Start with the window: How Liam Spenser works back from the irrigation window to shot size, dryback, EC, and VPD

Liam Spenser started at Curaleaf's Florida facility in 2018, worked up to cultivation manager, then ran two Colorado facilities (about 300 lights between them) before moving to the sensor side of the industry and then to AROYA. He sat down with Green Thumb Depot's grower podcast to talk crop steering. This post pulls out the parts worth keeping, in grower terms.

One note before we dig in. This is how Liam approaches crop steering, from his own experience. It is not a single AROYA playbook, and other growers, including others on the AROYA horticulture team, may weigh things differently. Treat it as a starting framework to adjust for your cultivars, your substrate, and your system.

Everybody is already crop steering

Crop steering is a precision cultivation method. You use data and feedback to decide what to change, then manage how the plant responds. That covers irrigation and environment.

The point Liam kept coming back to: you are already doing it. Defoliation is crop steering. You create a stimulus and the plant responds. The difference is how intentional you are about it, and whether you can see the response well enough to repeat it.

Irrigation is the fastest cue. Light sets the ceiling.

A common misconception is that crop steering is only irrigation. Irrigation is the fastest cue you can send, because you are delivering events all day. But the environment is the context those decisions happen in.

If the room is too hot or too cold, transpiration slows and the plants drink less. You adjust input EC, volume, shot count, and window to compensate. Underneath all of it, light intensity is the biggest driver of photosynthesis. Raise light and the plant needs more CO2, more feed, and more irrigation to keep up. As Liam put it, the plant has a genetic ceiling, and everything else has to follow the light to reach it.

Start with the window, then work back to shot size

Asked what matters most, Liam's answer was that he pays the most attention to the irrigation window. That is where he always starts. It does not mean shot size takes a back seat. Shot size is a big part of the stimulus, and in rockwool it may be nearly as important as the window.

Here is how he thinks about it. Growers can get lost in the details, and the right answer is specific to the situation. So he zooms out and asks what he is really trying to do: stimulate the plant through the pressure balances in the root zone. The thing you can manage is how long you water for. Inside that, you decide how big each shot is, how far apart they are, and how many events you deliver. The window is the critical starting point, and shot size, interval, and shot count all get worked out backward from it.

Liam sets two windows:

• Irrigation window: first shot of the day to last shot of the day.
• Dryback window: last shot of the day to first shot the next day.

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Manage the irrigation window, and you manage the dryback window, and that sets your dryback percentage. EC stacks as a result.

A worked example. Say your block transpires about 2% per hour. Over 24 hours with no irrigation, that is a 48% dryback. From a 70% field capacity, you would land at roughly 22%, which is far too deep. If you want a dryback closer to 35%, you need about two to four hours of irrigation to bring it back.

From there, pick your shot size and do the math. With 8% shots, regaining 35 points takes a little over four shots, so call it five. The extra covers runoff.

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Why shot size still matters. In rockwool especially, shots that are too big or too small can both cause channeling. The window tells you how much time you have to work with. Shot size decides how well that water actually moves through the block.

For scale, a 1% shot on a one gallon pot is about 38 mL. Liam suggests sizing emitters so a 1% shot runs close to one minute, which makes the math easy: a 5% shot is five minutes, a 7% shot is seven. He prefers smaller emitters for rockwool and smaller coco blocks so you can fine-tune. Watch for overlap if your zones fire in series, since a long shot in each zone can stretch the last table's start by an hour.

Know your floor

Field capacity is the most water a block holds at full saturation before it drains. Liam's rule of thumb in coco is around 65–70% on a typical 70/30 chip to pith blend, and closer to 50–55% on a coco and perlite mix. Rockwool can read higher, up to about 80% on AROYA sensors. Different sensors use different technology and calibrations, so treat these as ranges and not absolutes.

On the low end:

• Coco: he rarely loses more than half of the available water in a day. If field capacity is 70%, he rarely goes below 35%.
• Everything: he tries never to go below 30% water content. Rockwool can handle it, but it may not come back. You can get channeling, and the pore space may not close back up.
• Rockwool in veg: dryback of about 10–15%, and very rarely below 45%.
• Rockwool late in flower: during the ripening push, dipping into the 30s is on the table.

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In his words, some cultivars can take an aggressive dryback and some cannot. Know which is which before you push.

Three phases of flower, two kinds of steering

Liam breaks flower into three phases for simplicity.

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Vegetative steering makes life easy for the plant. Plenty of access to water and nutrients, very little pressure. Generative steering pushes harder: deeper drybacks, higher substrate EC, more oxygen in the pore space. You are telling the plant to put its energy into reproduction, with tighter internodes and more bud sites and less leaf and stem.

He avoids the word stress. The goal is stimulus, not damage.

P1 and P2

Two terms worth having straight:

• P1: the first irrigation of the day, from peak dryback back up to field capacity, plus runoff.
• P2: maintenance shots that hold water content at or near field capacity.

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A generative P1 might be four or five shots of 6–8% about every 30 minutes. A vegetative P1 might be nine shots of 3% every 15 minutes. P2 spacing comes from your transpiration rate. At 2% per hour and a target 5% inner-shot dryback, you would space P2 shots a little over two hours apart.

EC is pressure, not food

Pore water EC builds osmotic pressure on the root system. That pressure changes how easy it is for the plant to move water up through the xylem to the stomata. Think of a straw: the higher the EC, the longer the straw.

Stacking EC means drying the block back and limiting runoff so salts accumulate faster than the plant uses them. Push more runoff and you flush those salts out. Less runoff and they build. That makes drain your tool for backing EC off when you need to.

Liam stacks harder in stretch and ripening, eases off in bulk, and avoids heavy stacking in veg, where you are building roots and a high EC would tell the plant to start making flower.

VPD is the accelerator, EC is the brake

VPD is the suction force pulling water out of the leaf. EC is the resistance in the root zone. You manage one in the context of the other.

Low VPD leans vegetative. High VPD leans generative, but push it too far and the plant closes its stomata to protect itself, which stops growth. If your VPD graph looks like a roller coaster, you cannot predict transpiration, and you cannot steer through irrigation.

The goal is predictability. Send the cue, do not just react to whatever the room does.

What we still don't know

On microbes, Liam was candid. The science on how drybacks affect microbial life cycles is not fully understood, partly because a flower cycle is short compared to the life cycle of fungi and mycorrhizae.

What he does see: in a purely synthetic feed, plants are not relying on microbes the way they do in soil, so many growers add humates, fulvic acid, and amino acid chelates to build a biological ecosystem in the root zone. Oxygen matters here too. If you use microbes, establish them early, at transplant, and keep in mind that high salinity can work against certain species. Coco has more buffering capacity than rockwool, which gives microbes more to hold onto.

Seeing it in the room

None of this math works without data you trust. Here is what AROYA measures:

• Substrate: water content, EC, and temperature with the TEROS ONE sensor.
• Climate and light: climate sensors and Apogee light sensors.
• Irrigation: the drip and drain station, which works like fill cups and drain cups, automated. It shows which shot hits drain, how much drain you get per shot and per day, when you hit field capacity, and whether you are channeling.
• Runoff: a runoff sensor that captures time series data for volume and EC, with a pH add-on.

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Everything runs wirelessly over a self-healing mesh network to a gateway, then into the software, where harvest groups let you line up what happened in each run against your yield and quality outcomes.

If you run another control system, AROYA connects through software integration, such as Modbus, not by plugging AROYA sensors directly into a third party controller.

On accuracy, Liam is direct. He has seen very little that comes close. The TEROS ONE uses complex dielectric technology, sending separate signals for water content and EC. Capacitance sensors derive both from a single signal, and in his experience EC readings can spike as water content drops below 30%. His view is that the TEROS ONE fixes that problem. It took over a decade to develop for high EC, container based cultivation.

He also cautioned that more data is not always better. Extra data points can become noise if you cannot act on them. The goal is the right data, in the context of your cultivars and your system.

Get a second set of eyes on your room

You can run these numbers yourself. You do not have to do it alone. The AROYA team offers crop steering calls where you can review your data together, plus troubleshooting and occasional on-site visits. Reach out to the AROYA team to set one up.

Listen to the full conversation on Green Thumb Depot's podcast: Crop steering: an interview with Liam Spenser

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