Every watering guide says "water when the pot feels light" and "don't overwater." That's good advice, but it doesn't explain WHY. Understanding the physics and biology of water movement in plants and growing media transforms watering from a guessing game into a decision you can reason about. This guide covers the science — not the schedule.
How water moves through a plant
The transpiration stream
Water moves through cannabis via a continuous column driven by evaporation at the leaf surface:
- Root uptake: water enters root hairs through osmosis (moving from low-concentration soil solution to higher-concentration root cell cytoplasm)
- Xylem transport: water moves upward through xylem vessels (dead, hollow cells that act as pipes). The driving force is not the roots pushing — it's the leaves pulling
- Leaf transpiration: stomata (pores on the underside of leaves) open, water vapor exits the leaf, and the resulting tension pulls the water column upward. This is the engine of the whole system
- Continuous column: water molecules are cohesive (they stick to each other via hydrogen bonds). The evaporation of one molecule at a leaf stoma pulls the next one up, creating an unbroken chain from root to leaf
Quantifying it: a mature cannabis plant in flower can transpire 1–3 liters of water per day under high-light conditions. On a hot, dry day with high VPD (vapor pressure deficit), transpiration increases — the plant uses more water. On a cool, humid day (low VPD), transpiration slows — the plant uses less.
This is why watering needs change with weather and environment, not just plant size. See our VPD guide.
What transpiration delivers
Transpiration isn't just about water — it's the delivery mechanism for dissolved nutrients:
- Mass flow: nutrients dissolved in soil water enter roots and travel upward with the transpiration stream. This is how most nitrogen, calcium, magnesium, and sulfur reach the leaves
- Active transport: some nutrients (potassium, phosphorus) are actively pumped into root cells against concentration gradients using cellular energy (ATP). This requires oxygen — which is why waterlogged roots can't absorb nutrients even when they're present in the medium
Calcium is a transpiration-dependent nutrient. It moves through the plant almost entirely via mass flow in the transpiration stream. It cannot be redistributed once deposited (it's "immobile"). This is why calcium deficiency shows up in new growth first and why low-transpiration environments (high humidity, low airflow) cause calcium deficiency even when calcium is abundant in the medium.
Osmosis and root pressure
How roots absorb water
Roots absorb water through osmosis — the movement of water across a semi-permeable membrane from lower solute concentration to higher. Root cell cytoplasm has a higher solute concentration than the surrounding soil solution, so water flows inward.
Root pressure is the slight positive pressure created by this osmotic gradient. On cool, humid mornings (low transpiration), root pressure can push water upward on its own. This is what causes guttation — those droplets that appear on leaf tips overnight. Guttation is a sign of healthy root pressure and good hydration (not overwatering).