis or Hemp Extracts has been in force since 26 April 2026. The Ministerial Regulation on Controlled Herbs (No. 2), in force since 30 April 2026, tightens the licence criteria for cannabis flower. Before you act, check which licence covers your activity.
References
- Chandra S, Lata H, Khan IA, ElSohly MA. (2008). Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO₂ conditions. Physiology and Molecular Biology of Plants, 14(4):299–306.
- Chandra S, Lata H, Mehmedic Z, Khan IA, ElSohly MA. (2015). Light dependence of photosynthesis and water vapor exchange characteristics in different high Δ9-THC yielding varieties of Cannabis sativa L. Journal of Applied Research on Medicinal and Aromatic Plants, 2(2):39–47.
- Eichhorn Bilodeau S, Wu B-S, Rufyikiri A-S, MacPherson S, Lefsrud M. (2019). An update on plant photobiology and implications for cannabis production. Frontiers in Plant Science, 10:296.
- Rodriguez-Morrison V, Llewellyn D, Zheng Y. (2021). Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Frontiers in Plant Science, 12:646020.
- Danziger N, Bernstein N. (2021). Light matters: Effect of light spectra on cannabinoid profile and plant development of medical cannabis (Cannabis sativa L.). Industrial Crops and Products, 164:113351.