Every number in a converted room was calibrated, deliberately or by habit, against the old lamp. The air temperature setpoint that produced a given canopy temperature under HPS produces a different one under LED, because the radiant term in the leaf's energy balance changed. The irrigation schedule that tracked an HID room's water use now tracks something slightly different, in a direction nobody can promise in advance. This page is the operating plan for that gap: what to measure before touching anything, what moves week by week, what the ramp-in is actually for, and how to tell on day 30 that the room has been re-anchored rather than merely relamped.
The Room You Inherited Was Tuned to a Different Lamp
An HID lamp does two jobs at once: it delivers photons and it heats the canopy from above. HPS fixtures run at high lamp temperature and emit longwave radiation in the same direction as their photosynthetic light, straight down at the crop, and leaf tissue absorbs roughly 95 percent of the longwave that strikes it. A large share of the climate behaviour growers attribute to "the room" was actually the lamp's radiant term: the leaf temperature that drove transpiration, the water the crop pulled per day, the dehumidification load that followed.
An LED fixture delivers the photons and disposes of its heat differently. Most of it leaves through the heat sink by convection, into the air and away from the leaf plane. The radiant term largely disappears. That is the whole mechanical story of the retrofit, and every adjustment in the first 30 days is a downstream consequence of it.
What Actually Changes at the Canopy
The honest magnitude first, because this is where retrofit advice goes wrong most often. In an energy-balance study across four radiation sources, Nelson and Bugbee found that an unstressed leaf sits within about 2 °C of air temperature under every one of them, field sunlight included, and that LED leaves run about 2 °C cooler than HPS leaves under typical indoor conditions: a real but modest difference, smaller than folklore assumes. Plant water status dominated the model more than the lamp did. In the water-stressed, low-airflow worst case, leaves ran 6 to 12 °C above air temperature depending on source, with HPS the hottest and LED far from the top of that range.
The practical translation: the retrofit does not license a blanket instruction to raise the room several degrees. It licenses one measurement. Read the canopy with an infrared thermometer at the working light level, compare it to the air reading at canopy height, and move the air setpoint only as far as that comparison demands. The leaf-temperature measurement habit, and what its readings separate, is covered in our light burn guide; here it is simply the first instrument out of the box.
Transpiration is the second surprise, because it can move either way. The widespread expectation is that plants drink less under LED once the radiant heat is gone. A Wageningen-led tomato trial comparing LED and HPS supplementary lighting at high lamp fractions measured the opposite: plants under LED showed lower leaf temperature but higher stomatal density, higher stomatal conductance and higher transpiration than plants under HPS, with the differences shrinking as the lamps' share of all light fell. The direction in your room depends on leaf temperature, stomatal acclimation and the vapour pressure at the leaf surface, which is why the irrigation section below insists on substrate evidence rather than folklore.
The Two Clocks of Acclimation
Plants adapt to a new light environment on two clocks at once, and retrofit plans go wrong by judging both on one timescale. The fast clock runs in hours to a day: stomatal conductance, chloroplast position and gas exchange measurable shifts complete themselves within a day of a light change in classic soybean work by Bunce and colleagues. The slow clock runs in weeks: in Arabidopsis work, low-light-adapted plants given periodic high-light exposure showed clear photoinhibition at the outset and needed about two weeks for photosynthetic efficiency to match unstressed controls, and classic ivy work found net photosynthesis recovering over 10 to 20 days with structural leaf change, a new palisade cell layer, still accruing at 40 to 65 days.
Two consequences shape everything below. First, a step up in intensity can look wrong on day one and right by day 14, so the plan holds checkpoints instead of reacting daily. Second, an over-step is recoverable: in the same Arabidopsis work, photosynthetic efficiency recovered within hours once the excess light was removed and fully overnight. The ramp-in in week two is designed around both facts.
Day 0: Baseline the Room Before You Change a Setpoint
The install crew leaves, the fixtures power up, and the temptation is to start dialling. Resist it. Set the new fixtures to the output that matches the light the old room was actually delivering, using a delivered daily light integral comparison rather than a wattage comparison, with the stage targets in our PPFD and DLI guide as the reference. Matching, not exceeding: the plants are already adapting to a new spectrum and a new thermal environment, and adding more photons during that window stacks a second variable on the first. Then take the baseline in Table 1. Every later decision in the 30-day plan is priced against these numbers.
| Reading | Instrument | Where | Why it decides the next 30 days |
|---|---|---|---|
| Canopy PPFD map | Quantum sensor, grid walk | Even grid at canopy height, fixtures at working output | The number every intensity decision prices; uniformity here is the retrofit's delivery promise |
| Leaf temperature | Infrared thermometer | Upper canopy: directly under a bar, and between bars | Separates the lamp effect from the air effect; decides whether air setpoints move at all |
| Air temperature and humidity at canopy | Aspirated sensor or verified probe | Canopy height, not the wall | The numbers the controller acts on; a wall sensor reads the room, not the crop |
| Substrate water content and dryback | Scale or substrate sensor | Representative containers in each zone | Transpiration change shows up here first, in either direction |
| Irrigation events and volume | Timer log or flow meter | Per zone | The baseline you will compare against, not replace, in week three |
| Dehumidifier runtime and dark-period humidity | Controller log plus overnight trace | Per unit; canopy height overnight | The load follows the water the crop puts into the air; the lights-off spike is where condensation risk lives |
The First 30 Days, Week by Week
| Window | The move | What confirms it | Roll back if |
|---|---|---|---|
| Days 0–3 | Hold output at the matched level; take the Table 1 baseline; change nothing | All six readings logged; dark-period humidity trace captured at least one full night | Serious stress appears: drop output toward the old level and diagnose before proceeding |
| Days 4–7 | Setpoint pass: air temperature adjusted to the leaf-temperature reading, VPD recomputed at leaf temperature, irrigation held | Leaf temperature sits where the reading says it should; dryback curve repeating | Leaf-to-air gap widens or the dryback curve breaks pattern: undo the setpoint change and re-read |
| Days 8–14 | Intensity pass: step output toward the new target in stages, checkpoint at each | No interveinal bleaching, cupping or tip burn; PPFD remap uniform; climate held | Stress signs at the canopy: step back down the same day; recovery is fast, damage compounding is not |
| Days 15–30 | Crop-follow levers: irrigation frequency trimmed to the measured dryback, dehumidification schedule matched to the new load, SOP written | Dryback repeats cycle to cycle; dehumidifier runtime explainable; night humidity below condensation | Any lever change degrades a number that was stable: revert and change one variable at a time |
The four windows earn their shape from the acclimation clocks. The baseline week exists because the fast clock runs whether or not you are watching, and readings taken after setpoints have moved can no longer tell you what the retrofit itself did. The setpoint pass comes before the intensity pass because air temperature and humidity determine what the coming photons will cost the leaf; raising intensity into an unretuned climate stacks stressors. The intensity pass sits inside the two-week window the physiology literature gives photosynthetic capacity to rebuild, which is also why it ends at day 14 rather than drifting. The back half of the month is quieter on purpose: levers that follow the crop, and the writing down of what was learned.
Irrigation and Dehumidification Follow Transpiration, and Transpiration Can Go Either Way
The dehumidifier's load is the water the crop puts into the air, plus evaporation from the substrate, and the irrigation schedule is the upstream supply of both. Change the lamp and you change that water balance, but the direction is not guaranteed. The tomato trial above measured more water use under LED, not less, at high lamp fractions; rooms converting from dimmer or older installations to much higher delivered light can land either side of their old number. What is predictable is only that the old schedule is no longer self-evidently correct.
So the plan treats irrigation as an evidence problem. Hold the event schedule through the baseline week. Watch the dryback curve for three to five days: if it repeats its old shape, the crop's water use did not materially move and nothing needs trimming. If it shallows, the plants are drinking less per day and the frequency, not the volume per event, is what trims first. If it deepens, the reverse. Feed concentration stays untouched through all of it; the first 30 days are for water balance, and substrate chemistry moves on its own slower clock.
The dark period deserves its own line of attention. Transpiration does not stop when the lights do, and the old lamp's residual heat used to keep the air moving and the surfaces warm through the first hours of darkness. LED rooms lose that blanket, so the humidity rise at lights-off can be sharper than the room's history suggests, and that is precisely the window where condensation and mould pressure develop unnoticed. The overnight trace from the baseline week is the receipt; the dehumidification schedule is set against it, not against a daytime average.
The Ramp-In: Step When You Raise, Hold When You Match
A ramp is not a ritual; it is a response to one specific situation. If the retrofit's target light level exceeds what the old room was delivering, the canopy needs time to rebuild photosynthetic capacity, and the two-week clock from the acclimation section is the reason the intensity pass occupies days 8 to 14. If the retrofit matches the old delivered light, there is nothing to ramp: the plants adapt on their own clocks regardless, and manufacturing a ramp would only delay the baseline.
When stepping, each step carries the same three checks before the next one is allowed: the plant response at the canopy, meaning no interveinal bleaching, no persistent cupping and no tip burn on new growth; the PPFD remap, because a fixture swap can move the uniformity as well as the quantity; and the climate, meaning VPD held in band at leaf reference and no condensation forming. A step that fails any check gets rolled back the same day, and the physiology says that is cheap: photosynthetic efficiency recovered within hours of the excess light being removed in the trials above, with full overnight recovery. What is not cheap is compounding an over-step across consecutive days.
Enforcement belongs to the controller. A staged dim curve loaded once beats hand-walking the room, and grouped zones let the vegetative room and the flower rooms step on different calendars if their gaps differ; our controller page covers the grouping mechanics. Dimming held back during the hours that set a demand charge is a separate lever with its own arithmetic, covered in our demand charges guide; the two schedules should be built together rather than tuned against each other later.
What Not to Change in the First 30 Days
The retune fails most often not from a wrong setting but from three changes landing in the same week, none of them traceable afterwards. Four things stay frozen while the plan above runs.
The feed programme, first: EC targets and additives hold through the window, because substrate chemistry is re-stabilising around a new water balance and a feed change would confound the dryback readings the whole plan depends on. The spectrum, second: no UV bars, no supplemental far-red, no new channels switched on mid-retrofit; the plants are acclimating to one new spectrum already, and our spectrum guide's levers apply one change per cycle, not three. The defoliation and training calendar, third: run it as written so the canopy architecture stays comparable to previous cycles. And the hardware, fourth: no re-hanging fixtures for spread, no HVAC swaps, until the PPFD map and the climate logs say what they are for.
One temptation deserves its own sentence: chasing calcium. A converted room can show tip burn or pale new growth that looks like a feed deficiency and is actually a delivery shift, because calcium moves with the transpiration stream and the stream changed. The mechanism and the two-measurement check that settles it are in the light burn guide. Touch the feed schedule only after that check, not before.
Day 30: What a Retuned Room Looks Like
The month ends with a room that can be described in receipts rather than impressions. The dryback curve repeats cycle to cycle at a frequency you chose deliberately. Leaf temperature sits a small, stable distance below air temperature across the footprint, not just under the centre of a bar. VPD holds in band at leaf reference through the photoperiod, and the overnight humidity trace stays clear of condensation with the dehumidifier runtime you can explain. The PPFD map is uniform at the final output after the last height or dim adjustment, and the canopy under it shows no interveinal bleaching, no tip burn on new growth and no shade-avoidance stretch at the bottom. The SOP, the dim curve, the checkpoint readings and the sign-offs, is written down while the numbers are still fresh, because the next room converts faster on paper than on memory.
One honest caveat closes the page: day 30 is the end of the retune, not the end of the adaptation. Classic physiology work found structural leaf change still accruing well past a month after a light change, with photosynthetic rates eventually exceeding the starting point. Hold the yield judgement until a full crop cycle has run under the new lamp, and let the 30-day receipts, not the first-week impressions, be the record the next conversion is built on. The economics of that conversion, and the cost lines that hide inside it, live in our LED versus HPS guide; the room-level case it draws from is in our case file.
After an LED Retrofit FAQ
Do I need to raise my grow room temperature after switching from HPS to LED?
Only as far as a leaf temperature reading demands. Under typical indoor conditions leaves run about 2 °C cooler under LED than under HPS, because the longwave radiation that used to warm the leaf directly is gone, and leaves absorb roughly 95 percent of the longwave that strikes them. Measure the canopy with an infrared thermometer at the working light level first. If the leaf sits where it should relative to air, keep the air setpoint; if it sits low, raise the air a degree or two and re-measure. Blanket advice to add several degrees belongs to folklore, not to your room.
How long does it take plants to adjust to LED grow lights?
On two clocks at once. Stomatal behaviour and gas exchange adjust within about a day of a light change, while the capacity side, the photosystems and the leaf's internal structure, rebuilds over roughly two weeks: low-light-adapted plants given periodic high light showed photoinhibition at first and matched control plants' photosynthetic efficiency after two weeks. Structural change continues past that, so judge the crop at the 30-day mark and the yield at a full cycle, not on any single afternoon.
Should I run my new LEDs at the same light level as my old HPS?
Match the delivered daily light integral first and hold there through the baseline week, even if the new fixtures could run harder. The plants are already adapting to a new spectrum and a new thermal environment, and adding more photons during that window stacks a second variable on the first. If the plan calls for more light than the old room delivered, raise it in steps during days 8 to 14 with a checkpoint at each step.
Why are my leaves curling after an LED retrofit?
Curling is a response, not a diagnosis. Upward curl with bleaching at the top of the canopy points at light; downward curl or limp growth points at humidity or root-zone trouble; and in the first weeks after a conversion, a leaf-temperature shift can move calcium delivery and mimic a feed problem. Our light burn guide separates the two gradients with two measurements, a PPFD reading at the canopy and a root-zone EC and pH check, before anyone touches the feed schedule.
Will my irrigation schedule change after an LED retrofit?
Not on day one, and maybe less than you expect. The folklore says plants drink less under LED because the radiant heat is gone, but a greenhouse trial under high lamp fractions measured higher transpiration under LED than under HPS, driven by stomatal behaviour rather than leaf temperature alone. Hold the event schedule, log the dryback for three to five days, and trim frequency only if the dryback curve has visibly shallowed. The substrate settles the argument; the folklore does not.
Does VPD change under LED lighting?
The room numbers may not move, but the leaf's numbers do. VPD charts assume the leaf is at air temperature; under LED an actively transpiring leaf commonly runs a degree or two cooler, which shifts the vapour pressure the leaf actually experiences. Compute or verify against leaf temperature rather than the wall sensor, and note the dark period: with the old lamp's thermal blanket gone, humidity can climb faster at lights-off than the room's history suggests. Our grow room terms guide carries the vocabulary.
Do I need a ramp-in schedule for new LED fixtures?
Only if the new target exceeds the light the room was already delivering. If you match the old daily light integral at installation, there is nothing to ramp; the plants adapt on their own clocks regardless. If you are raising intensity, step it across days 8 to 14: at each step, check the plant response at the canopy, remap the PPFD uniformity, and confirm the climate held, then continue. Over-stepping is recoverable: photosynthetic efficiency bounced back within hours in trials once the extra light was removed.
What should I measure in the first week after installing LEDs?
Six readings, taken before any setpoint moves: a PPFD map at the working output, leaf temperature at the top of the canopy with an infrared thermometer, air temperature and humidity at canopy height rather than the wall, substrate water content with its dryback curve, the irrigation log, and dehumidifier runtime against the dark-period humidity trace. Every later decision in the 30-day plan is priced against this baseline, which is why the readings come before the changes.
Why does my dehumidifier run differently after the retrofit?
Because its load follows the water the crop puts into the air, and that water balance moved with the lamp. Transpiration can rise or fall depending on leaf temperature, stomatal acclimation and how much radiant heat disappeared, and the lights-off humidity spike sharpens once the old lamp's heat is gone. Hold the dehumidification setpoints, watch a full week of runtime and the night-time humidity trace, and change the schedule only once the new pattern is explainable.
Planning a Conversion and Want the Retune Handled Like a Commissioning?
Tell us the rooms, the fixtures coming out and the targets going in: we will come back with a matched lighting plan, a staged ramp-in curve and the baseline checklist your team runs on day 0.