How tall do cannabis plants get indoors?

Short answer

Indoor cannabis usually finishes about 60–150 cm (2–5 ft) tall, because you decide. A photoperiod plant grows 50–100% taller after the flip, and stretchy cultivars up to 3×, so final height ≈ height at the flip × 1.5–3. Autoflowers usually fit a 140–160 cm (4.6–5.3 ft) tent. Grower practice, anchored to stretch research.

Why

Veg height is your choice. A photoperiod plant stays vegetative under long days until you flip it, so veg time sets the starting height.

The stretch adds the rest. Under short days, stems and internodes lengthened fastest from about day 5 to day 10 ^[1]. A plant-patent description puts the gain at 50–100% over the first two to three weeks after induction ^[2]; growers report 1.5× to 3×.

Light shapes the plant. Higher light in veg gave shorter internodes and more compact plants ^[3]. A dim or distant light makes plants stretch and gap.

Outdoors, there's no ceiling. Unrestricted plants commonly reach 1–5 m (3–16 ft), depending on genotype and season ^[4].

The numbers

Height at flip	Final, 1.5×	Final, 2×	Final, 3×
30 cm (1 ft)	45 cm	60 cm	90 cm
45 cm (1.5 ft)	70 cm	90 cm	135 cm
60 cm (2 ft)	90 cm	120 cm	180 cm

Tent height	Usable plant height (about)
140–160 cm	55–90 cm
180–200 cm	95–130 cm

Usable height = tent − pot (~30 cm) − 30–45 cm light gap − fixture and hangers (~10–15 cm).

What to do

Work out your usable height before you plant.
Flip photoperiods at about half of it; a third for very stretchy cultivars.
Train flat with LST from early veg.
Keep light strong enough to hold internodes short.
Keep a little headroom for the light to be raised in week 3.

Common mistakes

Vegging by the calendar and running out of room in week 3.
Assuming the breeder's "medium height" fits your tent.
Raising a weak light higher, which makes stretch worse.

Full article: The cannabis life cycle
Answers: when to flip · plant too tall for the tent · what size tent
Article: Stretchy, leggy cannabis
Terms: stretch · internode

Follow-up questions

How much do plants stretch after the flip?

Cannabis plants usually stretch 50–100% in the first two to three weeks after the flip to 12/12, and grower reports range from about 1.5× to 3× the height at the flip. Stems and internodes lengthen fastest from about day 5 to day 10 of short days. Plan for it by flipping at about half the height you can fit.

How do I keep my plants short indoors?

Keep cannabis plants short indoors by flipping early, at about half your usable height, and training them flat. Low-stress training bends the main stem sideways; topping and a net (SCROG) spread the canopy. Strong light close to the canopy also keeps internodes short, while a weak or distant light makes plants stretch and grow gappy.

How tall should my grow tent be?

A grow tent for photoperiods should be about 180–200 cm (6–6.6 ft) tall, because plants commonly double in height after the flip. Autoflowers and short, trained plants fit a 140–160 cm (4.6–5.3 ft) tent. Allow for the pot, a 30–45 cm (12–18 in) gap to the light and the fixture itself when working out the plant's usable height.

Sources

Alter H, Sade Y, Sood A, et al. 2024. Inflorescence development in female cannabis plants is mediated by photoperiod and gibberellin. Horticulture Research 11(11):uhae245. doi.org/10.1093/hr/uhae245

US Plant Patent PP30,434 (cannabis plant 'LW-BB1'), botanical description of post-induction stretch. image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/PP3

Moher M, Llewellyn D, Jones M, Zheng Y, 2021. High light intensities can be used to grow healthy and robust cannabis plants during the vegetative stage of indoor production. Preprints 2021040417 (not peer reviewed). doi.org/10.20944/preprints202104.0417.v1

Small E. 2015. Evolution and classification of Cannabis sativa (marijuana, hemp) in relation to human utilization. Botanical Review 81:189-294. doi.org/10.1007/s12229-015-9157-3