Iron Deficiency in Cannabis Plants

Iron deficiency is a common problem in home cannabis grows, and it is easy to spot once you know where to look. Kind Seed Co covers the signs, the usual causes and the steps that get your plants growing green again.

Signs of iron deficiency

New leaves turn yellow

The first sign usually shows on the newest leaves at the top of the plant. Yellowing starts at the tips and edges and spreads inward while the veins stay green. Iron helps the plant make chlorophyll, so less iron means paler leaves.

Green veins on yellow leaves

This pattern of yellow between green veins is called interveinal chlorosis. Magnesium shortages can look similar, but they start on older, lower leaves. When the pattern starts on fresh growth at the top, iron is the more likely cause. Our guide to magnesium deficiency in cannabis plants shows the difference.

Slow growth

If the shortage goes on for a long time, new leaves come in smaller and more twisted than usual. With less chlorophyll the plant makes less energy, and growth slows down.

What causes iron deficiency

pH out of range

Cannabis takes up iron best at a pH of about 6.0 to 6.5 in soil and 5.5 to 6.0 in hydro. When pH climbs too high, roots cannot absorb iron even if there is plenty in the soil or water. That makes pH the first thing to check. See our guide to understanding pH and how it affects cannabis plants.

Overwatering and poor drainage

Waterlogged soil cuts off oxygen to the roots, and roots short on oxygen move nutrients poorly, iron included. Our guide on overwatering cannabis explains how to spot and stop it.

Soil makeup

Some soils simply hold little iron. In others, high levels of another nutrient get in the way. Too much phosphorus, for example, can interfere with iron uptake.

How to fix iron deficiency

• Correct the pH. Bring your soil or hydro solution back into range so the roots can take up iron again. This is the first step.
• Use an iron chelate or iron fertilizer. These supply iron in a form plants absorb easily. Use them as a short term boost while you fix pH and other nutrient levels.
• Improve drainage and air flow in the soil. Mixing perlite or sand into the soil helps it drain and keeps it from getting waterlogged.

How to prevent it

• Add iron rich amendments. Peat moss or well rotted manure help keep a steady supply of iron in the soil.
• Keep checking pH. Regular checks matter most in hydro, where pH can drift quickly.
• Water and feed with care. Avoid overwatering and stick to a balanced feeding schedule.

Iron is one of several nutrients that show up first in the leaves. Our guide to spotting common deficiencies by their leaves helps you rule out the rest.