Crop steering

Crop steering is the control of plant growth with irrigation and climate. Each setting gives a signal to the plant. Some signals make the plant grow more roots, stems and leaves: this is vegetative growth. Other signals make the plant grow more flowers and fruits: this is generative growth.

The method comes from greenhouse tomatoes, cucumbers and sweet peppers on stone wool. Many cannabis growers also use it, with sensors that measure the water content and the EC of the substrate.

Crop steering is not a recipe. Select a direction and make a small change. Then examine the plants, and adjust the settings again.

Leaves use light to make sugars. The plant sends the sugars to the parts that grow: roots, stems, new leaves, flowers and fruits. Flowers and fruits get a large part of the sugars. In a tomato crop, the fruits get approximately two thirds.

Vegetative growth gives the plant its roots, stems and leaves. A young plant must grow these first, because the leaves make the sugars. Generative growth gives the harvest. In flower, the plant sends most of its sugars to the flowers and fruits.

Crop steering keeps the two types of growth in balance. The plant must have sufficient leaves to make sugars, and sufficient flowers or fruits to use them.

A plant that is too vegetative makes large leaves and long stems, but not many flowers. Its harvest comes late. A plant that is too generative stays small. It cannot make sufficient sugars for its flowers and fruits.

For leafy greens, the harvest is the leaf. Keep these crops vegetative.

The plant shows its balance. Examine the same plants each week, at the same time of day. Record what you see. A trend during two or three weeks tells you more than one measurement.

Measure the tomato stem 10–15 cm below the top. Beef tomatoes have thicker stems than cherry tomatoes. The cannabis signs come from grower practice, not from trials.

You steer with the root zone and with the climate. The table shows the direction of each tool. On stone wool and coco, the irrigation is the main tool.

Temperature is not in the table, because the sources do not agree. Refer to How to steer with VPD and temperature?

Light and CO₂ control the quantity of sugars that the plant can make. With more light, the plant can support more flowers and fruits. In a cannabis trial, more light gave more yield, and the flowers got a larger part of the plant mass.

Tomato growers also steer by hand. More fruits on each truss make the plant more generative. More leaves make it more vegetative.

On stone wool and coco, you control when the irrigation starts and stops, the size of each shot, and how much the substrate dries.

VWC

Volumetric water content: the percentage of the substrate volume that is water. One VWC point is 1% of the substrate volume.

Shot

One short irrigation.

Runoff

The water that flows out of the pot or slab. Greenhouse growers call it drain.

Dryback

The decrease of the VWC between two shots, in VWC points. The overnight dryback starts at the last shot of the day and stops at the first shot of the next day.

Substrate EC

The electrical conductivity of the water in the substrate. It shows the quantity of nutrient salts at the roots.

Growers divide the day into irrigation phases. The phases P1, P2 and P3 come from stone wool growing. Cannabis growers added P0.

P0

From lights on to the first shot. The plant starts to transpire, and the substrate continues to dry.

P1

The first shots of the day. They increase the VWC to the peak, or until the first runoff.

P2

Shots that keep the VWC near the peak.

P3

From the last shot to lights on. The substrate dries during the rest of the light period and during the night.

To steer vegetative, start the irrigation early and stop it late. P0 and P3 are short, and the dryback is small. To steer generative, start the irrigation late and stop it early. P0 and P3 are long, and the dryback is large.

Supplier values for cannabis on stone wool or coco: Grodan for the times and the shot size, AROYA for the dryback. They are starting points, not trial results.

Some suppliers give the dryback as a percentage of the peak VWC. Athena gives 30–40% for a vegetative steer and 40–50% for a generative steer. A 40% dryback from a peak of 60% VWC is 24 VWC points.

Shots that make runoff flush salts out of the substrate. The substrate EC then decreases to near the EC of the nutrient solution. Shots that make no runoff leave the salts in the substrate, and the substrate EC increases. Growers call this EC stacking. It is a generative tool.

When the substrate dries, the salts stay in it. Thus the substrate EC is highest at the end of the dryback, and it decreases during P1. Compare the substrate EC at the same time each day.

The blocks are not all the same. Put sensors in more than one block. Then use the average of the sensors.

Athena gives a substrate EC of 3–5 mS/cm in veg and 4–10 mS/cm in the stretch. Do not use a very high EC to make the plants shorter. In a trial, a nutrient solution with an EC of 8 mS/cm instead of 4 made cannabis plants 15% shorter. But the flower yield decreased by up to 20% (Allred et al. 2025).

Without a substrate sensor, use a scale:

1. Irrigate a typical pot until runoff starts. Wait until the runoff stops.
2. Weigh the pot. This is the weight at the peak VWC.
3. Weigh the pot again before the first shot of the next day.
4. Divide the weight decrease in grams by the substrate volume in mL. Multiply the result by 100.

The result is the dryback in VWC points. For example, a pot with 4 L of substrate that becomes 600 g lighter has a dryback of 15 points.

To measure the runoff, put two or three typical plants on trays. Measure the runoff volume and the runoff EC at midday. The first runoff after the night has a higher EC.

In living soil, you have less control. The soil holds nutrients and releases them slowly. Thus the runoff EC does not show the EC at the roots. Steer with the irrigation times, the dryback and the climate. EC stacking is a tool for stone wool and coco.

VPD controls the transpiration. With a low VPD, the cells stay full of water, and the leaves expand. The plant makes large, soft leaves and long stems. With a high VPD, the transpiration increases, and the plant stays more compact.

Use the chart's zone for the growth stage. To steer vegetative, keep the VPD in the lower half of the zone. To steer generative, keep it in the upper half. Stay in the zone.

To find the RH and the temperature for a VPD, set a target on the chart. On a computer, right-click a cell and select “Set target”. On a phone, touch and hold the cell. Then select “Show me how”.

A higher VPD also gives a larger dryback, because the plant uses more water. Thus a higher VPD and a later first shot give the plant the same signal two times. When you change the VPD, measure the dryback again. Then adjust the irrigation.

The chart's zones use leaf VPD. Many steering guides give air VPD, with the leaf at air temperature. A leaf that is 2 °C (3.6 °F) cooler than the air has a VPD that is approximately 0.3–0.4 kPa lower.

At lights off, the air cools, and the RH increases. For example, when the air cools from 24 °C (75 °F) to 18 °C (64 °F) with the same water vapor, the RH increases from 53% to 77%. Make sure that the leaves and flowers stay warmer than the dew point.

In late flower, keep the RH low. Move air through the canopy. Botrytis (bud rot) can destroy the flowers quickly when the RH is more than 70% at 17–24 °C (63–75 °F) (Mahmoud et al. 2023). The inside of a dense flower is more humid than the room. In a trial, more air movement around the flowers decreased bud rot by 66–92% (Buirs et al. 2025).

Temperature. In tomato, a higher 24-hour average temperature makes the trusses develop faster. More fruits then use more sugars, and the plant becomes more generative. If the temperature is too high for the light, the plant becomes long and thin.

Many cannabis guides use the opposite rule: a warmer room for a vegetative steer, and a cooler room for a generative steer. We did not find a trial that compares the two rules.

The DIF is the day temperature minus the night temperature. In research, a larger positive DIF makes the internodes longer. A DIF of zero keeps the plants more compact. A temperature drop of 3–8 °C (5–15 °F) for 2–3 hours, from 30 minutes before lights on, has a similar effect.

Many steering guides call a large DIF generative. In vegetable growing, this means faster flowering and ripening, not shorter plants. In the only cannabis trial that we found, hemp with a cool day and a warm night made much less flower mass. Equal day and night temperatures gave the most flower mass (Bok et al. 2023).

Test a DIF change on a small number of plants first. A cooler night also increases the RH.

The direction changes with the growth stage. For cannabis, flower week 1 starts when you change the light period to 12 hours. The chart points use the Cannabis crop, with the leaf 2 °C (3.6 °F) cooler than the air.

1. Propagation and early veg

   Vegetative

   Roots and leaves come first. After transplant, do not irrigate again until the block is 20–30% lighter than when it was fully wet (Grodan). This helps the roots grow into the block. Keep the VPD in the Early Veg / Propagation zone.

   On the chart: 0.60 kPa · 26 °C (79 °F) · 71% RH

2. Late veg

   Vegetative

   Grow a full canopy. Keep the VPD in the lower half of the Late Veg / Early Flower zone.

   On the chart: 0.90 kPa · 26 °C (79 °F) · 62% RH

3. The first 3–4 weeks of flower: the stretch

   Generative

   The plants stretch and make their flower sites. A generative steer limits the stretch: a later first shot, an earlier last shot and a larger dryback. Keep the VPD in the upper half of the Late Veg / Early Flower zone.

   On the chart: 1.10 kPa · 26 °C (79 °F) · 56% RH

4. Mid flower: the bulk

   Vegetative

   When the stretch stops, the flowers gain mass. A vegetative steer gives them water: more P2 shots, a smaller dryback and more runoff. Keep the VPD in the lower half of the Mid / Late Flower zone.

   On the chart: 1.30 kPa · 26 °C (79 °F) · 50% RH

5. The last 1–2 weeks: ripening

   Generative

   The sources do not agree. Many growers use a larger dryback and a VPD in the upper half of the Mid / Late Flower zone. A lower RH also decreases the risk of bud rot. If you decrease the EC, decrease it gradually. In a trial, a longer flush (a low EC before harvest) decreased the flower mass and the CBD yield (Alden & Faust 2025).

   On the chart: 1.45 kPa · 25 °C (77 °F) · 43% RH

For tomatoes, cucumbers and sweet peppers, keep the balance for the full season:

• Each week, measure the stem at the top and the distance from the top to the open flowers. Steer to keep them in the normal range for the variety.
• With many fruits on the plant, steer more vegetative. This keeps the top of the plant strong.
• Grodan steers the root zone more vegetative on bright days and more generative on dark days.

Leafy greens: the leaf is the harvest. Keep the crop vegetative for the full cycle.

Crop steering uses mild stress. Too much stress decreases the yield. Use these rules:

1. Change one tool at a time.
2. Make small changes. Stay in the chart's zone for the growth stage.
3. Wait some days. Then examine the plants before the next change.
4. Record the settings and the plant signs.
5. Test a new strategy on a small number of plants first.

Most steering numbers come from suppliers and growers, not from trials. The trials show this:

• Stress can make a shorter plant, but it costs yield. In cannabis, a nutrient solution with an EC of 8 mS/cm instead of 4 made the plants 15% shorter. The flower yield decreased by up to 20%, and the cannabinoid content did not change (Allred et al. 2025).
• A higher EC did not give more yield or quality. In cannabis, a nutrient solution with an EC of 4 mS/cm instead of 2 gave no significant increase (Hershkowitz et al. 2025).
• A longer flush before harvest decreased the flower mass and the CBD yield (Alden & Faust 2025).
• In tomato, each 1 mS/cm of EC more than 2 mS/cm decreased the marketable yield by 5.1%. The number of fruits did not change, but the fruits were smaller (Li et al. 2001).
• A moderate drought saved water without a loss of yield. A severe drought decreased the yield and gave no advantage (Morgan et al. 2024, Crispim Massuela et al. 2026).
• One trial found more cannabinoids after a controlled drought in flower week 7 (Caplan et al. 2019). Later trials did not find this increase (Morgan et al. 2024, Pena et al. 2025, Crispim Massuela et al. 2026).
• In cannabis, the yield increased linearly with the light, up to 1800 µmol/m²/s. The potency did not change (Rodriguez-Morrison et al. 2021).
• Between 0.2 and 1.0 kPa, the humidity has little effect on most horticultural crops (Grange & Hand 1987). In cannabis at 78–98% RH, the flowering started 3 weeks later, and the flower mass decreased by 71% (Corredor-Perilla et al. 2025).
• A larger positive DIF makes the internodes longer (Erwin & Heins 1995, Myster & Moe 1995).
• The flowers and fruits control most of the distribution of the sugars. Light has an indirect effect: it changes the number of flowers and fruits (Marcelis 1996).

Use crop steering to control the shape and the timing of the crop. Do not expect stress to increase the yield.

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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. https://doi.org/10.1186/s42238-025-00351-2

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