How Long to Veg — and Why Light Builds the Skeleton
Veg length sets your plant count, canopy shape, and yield ceiling. Learn the 2-4 week clone window and how PPFD intensity dictates internode length and node count.
Veg is where you build the skeleton the flower will hang on
Vegetative growth is the phase between a rooted clone and the flip to 12/12. Everything you do here is about structure: how many bud sites, how evenly they sit under the light, and how much biomass the plant can carry into flower. A commercial room lives or dies on repeatability, so veg is run to a target architecture — not to a calendar guess. The two dials that control that architecture are time in veg and light intensity.
For clones off a mother, the commercial window is ~2-4 weeks of veg. Clones arrive already sexed and genetically identical, so you are only building size, not waiting on maturity. Seedlings need longer (they must clear juvenile growth before they respond to a flip), which is one reason production rooms run clones.
Light intensity writes the plant's shape
The single most useful architecture finding for commercial growers comes from the Guelph / Zheng light-and-architecture work: higher PPFD in veg produces shorter internodes, thicker stems, and more nodes. Same genetics, same weeks in veg — the light level decides whether you get a loose, stretchy plant or a compact one packed with bud sites.
| Veg PPFD | Architecture | Best for |
|---|---|---|
| ~600 µmol·m⁻²·s⁻¹ | Open, taller, longer internodes | Fewer, larger plants; SCROG fill |
| ~900 µmol·m⁻²·s⁻¹ | Compact, short internodes, more nodes | Commercial-preferred; tight, uniform canopies |
Cannabis is light-limited at the whole-canopy level far beyond where a single leaf saturates — the Bugbee data shows near-linear yield from PPFD 120 to 1,800. In veg, that extra light doesn't get 'wasted' making the plant tall; the plant banks it as shorter internodes and additional nodes. Each node is a future bud site, so a high-light veg literally pre-loads your flower yield.
Practically: start veg at PPFD 300-600 (18/6, DLI ~20-40) while the young plant is establishing, then ramp toward 600-900 as the canopy fills and the plant can use the photons. You do not flip at max intensity — you arrive at it.
No CO₂ in a tent? Cap veg PPFD around 800-900 — above that without enrichment you're paying for photons the plant can't convert. You lose essentially nothing on architecture. A cheap PAR meter or your light's height chart gets you close; aim for the top of the 300-600 band on young plants and creep up as they fill in.