T5 Fluorescents for Clones

What a T5 fluorescent is

T5 refers to a tubular fluorescent lamp 5/8 inch (about 16 mm) in diameter. The "HO" variant — high output — runs at higher current and produces more light per tube than standard T5s. A typical horticultural fixture holds 2, 4, or 8 tubes in a reflector, with each 4-foot HO tube drawing about 54 W ^[1].

For clones, the relevant traits are: low radiant heat at the canopy, broad even coverage (no hotspots), and a moderate photosynthetic photon flux density (PPFD) of roughly 100-250 µmol/m²/s at typical hanging distances Strong evidence. That intensity range is well-matched to unrooted cuttings, which cannot transpire normally and are easily stressed by stronger light ^[2].

Why growers use them for clones

Clones are in a fragile state. They have no roots, so they cannot replace water lost through their leaves. The job of the propagation light is to give them just enough photons to keep photosynthesis ticking over while they build a root system — not to drive aggressive growth.

T5 HO fixtures hit that target well:

• Low heat at canopy. You can run a T5 4-6 inches above a humidity dome without melting it or scorching tips.
• Diffuse, even light. Multiple long tubes spread photons across a tray with no hotspots.
• Cheap and replaceable. Bulbs are a few dollars each. Ballasts last years.
• Forgiving spectrum. 6500K "daylight" bulbs are standard for veg and clones; the broad phosphor spectrum covers what young plants need ^[3].

Modern LED bars and quantum boards can do the same job and use less electricity, but they often need to be dimmed or hung higher to avoid blasting cuttings. For a beginner, a T5 is harder to misuse Weak / limited.

When to start and when to stop

Start: The moment cuttings go into your medium — rockwool, peat plugs, aeroponic cloner, water glass, whatever. Turn the light on, set the timer, and walk away.

Photoperiod: 18 hours on / 6 off is the standard. Some growers run 24/0 during the rooting phase on the theory that more light equals more carbohydrate production Weak / limited. There is no strong controlled evidence that 24/0 outperforms 18/6 for rooting cannabis cuttings, and 18/6 saves electricity and gives the bulbs (and you) a rest.

Stop: Move clones off the T5 once they have:

1. Visible white roots reaching out of the plug, and
2. Two to three new sets of leaves above the original cutting.

At that point they can handle stronger light. Transition them to your veg fixture by either hanging the new light higher than normal for the first few days, or dimming it, to avoid bleaching. See Hardening Off Clones.

How to do it: step by step

1. Mount the fixture. Hang it from chains or ratchet hangers so you can adjust height. Above a 2x4 ft tray, a 4-bulb 4-foot T5 HO is the standard size.

1. Load bulbs. Use 6500K ("daylight") tubes. Some growers mix in 3000K for a fuller spectrum; for clones, straight 6500K is fine Weak / limited.

1. Set height. Start with the fixture 6-12 inches above the tops of the cuttings (or above the humidity dome). T5s do not burn at this distance, but too close still wastes light to the sides.

1. Set the timer. 18 hours on, 6 off. Pick a schedule that lines up with when you'll be checking on the room.

1. Check environment. Aim for 72-78 °F (22-26 °C) at canopy and 70-90 % relative humidity inside the dome ^[4]. The T5 itself contributes very little heat, so you usually do not need extra cooling.

1. Vent the dome daily. Crack it for a few minutes once or twice a day to exchange air. This is unrelated to the light but is the most common thing new growers forget.

1. Adjust height as roots appear. Once roots show, you can lower the fixture closer (3-6 inches) to push more light into the young plants, or move them to a stronger fixture.

1. Replace bulbs annually. T5 HO tubes lose meaningful output by 10,000-20,000 hours of use ^[1]. If yours are more than a year of heavy use old and clones are stretching, replace them before troubleshooting anything else.

Common mistakes

• Hanging too high. A T5 four feet above clones is essentially decorative. Keep it within 6-12 inches.
• Hanging too close without a dome. Possible but rare to cause light stress; the bigger risk is drying out leaves. Watch for curling tips.
• Using only one or two bulbs in a 4-bulb fixture to "save power." This creates uneven coverage. Run all bulbs or use a smaller fixture.
• Wrong color temperature. 2700K "warm white" bulbs intended for flower are not ideal for clones. Use 6500K.
• Old bulbs. Fluorescents dim gradually. If your clones look pale and stretchy and your environment is dialed in, the bulbs are likely tired.
• Skipping the timer. Manual on/off leads to inconsistent photoperiods, which stresses plants. A $10 mechanical timer fixes this.
• Assuming T5 = no heat at all. A 4-bulb T5 HO still puts out ~216 W of heat into a small space. In a sealed closet it can raise temps several degrees.

Related techniques and alternatives

T5s are one of several reasonable propagation light choices. Compare:

• LED Clone Lights: More efficient per watt, often dimmable, but easier to misuse by running too hot or too close.
• CFLs (compact fluorescents): Cheaper and fine for a handful of clones; less even coverage than a tube fixture.
• Veg-stage LED panels at low power: Workable if you can dim to 25-40 %.
• Sunlight in a shaded spot: Free, works, but hard to control humidity and photoperiod.

Related cultivation reading: Taking Cuttings, Humidity Domes, Rooting Hormones, Photoperiod Basics, Hardening Off Clones.

The broader point: clone-stage lighting is a low-stakes problem. Almost any low-intensity, full-spectrum light source within a sensible distance will root cuttings if your humidity, temperature, and medium are right. The T5 is popular because it makes those conditions easy to maintain, not because it has any magic spectrum No data.

Sources

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