Hemp is a photoperiod-sensitive plant, which means its life cycle is guided not by age, but by light. Unlike crops that follow a strict number of days to maturation, hemp reads its environment through daylight patterns and uses those signals to decide when to grow, when to stretch, and when to bloom.
Whether you’re growing boutique craft flower or managing full outdoor acreage, understanding hemp’s life cycle — and how it responds to day length — gives you the ability to predict growth patterns, prevent early flowering, manage stretch, and produce consistent results year after year.
This chapter explains how hemp moves through its major stages, how light controls those transitions, and how growers can work with the plant’s biology instead of fighting it.
The Four Life Stages of Hemp
Hemp progresses through four primary stages:
- Germination
- Vegetative Growth
- Transition (Stretch)
- Flowering
Each stage has its own biological markers and environmental needs.
1. Germination: Awakening the Seed
The life cycle begins when moisture triggers the seed to release enzymes that soften the shell. The taproot emerges first, anchoring the plant and seeking water.
Ideal germination conditions for hemp are:
- Warm soil (70–80°F)
- Light moisture
- Good oxygen flow
- Stable environment
Once the first true leaves appear, the plant moves into vegetative growth.
2. Vegetative Stage: Building the Plant’s Body
In veg, hemp focuses on:
- Root expansion
- Leaf production
- Stem thickening
- Branch development
- Establishing structure
Vegetative hemp grows faster in long-day conditions.
Outdoor Veg Lighting
Hemp stays in veg when days are 14+ hours long.
In most regions, this is spring through midsummer.
Indoor Veg Lighting
Growers use 18/6 or 20/4 light cycles to maintain consistent veg growth.
During this stage, the plant establishes all structural characteristics it will carry into flower — like height, branch spacing, and potential canopy shape.
3. Transition: The Photoperiod Trigger & Stretch Phase
Hemp begins transitioning when it detects a reduction in daylight hours. This is not based on sunrise/sunset alone — hemp measures uninterrupted darkness using internal photoreceptors.
The Critical Moment: Night Length
Hemp begins flowering when nights grow long enough to trigger hormonal changes.
For most varieties, this is 12–14 hours of darkness.
This is why:
- Outdoor hemp begins flower mid–late summer
- Early planting outdoors risks preflower if nights are still long
- Light pollution at night can cause re-veg or hermaphroditism
- Greenhouse operations must manage light leakage carefully
Stretch Phase
The first 2–3 weeks after floral induction involve rapid vertical growth.
This stretch is driven by gibberellins, the hormones responsible for stem elongation.
Hemp may double in height during this window.
4. Flowering Stage: Resin, Aroma & Reproduction
Once the plant fully transitions, it focuses almost entirely on flower development.
Biological priorities shift toward:
- Trichome formation
- Terpene production
- Resin and secondary metabolite synthesis
- Seed production in non-sensimilla crops
- Defense chemistry
Light Needs in Flower
Outdoor hemp finishes as days naturally shorten.
Indoor/greenhouse growers use 12/12 lighting to maintain flowering.
Environmental Role
Cooler nights, steady moisture cycles, and good airflow support:
- Denser flower formation
- Richer aroma expression
- Strong resin profiles
Hemp’s flowering period typically lasts 6–10 weeks, depending on genetics.
How Hemp Measures Light: The Photoreceptors
Hemp uses specialized proteins called phytochromes to sense:
- Light duration
- Light quality
- Darkness length
- Seasonal change
Phytochromes act like internal timekeepers.
When the phytochrome system detects longer nights, it signals the plant to:
- Reduce vegetative hormones
- Increase floral hormones
- Shift metabolism toward reproduction
This is why even small amounts of nighttime light — streetlamps, greenhouse leaks, security lights — can disrupt flowering.
Autoflowers: The Exception to Photoperiod Rules
Autoflowering hemp flowers based on age, not day length.
They typically begin flowering:
- 3–5 weeks from sprout
- Regardless of light cycle
- With consistent size variation
Autoflowers are useful for short seasons or rapid turnover but do not respond to canopy training or photoperiod manipulation the same way as traditional hemp.
Final Thoughts
Hemp’s life cycle is a dialogue between the plant and the environment. Every shift in daylight, every change in temperature, and every stress signal influences how and when the plant grows.
Understanding these biological rhythms helps growers:
- Prevent early flowering
- Time planting correctly
- Manage stretch
- Support strong bloom development
- Produce consistent, expressive flower
Hemp is guided by the sky — growers simply learn how to read its signals.