T5 Fluorescents for Clones

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What a T5 fluorescent is

T5 refers to a tubular fluorescent lamp 5/8 inch (about 16 mm) in diameter. The "HO" (high output) variant runs at higher current and produces more light per tube than standard T5s. A typical horticultural 4-foot HO tube draws about 54 W.

For clone propagation, T5s are valued for three properties:

• Low radiant heat at the canopy: You can run a fixture 6 inches above a dome without melting plastic or scorching leaves.
• Broad, even coverage: Multiple long tubes spread photons across a tray with no hotspots.
• Moderate light intensity: T5 HO fixtures deliver roughly 100–250 µmol/m²/s photosynthetic photon flux density (PPFD) at typical hanging distances, which matches unrooted cuttings that cannot transpire normally and are easily stressed by stronger light ^[1].

Why growers use T5s for clones

Clones are physiologically fragile. They have no roots, so they cannot replace water lost through their leaves. The propagation light should supply just enough photons to keep photosynthesis active while roots develop—not drive aggressive top growth.

T5 HO fixtures are well-suited to this balance:

• Low heat output. A 4-bulb T5 HO produces roughly 216 W total, but the spectrum and diffuse nature mean minimal radiant heat at the canopy. Compare this to a high-powered LED hung at the same distance.
• Even, diffuse light. Tube fixtures spread photons evenly across a propagation tray.
• Forgiving spectrum. Standard 6500K "daylight" bulbs provide a broad phosphor curve suitable for vegetative growth and cloning ^[2].
• Low cost and simplicity. T5 bulbs cost a few dollars each; ballasts last years.

Modern LED clone lights can match or exceed this efficiency and use less electricity, but they require more careful dimming or placement to avoid stressing cuttings. T5s are more tolerant of height variation and less likely to cause light damage if you make a small setup mistake.

When to start and stop

Start: Turn the light on the moment cuttings enter your medium—rockwool plugs, peat, aeroponic cloner, water glass, or other substrate. Set the timer immediately to establish a consistent photoperiod.

Photoperiod: Use 18 hours on / 6 hours off. This is the standard for clone rooting. Some growers advocate 24/0, claiming continuous light increases carbohydrate reserves for root development. However, there is no strong controlled evidence that 24/0 outperforms 18/6 for cannabis clone rooting Weak / limited. The 18/6 schedule saves electricity and gives your equipment a rest cycle.

Stop: Move clones away from the T5 once they show:

1. Visible white roots emerging from the plug or substrate.
2. Two to three new leaf sets above the original cutting leaves.

At this stage, clones can tolerate stronger light. Transition them to a veg fixture by hanging the new light higher than normal for 2–3 days, or by dimming it, to avoid bleaching the leaves.

How to set up and use T5s: step by step

1. Mount the fixture. Hang it from chains or adjustable ratchet hangers so you can change height as needed. A 4-bulb, 4-foot T5 HO is the standard size above a 2×4 ft propagation tray.

1. Install bulbs. Use 6500K ("cool daylight" or "daylight") T5 HO tubes. Some growers add one or two 3000K tubes for a warmer spectrum; for clones alone, straight 6500K is sufficient and simpler.

1. Set initial height. Start with the fixture 6–12 inches above the tops of cuttings or the dome. T5s do not emit enough concentrated heat to burn at this distance, but too close wastes light to the sides without benefit.

1. Connect a timer. Set it to 18 hours on / 6 hours off on a schedule that aligns with your maintenance routine.

1. Monitor environment. Target 72–78 °F (22–26 °C) at canopy level and 70–90 % relative humidity inside the dome ^[3]. The T5 itself contributes minimal heat, so active cooling is rarely needed.

1. Ventilate the dome. Crack the dome lid for 1–2 minutes once or twice daily to exchange air and prevent fungal issues. This is a separate task from lighting but is critical—many new growers neglect it.

1. Adjust height as roots emerge. Once you see root tips, you can lower the fixture to 3–6 inches to deliver more light into the developing plants, or move them to a stronger fixture if they appear to stall.

1. Replace bulbs yearly. T5 HO tubes lose 20–30 % output after 10,000–20,000 hours of use. If clones look pale and stretchy despite proper temperature and humidity, old bulbs are likely the culprit.

Common mistakes

• Hanging the fixture too high. A T5 four feet above clones provides almost no usable light. Keep it within 6–12 inches.
• Running too few bulbs. Firing only one or two bulbs in a 4-bulb fixture to "save power" creates gaps in coverage. Either use a smaller fixture or run all four.
• Using warm-colored (2700K) bulbs. These are intended for flower and will promote stretching in clones. Stick with 6500K.
• Aged or dim bulbs. Fluorescents lose output gradually. Pale, stretched clones with otherwise good humidity and temp usually signal bulb fatigue. Replace them before troubleshooting anything else.
• No timer. Manual on/off creates inconsistent photoperiods, which stresses plants. A mechanical timer costs ~$10 and eliminates the problem.
• Assuming no heat output. A 4-bulb T5 HO produces ~216 W. In a small sealed space, this can raise air temperature several degrees. Monitor thermostat readings.
• Hanging too close without humidity control. While T5s rarely cause light burn, they will dry leaf surfaces if humidity drops. Watch for curled or papery leaf tips.

Related techniques and alternatives

T5s are one proven option for clone propagation. Alternatives include:

• LED clone lights: More efficient per watt and often dimmable, but they require careful adjustment of height or intensity to avoid stressing cuttings if you lack experience.
• CFLs (compact fluorescents): Cheaper, sufficient for small batches (under 10 clones), but coverage is uneven across a large tray.
• Full-spectrum veg LED panels at low power (25–40 %): Workable if your panel has a reliable dimmer, but most beginners overshoot the intensity.
• Sunlight in a shaded outdoor spot: Free and effective if you can maintain stable humidity and a consistent 18-hour photoperiod, but it is hard to control.

The broader principle: clone-stage lighting is not a high-stakes decision. Almost any low-intensity, full-spectrum light source at a sensible distance will root cuttings provided humidity, temperature, and substrate are correct. T5s are standard because they make those conditions easy to maintain, not because they possess a unique spectrum or effect No data. Success depends far more on dome humidity and consistent temperature than on the light source itself.

Sources

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