Lower leaves turning yellow: ranking the causes

Five things cause old, low leaves to turn yellow, and one of them needs no fix at all. Confusing that one for the other four is a common overfeeding mistake: a grower sees yellow, assumes hunger, and pushes more nitrogen into a plant that was already doing exactly what it should.

The pattern this page covers, and the patterns it doesn't

The pattern here is specific. Yellowing starts on the oldest fan leaves, low in the canopy, on both sides of the plant, and climbs upward over one to three weeks. If your plant is doing something else, the cause is different and this ranking won't help:

One-sided, or following a row or an irrigation line points to a mechanical or zone-specific problem, not a whole-plant one.
Sudden, on a leaf that was fine yesterday is usually physical damage, a chemical splash, or a cold draught, not a nutrient shortage.
Starting on the newest growth at the top is the opposite pattern; see new growth turning yellow.
Spots, speckles, or a powdery coating rather than plain yellowing means you want the visual key to nutrient problems instead of this page.

Fig. 1Mobile-nutrient shortages and senescence start in the same place on the plant. Telling them apart takes a second look, not a different location.Horus

Within the pattern this page covers, five causes explain almost every case, and they are not equally likely:

Rank	Cause	Most likely when	Fastest way to tell
1	Nitrogen shortage	Vegetative growth, early flower	Whole leaf yellows, veins included; several leaves at once
2	Senescence	Week 5 of flower onward	Only the lowest two or three nodes; one leaf at a time
3	Magnesium shortage	Any stage, worse on RO or soft water	Yellows between the veins; veins stay green
4	Root-zone problem	Any stage	Paired with stalled growth or wilting despite wet media
5	Self-shading	Dense canopy, veg onward	Only the physically shaded leaves, not all leaves that age

Nitrogen shortage is the most common cause in veg and early flower

Nitrogen is one of the four elements cannabis can move once it has already built it into a leaf, along with phosphorus, potassium and magnesium ^[1]. When the root zone can't supply enough, the plant strips nitrogen out of its oldest leaves and reroutes it to the growing tip. That is why the symptom starts low: the plant is choosing to sacrifice its most expendable tissue first, not failing randomly.

The tell is that the whole leaf fades, including the veins, moving from mid-green to pale to yellow before the leaf dies and drops. Controlled trials that withhold one nutrient at a time have tracked this in cannabis through defined stages, from the first pale leaves to advanced yellowing and tissue death, with leaf-tissue analysis confirming the nutrient status at each stage ^[2].

Test it today: compare your feed electrical conductivity (EC) against the stage target. In coco or hydroponics, most feeding charts sit around 1.2–1.6 mS/cm in vegetative growth and 1.6–2.2 mS/cm in flower; that range is grower and manufacturer convention rather than a tested standard, so treat it as a starting point. Soil growers should run lower again, since the medium already carries some nutrient charge, and are better off watching runoff than chasing an absolute number.

Fix it: raise the feed over your next one to two waterings rather than one large dose, and hold everything else constant so you can see whether it worked. In soil, the single most common cause is simply running out: many bagged mixes carry enough nitrogen for the first few weeks and nothing more, so plan to start feeding before the lower leaves ask for it.

Past mid-flower, losing the bottom two or three nodes is normal

From roughly week 5 of flower onward, the plant deliberately abandons its lowest, most shaded leaves and moves their nitrogen and other mobile nutrients into the developing flowers it is trying to finish. This is the same source-to-sink reallocation that drives nitrogen mobility generally ^[1]; late in flower, the developing flowers out-pull every other part of the plant for it.

Test it today: two things both have to be true. Only the very bottom fan leaves are affected, and the plant is past the midpoint of flower. If yellowing is happening in veg, or has already climbed past the lowest third of the canopy, it isn't senescence and you should keep working down this list.

Fix it: nothing. There is no feed change that stops a healthy plant from doing this, and trying to reverse it by pushing more nitrogen late in flower mostly buys you softer, wetter flowers and a longer finish. Once a leaf is more than half yellow, strip it for airflow if you like; leaving it a few more days changes nothing. A lower canopy that goes all at once, rather than a leaf here and there, is worth reading up on separately: see leaves dropping in late flower.

Magnesium shortage keeps the veins green while the tissue between them fades

Magnesium is the fourth mobile nutrient, so a shortage also shows on the oldest, lowest leaves first ^[1]. The pattern is different from nitrogen, though: chlorosis appears between the veins while the veins themselves stay visibly green, often starting near the leaf margin and spreading inward. Left uncorrected for long enough, the faded tissue can develop small rust-brown necrotic spots.

Magnesium shortages are disproportionately common with reverse-osmosis or naturally soft water, because both carry little or no magnesium on their own, and a standard NPK feed doesn't reliably make up the difference unless a cal-mag supplement is part of the recipe.

Test it today: check your water source and whether cal-mag or a separate magnesium source is actually in the feed, not just assumed. If you're on RO or soft tap water and running a base nutrient alone, this is the leading suspect.

Fix it: add a cal-mag product, or magnesium sulfate (Epsom salt) on its own, at the rate stated on the product; a commonly used starting point for Epsom salt is around 1–2 g/L in the reservoir. This is grower practice, not a dose tested against a control, so start at the lower end, hold it, and reassess in seven to ten days rather than repeating a full dose every feed.

Nitrogen shortageWhole leaf yellows, veins included; several leaves at once, climbing.
Magnesium shortageTissue between the veins fades first; veins stay green.
Senescence, week 5+ flowerOne leaf at a time, fully gold, on the lowest two or three nodes only.

Same starting position on the plant, three different pictures once you look closely.

Fig. 2The fastest way to split these three: whether the veins stay green, and whether one leaf is affected or several at once.Horus

When the roots can't take up nutrients at all

Overwatering, compacted media, and root disease all produce the same broad symptom for the same reason: the plant can't move enough water and nutrient through a damaged or oxygen-starved root system, so it reallocates from old growth exactly as it would in a true nutrient shortage, even if the feed itself is fine.

What separates this from causes 1 and 3 is that it rarely shows up as leaf colour alone. Growth stalls, and you may see wilting despite media that is visibly wet, both of which a straightforward nutrient shortage doesn't cause on its own.

Test it today: pull one plant and look at the roots. Healthy roots are white to cream; brown, grey or visibly rotting roots, especially with a sour or ammonia-like smell rather than an earthy one, point to root disease or prolonged waterlogging. Alongside that, compare runoff EC and pH to what you fed in: runoff far below the input, despite an adequate feed, points to poor uptake rather than an empty recipe.

Fix it: address the root cause before you touch the feed. Let the medium dry back properly between waterings, improve drainage or aeration, and remove or treat visibly diseased roots. Feeding harder on top of a root problem adds salt to a root system that is already struggling and typically makes things worse. The full workup for this branch is at brown, smelly roots.

Shaded leaves yellow for a reason that has nothing to do with feed

A leaf buried under a dense upper canopy can end up running at too low a light level to earn its keep. Once a leaf costs the plant more in upkeep than it returns in sugars, the plant reclaims what it can from that leaf, which looks identical to a nutrient shortage from a distance even though the feed and the roots are both fine.

The giveaway is that it tracks light, not age. A leaf of the same age near the edge of the canopy, getting real light, stays green while an equally old leaf buried in the middle fades. This can happen at any stage from mid-veg onward, unlike senescence, which is tied to flowering.

Test it today: part the canopy by hand and look. If the yellowing leaves are consistently the ones sitting in shade, and a same-age leaf with better light exposure nearby is still green, this is self-shading rather than a feed problem.

Fix it: light defoliation, removing only the most shaded, least productive leaves, or a training method that spreads the canopy so more of it sees direct light; more feed will not fix a light problem. See choosing a training method for your space for the options by canopy shape.

The same-day triage order: cheapest test first

Run these in order and stop as soon as one of them explains what you're seeing.

Diagnose lower-leaf yellowing

Where did the yellowing start, and what does it look like?Check the whole leaf, not just the tip or one side.

Fig. 3The same ranking this page explains in words, walked one question at a time.Horus

Check input and runoff EC and pH (5–10 minutes, a meter you already own). This alone separates nitrogen shortage, magnesium shortage and a root-zone lockout from each other most of the time.
Re-examine the exact leaf position and pattern (2 minutes, no tools). This is what tells senescence and self-shading apart from a real deficiency, and it costs nothing.
Pull one rootball and look and smell (10–15 minutes), only if the first two steps haven't explained it.

Confirming the fix, and knowing when to stop chasing it

If the cause was nutritional or a root-zone problem, new growth should come in a normal, even green within 5–10 days, and the line of yellowing should stop climbing. Leaves that are already yellow will not turn green again; remove them once they're mostly yellow, mainly for airflow, not because they're doing the plant any harm at that point.

If you want a number instead of a visual judgement, a leaf-tissue test is the way to get one: recognised sufficiency ranges for hemp and cannabis leaf tissue put nitrogen at roughly 3.3–5.0% and magnesium at roughly 0.30–0.65% of dry leaf mass ^[4], sampled from the most recently matured leaves, not the yellow ones. That's a commercial-scale tool more than a home one, since it means sending a sample to a lab, but it turns a guess into a measurement when a fix doesn't seem to be working.

Not every yellow leaf is a diagnosis. One isolated leaf on an otherwise vigorous, healthy plant, with no pattern climbing the canopy and nothing else off, is normal wear and doesn't need a cause. Save the troubleshooting for when the pattern is real: more than one leaf, a clear position on the plant, and a direction it's moving in.