Is this the fade, or a deficiency?

The short answer

• A fade is slow, starts at the bottom, works up evenly, and the flowers keep swelling. The plant is pulling mobile nutrients out of leaves it no longer needs and sending them into the buds. That is the plant doing its job.
• A deficiency is fast, patterned, or in the wrong place — interveinal, spotted, edged, on the newest growth, or on the leaves closest to the buds — and the buds stall while it happens.
• The one test: mark the yellowest leaf today and count the newly yellow ones in four days. A fade adds a leaf or two, low down. A deficiency adds a tier.

Why the plant yellows on purpose

Some nutrients a plant can move around inside itself and some it cannot. Nitrogen is the clearest mobile one, and it is also the one a plant needs least once it has stopped building leaves. In the last weeks of flower the plant stops investing in foliage and puts everything into the flowers, and part of that is reclaiming nitrogen from its oldest leaves and sending it upward. Those leaves lose their green because the plant took it, not because the plant ran out.

That is the fade, and it has a recognisable shape because of where the plant takes from first: the oldest, lowest, most shaded leaves, evenly across the whole blade, then the next tier up, over a week or two. The flowers themselves stay vigorous. The fan leaves nearest the buds are the last to go, if they go at all.

Worth saying plainly, because it is repeated as fact everywhere: there is no published evidence that the fade improves the smoke. The idea that low nitrogen at the end makes for a smoother product is borrowed from tobacco, where it is real and documented, and has never been tested on cannabis. So the fade is something to recognise, not something to chase. If your plant is fading in late flower it is fine. If it is not fading, that is fine too. Starving a plant to force it is a practice without a result behind it — the same is true of flushing.

What a deficiency looks like instead

The same mobility rule that produces the fade tells you when it is something else. A plant reclaims nitrogen evenly: the whole blade pales at once. Anything that leaves a pattern on the leaf is not that.

• Yellow between the veins with the veins still green, on lower leaves, is the classic magnesium presentation. The reference text notes it can also show on younger leaves, which is the honest limit of the position rule and why the pattern matters more than the position.
• Yellowing on the newest growth at the top is an immobile nutrient — iron or calcium — or, far more often in late flower, a root-zone pH that has drifted so the plant cannot take them up. Iron in particular locks out above about pH 6.5. A fade never starts at the top.
• Brown edges, rust spots, or tips that crisp are potassium or calcium territory, not a fade.
• Yellowing that races — a tier a day rather than a tier a week — is a plant that has genuinely run out or genuinely cannot absorb, and the buds will show it: they stop swelling and the sugar leaves go pale with the fan leaves.

The single most useful distinction, and it is ours rather than a published test: watch the flowers, not the leaves. A fading plant’s buds are fattening while its bottom leaves go. A deficient plant’s buds are stalled while its leaves go. The leaves look similar in a photo; the flowers do not.

The one that fools people after the flip

The same question turns up two to four weeks after switching to flower, and it is usually neither a fade nor a deficiency. The plant has just stretched hard, doubled its leaf area and started building flowers, and its demand for nitrogen and everything else jumped while the feed stayed the same. Lower leaves yellow because the plant is spending faster than it is being fed. That is a rate problem, it is early for a fade, and it usually answers to feeding rather than to waiting. The tell is timing: week three of flower is too early for a natural fade on almost any variety.

How to check this yourself

• Mark and count. Tie a twist-tie on the yellowest leaf. In four days count how many new leaves have gone yellow and where. One or two, low down, whole blade: fade. A whole tier, or anything up top, or anything patterned: not a fade.
• Look at the blade, not the plant. Whole blade paling evenly, edges included: reclamation. Veins staying green, spots, edges first, tips first: a specific shortage, and the position-and-spread page narrows which.
• Look at the buds. Still swelling, sugar leaves still green: the plant is fine. Stalled, sugar leaves paling too: it is short of something.
• Count the weeks. Yellowing before roughly the last three weeks of the variety’s flowering time is early for a fade. Yellowing at week three of flower is almost never one.
• If it is patterned or at the top, read the root-zone pH before you reach for a bottle. In late flower the medium has had months to drift, and a lockout looks exactly like a shortage — and feeding more makes a lockout worse.

What would show that we have this wrong

If the yellowing is even, bottom-up, slow, and the buds are still fattening, but the plant is only in week two or three of flower, it is not a fade whatever it looks like — it is being underfed for the stretch, and waiting will cost you. And if it is the last fortnight, bottom-up and even, but the buds have stopped growing, look harder at the root zone: a fade does not stall the flowers.

Where our device fits, stated narrowly

Croplock does not decide fade-versus-deficiency for you; that is a judgement about pattern and rate, and we would not trust a model to make it from one frame. Where it fits is the rate half: it photographs the same canopy every hour, so “is this a leaf a week or a tier a day” is a comparison across dated frames rather than a memory, and its record of the run’s phase and day tells you whether it is week three or week nine.

Sources

• Cannabis in Controlled Environment Production (CRC Press), Ch. 8 and 9. DOI 10.1201/9781003150442 — nutrient mobility, the nitrogen and magnesium presentations, the iron lockout above about pH 6.5. Restated in our own words.
• Cockson, P. & Whipker, B.E. (2019). “Nutrient Disorder Atlas.” Applied Sciences 9(20):4432 — an open-access set of photographs of each disorder, worth looking at before deciding what yours matches.
• Caplan, D., Matzneller, P. & Gutierrez, C. “Harvest and Post-Harvest.” Ch. 10 in the same handbook — the absence of published evidence for pre-harvest nutrient withdrawal on cannabis; Sumner & Moore (2009) for the tobacco origin of the idea.
• The rate-and-flowers test and the post-flip timing note are our own reasoning from the mobility axis, given as such rather than as a published finding.

More of these at Answers. The device is at croplock.com.

Related: Is it a deficiency or a disease? — the general position-and-spread key. Should I flush before harvest? — the evidence for late-flower withdrawal, graded.