Leaf Senescence in Cannabis – the Programmed Ageing Process During Flowering

A cannabis plant in its seventh week of flowering is losing green. The lowest fan leaves turn yellow, then straw-coloured, and many growers respond with additional fertilisation. In most cases that reflex is wrong: what is happening is not a disorder but a programmed process that the plant actively controls. It is called leaf senescence.

A process, not decay

Leaf senescence is the final life stage of a leaf and differs fundamentally from a nutrient deficiency or a disease. At the end of a leaf’s life the plant breaks down the chlorophyll – the green pigment that converts light energy into sugars – and disassembles the storage proteins in the leaf. The nutrients released in the process, above all nitrogen, phosphorus, and sulphur, are transported via the phloem (the plant’s sugar-conducting pathway) into other plant parts: younger leaves, storage organs, or – in a flowering plant – the flowers. The process is genetically regulated and follows a fixed programme.

That an ageing leaf yellows is therefore the visible sign of a retrieval, not of decay. The breakdown of chlorophyll reveals the other leaf pigments that were previously masked. The nutrient flow out of the old leaf is not waste but a harvest that the plant invests into its next structure.

The cannabis evidence

For cannabis a controlled piece of evidence comes from the 2023 study by Llewellyn and colleagues. In that nutrient withdrawal study on the THC-dominant cultivar “Gelato 29” in deep water culture – with a fully supplied control group receiving a modified Hoagland nutrient solution – moderate yellowing of the oldest fan leaves began from the sixth week of flowering onward. Neither the nutrient solution nor the leaf tissue analysis showed a deficiency. The authors explicitly attribute this yellowing to programmed senescence and not to a deficiency. The chlorosis progressed over the following weeks and moved from the bottom upward and, within the individual leaf, from the margin toward the midrib.

The six-week value applies to this one cultivar in this one system. The exact starting week varies with the flowering duration of the cultivar and the growing conditions; earlier than week five or later than week eight is rare. The pattern – bottom to top, leaf margin to midrib – is, however, consistent.

Nutrients move from the leaf into the flower

Where the nutrients go has also been measured. In the 2025 cannabis study by Hershkowitz, the nutrient concentrations in leaf and flower of the CBD cultivar “T1” were tracked weekly over eight weeks of flowering. At harvest the flower concentration of nitrogen, potassium, sulphur, iron, manganese, and molybdenum was roughly twice the leaf concentration; for phosphorus and copper roughly three times.

That is the other side of the leaf yellowing. What disappears from the old fan leaf turns up in the flower. The plant retrieves the material it needs for flower development from the leaves it will shed anyway. For phosphorus the accumulation is particularly pronounced – cannabis is a plant that draws especially large amounts of P into the flower during the reproductive stage.

Distinguishing senescence from deficiency

The most important practical question is whether late-flowering yellowing is senescence or deficiency. Four characteristics help with the decision.

Timing. Senescence in cannabis flowering typically begins from week five to seven and progresses toward harvest. A single-element nutrient deficiency can occur at any growth stage and is not tied to a calendar.

Pattern within the leaf. In senescence the chlorosis diminishes from the leaf margin toward the midrib – the margin yellows first, the midrib stays green for longer. In a magnesium deficiency, by contrast, the chlorosis is interveinal – between the leaf veins – while the veins themselves remain green. In a potassium deficiency the margin becomes necrotic, turning brown and dying, not just yellow. In a nitrogen deficiency the yellowing is uniform across the entire leaf surface.

Leaf tissue analysis. In senescence the leaf concentrations of several mobile nutrients drop in parallel, because the plant withdraws them from the leaf together – nitrogen, phosphorus, potassium, and magnesium decline simultaneously. In a true single-element deficiency one value drops far while the others remain within the sufficiency range.

Nutrient or substrate solution analysis. In senescence the nutrient solution and substrate solution are unremarkable – the supply is there, the plant simply no longer uses it in the yellowing leaf. In a deficiency the deficient nutrient is also low there. This single check usually decides.

Why the confusion is so persistent

The leaf yellowing of senescence resembles the picture of a nitrogen or magnesium deficiency because both move from the bottom upward – and the picture of a redistribution resembles that of a decline because the leaf loses nutrients in both cases. The confusion is further encouraged by the fact that many growers reduce or stop fertilisation in exactly this period (see the article on cannabis flushing) – programmed senescence and reduced supply then run in parallel, and the visual effect becomes even more pronounced. Anyone who reads the picture as a deficiency and responds with additional fertilisation is intervening in a process that cannot be corrected: the plant will shed the leaf regardless, and the extra fertiliser ends up in the younger leaves and in the flower, where it is not needed.

What senescence is not

Senescence is not the same as necrosis. Necrosis is cell death, often triggered by drought stress, salt stress, frost, or disease; the leaf dies rapidly, usually with brown dry patches. Senescence, by contrast, is a slow, controlled process that lasts weeks and begins with steadily advancing yellowing. Necrosis is also not the end point of senescence; the senescent leaf turns fully yellow, loses its structural integrity, and drops – in cannabis cultivation often only after harvest, when the plant has already been cut.