Short answer, in plain words. A cannabis plant makes its own sugar. Its leaves turn light, air and water into sugar, then send it down to the roots and up to the buds [1]. Sugar you pour into the pot doesn't go into the plant; the microbes in the soil eat it within days [2]. In living soil that can wake the microbes up, which is fine. In clean hydro it isn't needed, and it can feed the wrong bugs. GenetixBay's position: molasses and sugars feed microbes only, not the plant (founder's field experience) [6].
Where does a cannabis plant get its sugar?
From its own leaves. Photosynthesis in the chloroplasts builds sugars from CO₂ and water using light energy, and the phloem carries them to roots, new growth and flowers [1]. Established.
Light- Leaves make sugarCO₂ + water + light → sugars
- Phloem carries itTo roots, shoots and buds
- Roots and buds grow
Source: [1]
What happens to sugar poured into the pot?
Microbes eat it. Sugar is the easiest food a soil microbe can find; adding fresh, easily used carbon sets off a burst of microbial growth and activity, sometimes speeding up the breakdown of older organic matter too (a "priming effect") [2]. Established (soil biology).
Roots can absorb some sugars, which is why sterile plant tissue-culture media include sucrose [3]. In a pot or reservoir full of microbes, though, there's no evidence that cannabis takes up meaningful sugar from the root zone. Supported.
Does molasses make buds sweeter or bigger?
No evidence says so. Supported.
- Flavour and aroma come mostly from terpenes made in the glandular trichomes from the plant's own carbon [4]. Sugar in the soil isn't on that path.
- Yield and potency: we found no controlled cannabis study showing molasses raises either.
- Nutrients: molasses carries some potassium, calcium, magnesium and iron, but far less precisely than a balanced feed.
Living soil vs hydro
Living soil
Microbes are the point
- +Quick carbon for soil microbes
- +Can boost microbial activity for a short time
- −No proven yield, potency or taste gain
- −Easy to overdo: sour smell, fungus gnats
Sterile or clean hydro
- −Not needed: the plant makes its own sugar
- −Feeds bacteria and biofilm in the reservoir
- −Bacteria use up dissolved oxygen around the roots
- −Conditions Pythium root rot likes
In hydro, root rot from Pythium and Fusarium is a real problem for cannabis [5]; feeding sugar into the water feeds microbial growth that competes with roots for oxygen. If you run a beneficial-microbe reservoir (a house-endorsed choice), use products made for it (see bottled microbes).
Common mistakes
- Adding molasses to a hydro reservoir "for the plant".
- Expecting sweeter smoke from sugar in the soil.
- Brewing compost tea with molasses and spraying it on plants (it can grow human pathogens; see does compost tea work?).
- Using molasses instead of fixing a real potassium or magnesium shortfall.
