What humidity and light do clones need?

Q: What humidity and light do clones need?

Unrooted cannabis clones do best at 75–90% RH (a leaf VPD of about 0.2–0.5 kPa), a medium temperature of 22–26 °C (72–79 °F), and gentle light of 50–150 µmol/m²/s on 18–24 hours, a DLI of about 3–10 mol/m²/day. Once roots show, lower humidity over 3–5 days toward 65–75% and raise light toward 150–300 µmol/m²/s.

Short answer

Unrooted cannabis clones need 75–90% RH, a medium at 22–26 °C (72–79 °F), and gentle light of 50–150 µmol/m²/s for 18–24 hours. After roots appear, step humidity down toward 65–75% and light up toward 150–300 µmol/m²/s. Grower practice, consistent with propagation research.

Why

A rootless cutting can't replace much water. High humidity shrinks the vapour pressure deficit, the pull on its leaves; at 25 °C the gap between 85% and 60% RH is large ^[3].

Light is needed, but only a little. Horticultural guidance keeps cuttings at roughly 100–150 µmol/m²/s at sticking ^[1]. Hemp cuttings rooted in 12 days at about 54 µmol/m²/s on 24-hour light ^[2].

Too-dry air shows up fast. That same hemp trial, at about 64% RH, had to resaturate media and remove leaves to stop cuttings losing turgor ^[2].

Roots change the rules. Once they can supply water, lower humidity and more light build a sturdier plant.

The numbers

Phase	RH	Leaf VPD	PPFD (µmol/m²/s)	Hours	DLI (mol/m²/day)
Days 0–7, unrooted	80–90%	0.2–0.4 kPa	50–100	18–24	3–8
Days 7–14, rooting	75–85%	0.3–0.5 kPa	100–150	18–24	6–10
Rooted, hardening	65–75%	0.4–0.8 kPa	150–300	18	10–19

Temperature: medium 22–26 °C (72–79 °F); air 23–26 °C (73–79 °F).

What to do

Mist the dome inside and set the tray on a thermostatted heat mat if the room is cool.
Measure PPFD at the leaves; dim or raise the light to 50–150.
Vent the dome once a day and wipe heavy condensation.
When roots show, open vents, then prop the lid, over 3–5 days.
Raise light gradually as the clones harden.

Common mistakes

Running clones under the veg light at full power.
Sealed domes with standing water and no venting.
Reading air temperature and ignoring a cold plug.
Hardening off before roots are visible.

Full article: Clone-only cuts and how to preserve them
Humidity by stage · PPFD and DLI by stage
Why are my clones wilting?
Terms: cloning dome · VPD

Follow-up questions

Do clones need light to root?

Yes, but not much. Unrooted cannabis clones need gentle light, about 50–150 µmol/m²/s on 18–24 hours, so the leaves can make sugars for new roots without drying out. A hemp trial rooted cuttings in 12 days at just 54 µmol/m²/s on 24-hour light. Full veg intensity on rootless cuttings makes them wilt and burn.

Should clones get 24/0 light?

Clones can run on 24/0 or 18/6; both work. Long days are common in propagation because a low-intensity light kept on longer gives a steady, gentle daily dose. A hemp trial rooted cuttings on 24-hour light at about 54 µmol/m²/s. The house position is that cannabis needs no sleep period in veg, so pick the schedule your mothers or nursery already run.

What temperature do clones need?

Cannabis clones root best with the medium at about 22–26 °C (72–79 °F) and the air at 23–26 °C (73–79 °F). The medium temperature matters most: a cold plug slows rooting badly. A heat mat with a thermostat solves a cold room, but measure inside a plug, because mats can easily push the medium above 28 °C (82 °F) and dry it out.

Sources

Michigan State University. Lighting of cuttings (rooting-phase PPFD). canr.msu.edu/uploads/resources/pdfs/lighting-of-cuttings.pdf

Campbell SM, Anderson SL, Brym ZT, Pearson BJ, 2021. Evaluation of substrate composition and exogenous hormone application on vegetative propagule rooting success of essential oil hemp (Cannabis sativa L.). PLOS ONE 16(7): e0249160. doi.org/10.1371/journal.pone.0249160

Allen RG et al., 1998. FAO-56 Chapter 3 (vapour pressure). fao.org/4/x0490e/x0490e07.htm