How much light do cannabis clones need?
Cannabis clones need 100-200 PPFD (low intensity) during the rooting phase — typically the first 7-14 days. Once roots are established and you've transplanted, ramp up to 300-500 PPFD for vegetative growth. Light schedule should be 18 hours on, 6 hours off (18/6) throughout the clone and veg phases. Too much light during rooting is one of the most common causes of clone failure.
Shop HLVD-Tested Clones →Why low intensity matters during rooting
A cannabis clone without an established root system has no way to replace water lost through its leaves. Strong light forces the leaves to transpire harder, but the underdeveloped or absent root system can't supply enough water to keep up. The result is wilting, leaf curl, and eventually death. Keeping light at 100-200 PPFD during rooting reduces transpiration demand to a level the clone can sustain on the moisture absorbed through its leaves and the small amount of water in its rooting medium.
How to measure 100-200 PPFD without a meter
PAR meters are accurate but expensive. Common rule-of-thumb equivalents: a single T5 fluorescent fixture at 6-8 inches above the clones, a 100W LED light at 24-30 inches, or a 200W LED at 36 inches. If you only have full-power flower lights available, raise them to 36-48 inches above the clone tray. Most modern dimmable LED fixtures should be set to 25-40% intensity for the rooting phase.
Ramping up after rooting
Once roots are visible and the clone has been transplanted, increase light intensity gradually over 3-5 days. Jump from 200 PPFD to 500 PPFD overnight and you'll bleach the leaves; ramp up at 50-75 PPFD per day and you'll get clean transition. Standard vegetative range is 300-500 PPFD. Avoid pushing higher than 600 PPFD during veg unless you're supplementing CO2 — additional light intensity without supplemental CO2 doesn't increase growth rate and may stress the plant.
Spectrum considerations
Spectrum matters less than intensity at the clone stage. Standard 'full spectrum' or 4000-5000K white LEDs work well. Some growers prefer slightly cooler color temperatures (5500-6500K) for vegetative work because plants stretch less under cooler light, but a difference of 500K is rarely visible in actual clone outcomes. The single biggest spectrum mistake is using purple-only 'blurple' LEDs designed for flower at the clone stage — these have insufficient green and yellow light for the leaves to read accurately and can cause stretchy, weak clones.
Light schedule: 18/6 for clones and veg
Run an 18 hours on, 6 hours off schedule throughout the clone phase and the rest of vegetative growth. The 6-hour dark period gives the plant time to translocate stored sugars and complete cellular respiration. Some growers run 24/0 (continuous light) during early veg for slightly faster growth, but most experienced growers stick with 18/6 because it produces healthier plants long-term and saves electricity costs.
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