Leaf Senescence in Cannabis – Yellowing with Age in Flower

In the sixth week of flowering the lowest fan leaves, the large, fingered leaves on the main stem and branches, turn yellow. Growers who feed more at that point are taking the picture for a deficiency. Yet in one trial the same yellowing also appeared with full supply, and neither the nutrient solution nor the leaf showed a deficiency there. This yellowing with age is called leaf senescence.

How do I recognise leaf senescence?

You recognise it by the oldest fan leaves, which yellow from the bottom of the plant upwards in late flower. Within each leaf the yellow runs from the margin to the midrib, the main vein in the middle of each leaflet. The margin therefore yellows first, and the midrib stays green the longest. In the trial, moderate yellowing began in the sixth week of flowering and increased week by week until harvest.

That applied to the variety “Gelato 29” in deep water culture, where the roots hang in an aerated nutrient solution, meaning water with dissolved fertiliser. Week six is therefore a value for one variety in one system. Ripening time depends on the variety, and a measurement across varieties is still missing. Yellowing that starts at the top of the plant does not fit this pattern.

Note the week of flowering in which the yellowing begins.
Check whether it starts at the oldest fan leaves at the bottom or at new growth. New growth means the youngest leaves at the shoot tips.
Check in each leaflet whether the margin yellows first and the midrib last.

Why does the leaf turn yellow even though there is enough fertiliser?

In the researchers’ reading, the old leaf hands its nutrients over to the flower. Contents in flower and leaf were measured on “T1”, a high-CBD variety; CBD is an active compound that does not get you high. The plants grew in deep water culture, and the measurements ran over eight weeks of flowering. At harvest many nutrients were markedly higher in the flower than in the leaf, which the researchers read as ongoing uptake plus relocation from ageing leaves. This relocation would run through the phloem, the pathway for sugars and dissolved substances. Meanwhile the leaf loses its chlorophyll, the green leaf pigment, and turns yellow.

Nutrient	Content in the flower compared with the leaf
Nitrogen, potassium, sulphur	about twice as high
Iron, manganese, molybdenum	about twice as high
Phosphorus	about 2.5-fold
Copper	2.8-fold

If the nutrient solution is unremarkable, do not treat late yellowing of the oldest fan leaves as a deficiency.
Keep feeding the nutrient solution as planned.

How do I tell senescence from a deficiency?

Pattern and timing give the first clues, and a leaf and solution analysis confirms them. In a leaf analysis a lab measures the nutrients in the dried leaf. A solution analysis measures them in the nutrient solution or in the runoff, the water that drains out of the bottom of the pot. A mobile nutrient is one the plant can pull out of old leaves, and a shortage of it hits the lower leaves first. A potassium deficiency also starts at the margin, but there the tissue soon turns brown and dies. In the trial a calcium deficiency in flower also began at the bottom, so location alone does not decide. Without analyses, the age yellowing in the trial would have looked like an early nitrogen or magnesium deficiency.

Feature	Age yellowing	Deficiency
Timing	late flower	possible at any stage
Start	oldest fan leaves at the bottom	at the bottom or top, depending on the nutrient
Course within the leaf	yellow from margin to midrib	magnesium between the veins, nitrogen evenly
Pace	slow, over weeks	faster (mobile nutrients)
Leaf analysis	several mobile nutrients drop together	one value collapses on its own
Solution analysis	unremarkable	the missing nutrient is low there too

Read the leaf picture, the leaf analysis and the solution analysis together.
If the picture is unclear, send a leaf sample to a lab. Sampling is covered in the article on nutrient diagnosis.
Measure the nutrient solution or the runoff and check whether the nutrient in question is normal there.
Compare the deficiency patterns and how they progress in the article on identifying nutrient deficiency.

Should I feed more when leaves yellow late?

No, not if it is age yellowing: judging by the fully supplied control plants of the trial, extra feeding brings nothing here. By the same conclusion, an extra dose only reaches the younger leaves that are still taking up nutrients.

With age yellowing, do not increase feeding.
Only feed more once a leaf or solution analysis shows a deficiency.
Watch the new growth, and if it yellows, look for another cause.

Does flushing before harvest make the yellowing worse?

That is possible, but it has not been measured. The researchers of the senescence trial consider it possible that flushing, meaning plain water only in the last weeks before harvest, intensifies the deficiency sign or speeds up ageing. They did not test flushing itself, however.

Read yellow fan leaves after flushing neither as success nor as deficiency.
Before flushing, look up its benefit in the article on cannabis flushing.