The Flowering Phase — The Most Important Stage in Cannabis Growing

## Introduction

The flowering phase is the decisive stage in the cannabis plant's life cycle. During this period, the inflorescences form, in which cannabinoids, terpenes, and flavonoids are produced and stored in the trichome glands. Depending on genetics, this phase lasts 7 to 12 weeks and requires precise control of environmental conditions.

## The Light Cycle: Trigger for Flower Formation

In photoperiodic cannabis varieties, flowering is initiated by a change in the light cycle. Switching from 18 hours of light / 6 hours of darkness (vegetative phase) to 12 hours of light / 12 hours of uninterrupted darkness (12/12) simulates the shortening autumn days and activates flower formation.

The critical factor is the length of uninterrupted darkness. The plant's phytochrome system — a light-sensitive protein pair (Phytochrome Red and Phytochrome Far-Red) — registers the dark duration. Even brief light interruptions during the dark period can disrupt the phytochrome equilibrium and delay flowering or trigger hermaphroditism.

Autoflowering varieties are an exception: they begin flowering based on age, regardless of the light cycle, as they carry genetic components from Cannabis ruderalis.

## Week by Week: Flower Development

Week 1–2 (Stretch Phase): After switching to 12/12, the plant undergoes a significant growth spurt known as "the stretch." The plant may increase its height by 50–100%. First female flower characteristics (white hairs, called pistils or stigmas) become visible at the nodes.

Week 3–4 (Bud Development): Pistils condense into recognisable flower clusters. Trichome production begins — initially as clear, transparent gland heads on flower petals and small leaves (sugar leaves). Nutrient requirements shift: less nitrogen, more phosphorus and potassium.

Week 5–6 (Ripening Begins): Flower clusters gain density and weight. Under the microscope, trichomes appear milky-cloudy — indicating that cannabinoid production is running at full capacity. The smell intensifies significantly as terpene production reaches its peak. Pistils begin to change colour from white to orange or brown.

Week 7–9 (Maturation and Harvest Window): Trichomes undergo a colour transition: clear → milky → amber. This colour spectrum is the most reliable indicator of maturity. 70–80% of pistils have changed colour. Lower leaves yellow and die off — a natural process in which the plant redistributes nutrients to the flowers.

Week 10+ (Late Bloomers): Sativa-dominant genetics and certain landraces can flower for 10–14 weeks. These varieties often develop particularly complex terpene profiles.

## Trichome Maturation as Harvest Indicator

Examining trichomes under a loupe (30–60× magnification) or a pocket microscope is the most accurate way to determine the harvest time:

- Clear trichomes: Cannabinoid synthesis is not yet complete. Harvesting too early results in lower potency and an unbalanced effect profile. - Milky-cloudy trichomes: THC content at its maximum. Harvesting at this point yields the most potent effect profile. - Amber trichomes: THC has begun oxidising to CBN (cannabinol). A higher proportion of amber trichomes correlates with a more sedative effect profile.

A commonly recommended harvest target: 70–80% milky trichomes, 10–20% amber, fewer than 10% clear.

## Flushing

In the last 1–2 weeks before harvest, many growers water exclusively with plain water, without nutrients — a process known as flushing. The goal: to break down excess nutrient salts in the substrate and plant tissue, which is said to improve taste and smoke quality.

The scientific evidence for the effectiveness of flushing is limited. A widely cited study by RX Green Technologies (2020) found no significant difference in mineral content of flowers between flushed and non-flushed plants. Nevertheless, many experienced growers subjectively report smoother combustion and better flavour. The practice remains controversial in the growing community.

## Common Mistakes During the Flowering Phase

Light interruption: Even brief light exposure during the dark phase can disrupt flowering. All light leaks in the grow room must be sealed.

Over-fertilisation: Especially in late flowering, less is more. Nutrient burns manifest as brown leaf tips and can negatively affect flavour.

Harvesting too early: Impatience is the most common beginner mistake. Harvesting too early means lower potency and immature terpenes. Trichome inspection under a microscope is indispensable.

Excessive humidity: In late flowering, relative humidity should be below 40% to prevent Botrytis (grey mould). Dense flower clusters are particularly susceptible.

Heat stress: Temperatures above 28°C during flowering can destroy terpenes and impair trichome quality. Optimal temperatures are 20–26°C during the light phase.