Crop steering vendors quote exact targets, such as a set substrate EC and a 10% overnight dryback. Published cannabis studies support the general idea behind crop steering. None of them confirms those numbers. The only company trial that tested dryback found no yield gain from larger drybacks in three of four comparisons. This part explains why those numbers are unlikely to carry over to Thailand unchanged, and why a humid wet-season room may barely dry back. It also shows how to measure dryback and build targets from your own crop.
The terms used here:
- Crop steering: controlling how much water and fertiliser salt stays in the growing medium, to push the plant toward leaf growth (“vegetative”) or toward flowers and resin (“generative”).
- Dryback: how much the medium dries out between the last watering of one day and the first watering of the next.
- VWC (volumetric water content): the share of the medium’s volume that is water.
- Substrate EC: how much dissolved fertiliser salt is in the medium.
- Shots: the small, measured doses of water that crop steering uses.
This is Part 2. Part 1: Crop Steering by the Numbers: Where Those EC and Dryback Targets Actually Come From
What this article does not claim
The evidence in Part 1 is easy to misread as “crop steering does not work”. That is not the claim.
Substrate sensors are useful. Knowing the water content and EC of your medium in real time is clearly better than not knowing. Nobody disputes the measuring.
The plant science is sound. In horticulture it is well established that water stress and salt stress shift the plant’s resources toward reproduction. In cannabis, Caplan and colleagues found more cannabinoids after a controlled drought (Part 1).
The vendors are not lying. They may well be right. Commercial growers who use these routines report good results. A company with sensors in thousands of rooms holds real data.
The actual claim is narrower. Nobody independent has confirmed the specific numbers, yet they are taught and repeated as established fact. Suppose you adopt a widely quoted number and get a poor result. You cannot tell whether the number was wrong in general, wrong for your cultivar or wrong for your medium, because no published work exists to check it against.
What to do: whenever you adopt a quoted number, write the cultivar, growing medium and room conditions next to it in your log. If the result is poor, you then have something to trace it back to.
Why caution matters in this region
The studies already show that plant responses depend on genetics:
- In the Allred trial, the two cultivars reacted differently to the same EC treatment.
- In the Grodan/Royal Queen Seeds trial, under regular canopy work, the two varieties ranked the dryback settings in opposite order.
So crop steering thresholds are likely to depend on cultivar and on growing medium. This is a reasonable inference, not a trial result. The numbers mostly come from North American commercial rooms using stone wool. Of all crop steering advice, these numbers are the least likely to carry over unchanged to Thailand and Southeast Asia.
Translator’s note: the vendor numbers mostly came from stone wool. If you grow in coco coir, soil or a living-soil mix, the gap is probably even wider. Test any number separately for each cultivar and each medium you use.
Humid wet-season rooms: when the medium barely dries back
This section is about rooms without climate control, greenhouses and net houses in the wet season. It does not apply to well-sealed indoor rooms.
Why humidity controls dryback
Dryback is the total water the medium loses between waterings. Most of the loss is transpiration, the water the plant releases through its leaves. Drainage after the last shot and evaporation from the surface of the medium add to it.
Transpiration depends strongly on VPD (vapour pressure deficit). Evaporation depends on it to a lesser degree. VPD measures how much more moisture the air can take up at its current temperature. Low VPD means humid air that pulls little water out of the leaves.
In the Thai wet season, a room or greenhouse without active dehumidification can sit far below the VPD usually recommended for flowering cannabis. The plant then transpires slowly, so the medium dries back slowly.
As an example, at 30 °C and 90% relative humidity, VPD is about 0.42 kPa (the formula is in the box below). Under those conditions, a large overnight dryback target may be hard or impossible to reach. This follows from the plant science but has not been measured in a trial. The cause would not be a wrong watering strategy. The plant is simply moving very little water.
A sealed indoor room with air conditioning and a dehumidifier of adequate size is different. It can hold VPD close to target all year. In that room, dryback should behave much as the vendor material assumes.
Two consequences for rooms without climate control
Both points below are inferences, not measured results.
- A controller can hold back water for too long. An irrigation controller is the unit that switches pumps or valves on a timer or a sensor reading. If it is set to chase a dryback it cannot reach, it may delay watering longer than you intended. That depends on how its logic is written. Check what yours does when a target is not met.
- Dryback stops being a setting you control. It becomes partly a result of your humidity control.
Some vendor material discusses VPD alongside watering. No guidance was found for growers who cannot keep VPD near temperate-climate targets for part of the year. Most examples assume a greenhouse or sealed room with controlled climate.
Translator’s note: two Thai conditions affect your log.
- Power cuts: if the controller, sensors or dehumidifier lose power overnight, that night’s dryback reflects the outage, not your plan. Mark every outage in your log. After power returns, check whether the controller resumes its programme or drops back to a default schedule.
- Season changes: hot, drier months raise VPD. The same room can dry back quickly in one season and slowly in another. Keep separate records for each season.
What to do: for one week, record night-time VPD and dryback without changing anything. If dryback barely moves on humid nights, fix the humidity first. If you cannot fix it, accept a smaller dryback in those months.
What to do instead
The recommendations below are this article’s advice, not trial results.
Measure, and treat the numbers as your own. Install sensors if you can afford them. Log substrate EC, VWC and dryback. Then set your own targets from your own crop instead of copying someone else’s.
Always log VPD next to dryback. In this climate the two are linked. A dryback figure without a VPD figure is hard to interpret.
Change one thing at a time, across several cycles, and keep a comparison group. One group of plants stays on your current routine. The other group gets the change. This is slow work, but it is the only way to learn how your own crop really responds.
Translator’s note: a fair comparison needs plants with the same genetics. The safest source is clones cut from one of your own mother plants. Clones bought from another farm can bring in pests, hop latent viroid (HLVd) or the wrong variety. Any of these can ruin the comparison and infect your rooms.
Start from your current routine. No independently confirmed starting range exists to recommend. Log your current watering routine for one full cycle as a baseline. Then make any change in small steps from there.
Treat “generative” and “vegetative” steering as ideas to test on your genetics, not as settings.
Substrate sensors: what they measure and where to put them
A VWC sensor shows how much of the medium’s volume is water, as a percentage. A substrate EC reading shows the salt level in the medium. Many units measure both.
A substrate EC reading is not the same as the EC of your feed tank. The vendors’ EC targets for steering refer to substrate EC. The Allred trial set EC in the feed. Compare like with like.
How to place and read a sensor:
- Put it in a block or pot that is typical of the bench, not at the edge of the bench.
- Keep it in the same position for the whole cycle.
- Read the daily peak only after drainage has stopped, at the same point each day.
Translator’s note on equipment in Thailand: this article could not confirm whether the brands named here (AROYA, Growlink, TSRgrow and similar) have official distributors in Thailand, or what they cost locally. Before you buy, ask the seller about four things: local support, spare parts, calibration, and whether logging still works without a stable internet connection. You can also ask about other growers’ experience in the forum’s Equipment and Supplies section. If sensors are not available or not affordable, the pot-weighing method below gives you most of the same information.
Box: measuring dryback and air humidity
With a substrate sensor (VWC):
- Dryback = peak VWC after the last shot of the day − VWC just before the first shot the next day.
- Example: a peak of 60% VWC and a morning reading of 51% VWC give a dryback of 9 percentage points.
- Some controllers instead report dryback as a share of the peak (9 ÷ 60 = 15%). Check which method your software uses before you compare your figures with anyone else’s.
- Read the peak once drainage has stopped, at the same point every day. Otherwise drainage will make your number look bigger than it is.
- Place the sensor in a typical block or pot, not at the edge of the bench, and keep it there all cycle.
Log VPD next to it:
- Record air temperature and relative humidity at canopy height. Use the same interval as your VWC readings, and at least once an hour overnight.
- VPD (kPa) ≈ 0.6108 × e^(17.27 × T ÷ (T + 237.3)) × (1 − RH ÷ 100), with T in °C and RH in %.
- Worked example: at 30 °C and 90% RH, 0.6108 × e^(1.938) ≈ 4.24 kPa. Then 4.24 × (1 − 0.90) ≈ 0.42 kPa.
- The formula uses air temperature. Leaf temperature gives a more accurate result if you have an infrared thermometer.
- Each day, write the night’s average VPD next to that night’s dryback.
Without sensors (most home and small growers):
- Pot weighing. Mark one pot. Weigh it on a cheap hanging or platform scale right after the last watering, once drainage has stopped. Weigh it again just before the first watering the next morning.
- Overnight loss % = (evening weight − morning weight) ÷ (evening weight − dry pot weight) × 100.
- The dry pot weight is the pot filled with dry medium and no water. Weigh it once before you first water it.
- Illustration with made-up figures: evening 10.0 kg, morning 9.2 kg, dry pot 4.0 kg. The loss is 0.8 ÷ 6.0 × 100 ≈ 13%.
- The plant adds weight as it grows. Watch the trend over days, not any single figure.
- Using weight to decide when to water: after a few weeks, your log shows how much water the pot loses per night at a given VPD. You can then water when the pot drops to a weight that has worked for your plants, instead of watering by the clock or by a number from the internet.
- Runoff EC. Each day, measure the EC of your feed and of the water that drains out (runoff). If runoff EC keeps rising above feed EC, salts are building up in the root zone. If runoff EC stays close to feed EC, they are not.
- Notebook. Each day, record the date, feed EC, runoff EC, evening and morning weights, and the minimum and maximum temperature and humidity. One logged cycle is worth more than any number quoted online.
Be sceptical of any number given without a trial behind it, including the numbers in this article. The figures above are sourced in the References. Where this article reasons rather than reports, it says so.
What to do: pick one pot or one sensor position this week. Start recording dryback and night-time VPD side by side, every day, until harvest.
What would settle the question
The missing evidence is specific, and someone will eventually provide it. It is a peer-reviewed factorial trial in cannabis that combines substrate EC and dryback and repeats the test across several cultivars. It would need to report dryback as numbers, give cannabinoid and terpene results as measured amounts, and use enough plants. No such trial was found. Until one exists, crop steering rests on sound reasoning with untested numbers.
For this region, a narrower question matters more and is easier to answer. In rooms and greenhouses without dehumidification, does dryback-based steering work at all at monsoon-season VPD? Or is the method simply unavailable for part of the year?
Share your data below this topic. If you run substrate sensors in Thailand, post the dryback you actually reach in the wet season. Say whether you can reach a 10% overnight dryback when VPD is below 0.5 kPa. Say whether your room is sealed with air conditioning and a dehumidifier, or open. This question has a factual answer that nobody seems to have published, and the answer decides whether crop steering applies here for half the year. One logged wet-season cycle with VWC and VPD side by side would answer it.
References
- Caplan D, Dixon M, Zheng Y (2019). Increasing inflorescence dry weight and cannabinoid content in medical cannabis using controlled drought stress. HortScience 54(5): 964–969. hort
- Allred J, Fatzinger B, Bugbee B (2025). Crop steering through osmotic stress can reduce height but reduced yield in medical Cannabis. Journal of Cannabis Research 7: 92. Client Challenge
- Saloner A, Bernstein N (2021). Nitrogen supply affects cannabinoid and terpenoid profile in medical cannabis (Cannabis sativa L.). Industrial Crops and Products 167: 113516.
- RQS Editorial Team (Royal Queen Seeds) (2026, 10 July). Plant Maintenance and Overnight Dryback: Effects on Cannabis Yield, THC and Terpenes (company blog; trial run by Grodan at CRIC Labs, Québec, with Royal Queen Seeds; not peer reviewed). Plant Maintenance and Dryback: Cannabis Yield and THC - RQS Blog
- Examples of vendor training material: AROYA, Mastering dry backs: The key to optimizing yields and plant quality, Mastering dry backs to optimize yield and quality | AROYA; TSRgrow, Crop Steering with Irrigation: Dry Backs, Shots, EC, and more, Crop Steering with Irrigation: Dry Backs, Shots, EC, and more
This article is technical horticultural information for readers who operate legally where they live. Every country regulates cannabis growing differently. In Thailand, cannabis flower is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568. The Notification was issued under the Thai Traditional Medicine Protection Act B.E. 2542 and has been in force since 26 June 2025. Flower may be sold only against a PT 33 prescription, covering at most 30 days. Advertising and online sale of flower are prohibited. Research, export, sale and processing of cannabis flower require a licence. Nothing here is legal advice.