Senescence

Senescence is the genetically controlled ageing of plant organs. In leaves, chloroplasts are dismantled and mobile nutrients such as nitrogen, phosphorus, potassium and magnesium are moved through the phloem to growing tissue. Chlorophyll breaks down, unmasking yellow carotenoids and, in some cultivars, red and purple anthocyanins. Ethylene and abscisic acid promote it; cytokinin delays it.

In practice

In late flower many cultivars yellow from the bottom up as the plant redirects resources to buds. That "fade" is normal in the final weeks, especially with a nitrogen taper, and growers often find it photogenic.

The diagnosis question is timing and pattern: - Normal: gradual yellowing of older fan leaves in the last 2-3 weeks, buds still swelling. - Too early: lower leaves yellowing hard in mid-flower or veg, which points to nitrogen shortage, root-zone problems or pH, not ripening.

Don't confuse with

• Nitrogen deficiency: same look, but at the wrong time or severity.
• Ripeness: judged by trichomes, not by leaf colour.
• Chlorosis: yellowing from any cause.

Myth check

Plain-water pre-harvest flushing is often sold as a way to "force the fade". House position (Myth, and actively harmful): starving the plant in its most critical weeks costs yield and resin. Taper nitrogen while feeding everything else instead. See the flush debate.