9 min read

Yellow Leaves on Plants: Troubleshooting Brown Spots and Root Rot

Yellow leaves, brown spots and root rot usually trace back to water, pH or feed before anything else. Here is how to read the pattern on your plants, check causes in the right order and catch root rot early.

Thomas BrownDirector, The Grow Superstore

Published 5 Nov 2025Updated 23 Oct 2026

Yellow leaves, brown spots and root rot showing common plant problems caused by nutrient deficiency, pH imbalance, overfeeding, poor airflow, low humidity, poor aeration and stagnant water.

Why are my plant leaves turning yellow? Yellow leaves on plants are usually a watering, pH or feeding problem, and brown spots or crisp edges usually mean overfeeding, dry air or poor airflow. Check in this order: roots and water first, then pH, then feed, then the climate. Root rot, where roots turn dark, soft and smelly, is the serious one, so catch it early.

Root rot is the decay of a plant's roots, caused by organisms that thrive where the root zone is waterlogged, starved of oxygen or too warm. Healthy roots are pale and firm; rotting roots are dark, soft and smell unpleasant.

Who this guide is for: UK home and hobby growers, in soil, coco or hydro, who want to work out what a struggling plant is telling them. New to hydroponics? Start with our beginner's guide to hydroponics.

Why are my plant leaves turning yellow?

Look at where the yellowing starts. The pattern narrows the cause far faster than guessing at feed. The Royal Horticultural Society (RHS) notes that iron deficiency affects the youngest leaves first, while nitrogen, magnesium and manganese problems tend to start in the older leaves. A single old lower leaf yellowing now and then is normal ageing. Large numbers of yellow leaves are not.

Work through this order before you change anything:

Roots and water. Lift the pot to feel its weight, push a finger into the medium, and look at the roots (see the root rot section below).
pH. Wrong pH stops plants absorbing food that is already there. Check the feed and runoff against the targets in our pH, EC and water quality guide: 6.0-6.8 for soil, 5.5-6.2 for coco and hydro, where most growers hold 5.8-6.0.
Feed. Only then adjust nutrients, one change at a time. Plant nutrients explained covers what each element does.
Climate. Cold, heat and light all show up in the leaves too.

What you see	Likely cause	First fix
Older leaves pale and yellow, growth weak	Low nitrogen, or roots not taking food up	Check pH, then feed strength
Newest leaves yellow, veins stay green	Iron unavailable, often a pH problem	Correct pH before adding anything
Yellow between the veins of older leaves	Magnesium or manganese shortage; common in coco and with soft or RO water	Check pH; consider Cal-Mag
Yellow edges turning brown	Potassium shortage or salt build-up	Check EC and runoff; flush if high
Whole plant pale, medium stays wet	Overwatering and poor aeration	Let it dry back; check roots
Pale, drooping plant, light dry pot	Underwatering	Water thoroughly; check the schedule
Yellow with spots, webbing or sticky residue	Pests	Inspect leaf undersides
Slow growth, leaves lose colour in cold	Cold stress	Steady the temperature

How can you tell overwatering from underwatering?

Both make leaves yellow and droop, which is why they get confused. Use weight and roots rather than the calendar. A pot that feels heavy days after watering, with a medium that stays dark and damp, is overwatered. A pot that feels light, with a dry medium pulling away from the edge, is underwatered. The RHS makes the same point: avoid both soggy and bone-dry conditions, and if in doubt, check the roots.

Overwatering is easy to do. It starves roots of oxygen and invites rot, and it is especially common in oversized pots, where a small root ball sits in a large mass of wet medium. See our pot sizing and overpotting guide. Coco and soft or RO water often run short of calcium and magnesium, so a Cal-Mag such as Canna Calmag Agent or Dutch Pro Calmag can help. In hard-water soil you usually do not need it; our guide to coco calcium and magnesium buffering explains why. For a fuller deficiency and lockout diagnosis, use our nutrient deficiency and lockout triage guide.

Light matters too. Plants too far from the light turn pale and stretch, and a plant that has outgrown its pot yellows and stalls. If you are rooting cuttings or raising seedlings, our propagation masterclass covers the early-stage problems.

What causes brown spots and crisp edges on leaves?

Brown spots and crisp edges often point to something fixable. The usual suspects are:

Overfeeding and salt build-up. Burnt tips and scorched edges mean too much dissolved feed. Measure runoff EC, then flush with clean, pH-adjusted water and restart at a weaker strength. Our deficiency and flushing guide has the full method; an additives range covers flush products.
Dry air. Low humidity makes leaves brittle at the margins. Check your readings against the stage targets in our humidity control guide, and VPD explained shows how temperature and humidity work together. Our Humidity Control collection has the kit.
Too much moisture and still air. Damp, stagnant air invites fungal and bacterial leaf spot. Spots with a yellow halo that spread are the classic sign. Remove affected leaves, sterilise your scissors between plants, and do not compost them. Improve airflow with correctly sized ventilation.
Heat or light scorch. Bleached or crisp patches on the leaves closest to the lamp mean it is too close or the tent is too hot. See our grow tent too hot guide.
Calcium tipburn. Stagnant air slows water movement through the plant, so calcium struggles to reach new growth and the tips brown. Better airflow helps as much as extra calcium.
Pests. Aphids, spider mites, whitefly and root feeders can all cause spotting and yellowing. Check the undersides of leaves with a magnifier. For identification and control, use our pest and disease prevention guide or the Pest & Disease Control collection.

You cannot manage dry or damp air by feel, so measure it. A Hygrometer Temperature & Humidity Sensor With Probe at canopy height shows both at once, and our monitors and controllers guide covers automating the response.

How do you spot and treat root rot?

Pull a small plant gently from its pot, or lift a hydro net pot, and look. Healthy roots are pale and firm. Rotting roots are dark brown, soft or slimy and smell unpleasant. The RHS describes roots that are "brown, rotted and soft" as a sign to cut back on watering.

	Healthy roots	Rotting roots
Colour	White to cream (tinted if your solution is dark)	Brown or black
Texture	Firm, springy	Soft, mushy, sloughing off
Smell	Fresh or earthy	Sour or rotten
Above ground	Steady growth	Yellowing, wilting even when wet

The main causes are poor aeration, stagnant water and a warm root zone. In recirculating hydro, keep the reservoir at 18-20 °C (64-68 °F) and act above 22 °C (72 °F), when rot-causing organisms become active. Below 18 °C (64 °F) roots slow and take up less. Our reservoir management guide covers the detail, and DWC and reservoir temperature explains how deep-water systems differ. In pots, rot usually means too much water, too little air or an oversized container.

If rot has set in:

Stop watering, or drain the reservoir. Do not add more feed to a stressed root zone.
Trim dead roots. Cut back brown, soft roots with clean scissors, leaving any pale, firm ones.
Start fresh. Replace the nutrient solution and clean the reservoir, pipes and net pots thoroughly. For pots, move the plant into fresh, free-draining medium.
Add oxygen. An air pump with an air stone keeps water moving and oxygenated.
Support recovery. An enzyme product such as Cannazym helps break down dead root matter, but it supports the root zone rather than curing rot. Our root-zone health guide covers beneficial microbes and prevention.

Prevention is easier than rescue. Keep water moving and cool, keep kit clean between runs, and water to need, not to routine.

Why do plants struggle more in autumn and winter?

Cold, dark weeks change the picture. Roots in a cold reservoir or cold medium slow down, plants use less water so pots stay wet, and heated rooms dry the air. Overwatering, cold stress and brown tips all become more likely. Keep the root zone at or above 18 °C (64 °F), water less often in low light, and watch humidity. Our winter grow room tips cover heating and condensation, and the Heaters and Humidity Control collections have the kit.

Which tools make troubleshooting easier?

Catching problems early beats rescuing plants. The three measurements that solve most mysteries are pH, EC and temperature with humidity. Browse our pH & EC Testers to choose a meter that suits your setup; a combined meter such as the Bluelab Combo Meter reads several values at once. Add a hygrometer and a small kit of clean scissors and cleaning supplies from Grow Room Tools & Accessories. In our experience at the Peterborough counter, water and pH are the cause far more often than a missing nutrient, which is why this guide checks them first.

FAQ

Why are my plant leaves turning yellow? Most often it is water, pH or feed. Check the roots and moisture first, then pH, then feeding. Yellowing on older leaves points to mobile nutrients such as nitrogen or magnesium. Yellowing on the newest leaves points to iron or a pH problem.

Should I cut off yellow or brown leaves? Remove leaves that are mostly dead or spotted with disease, using clean scissors, because they will not recover. Leave partly yellow leaves a little longer, as the plant may still draw nutrients back from them. Do not compost leaves with spreading spots.

Can plants recover from root rot? If caught early, yes. Where some roots are still pale and firm, trimming, fresh medium or solution, and extra oxygen give the plant a chance. If most of the roots are soft and dark, recovery is unlikely and it is usually better to start again from a healthy cutting or seedling.

How do I tell overwatering from underwatering? Check weight and roots. A heavy pot, damp medium and soft dark roots mean too much water. A light pot, dry medium and wilting that perks up after watering mean too little.

What pH should I check yellow leaves against? Use 6.0-6.8 for soil and 5.5-6.2 for coco and hydro, with most growers holding 5.8-6.0. Our pH and EC guide has the full table and calibration steps.