Why Are My Microgreens Getting Mould?

Mould is the most common problem in microgreens growing and the one that discourages the most beginners. The good news is that it is almost always preventable, and its causes are well understood. Most cases of mould in microgreens trays are caused by one or two specific mistakes that are easy to correct. Understanding what is actually happening in the tray — and distinguishing real mould from root hairs that look similar — is the first step to solving it.

Root Hairs vs. Mould — Know the Difference

Many new growers see white fuzzy growth on the stems of their microgreens and immediately assume it is mould. In the majority of cases it is root hairs — thin, white, hair-like projections that emerge from the base of the stem to absorb moisture from the medium. Root hairs are entirely normal and a sign of healthy, actively growing plants. The key difference: root hairs are organised, uniform and attached to the stem; they grow in a pattern. Mould is disorganised, grey, green or black, and typically has a musty, sour or earthy smell. If there is no smell and the white fuzz is on the stems rather than the medium surface, it is almost certainly root hairs.

The Main Causes of Real Mould

Top-watering is the single most common cause. When water lands on the foliage and stem surface, it creates exactly the damp, warm microclimate that mould fungi need to colonise. Switching to bottom-watering (the growing tray placed in a water reservoir for 10 to 15 minutes then removed) eliminates this problem entirely for most crops. Never water from above once the tray is in the light phase.

Over-sowing is the second major cause. Too many seeds in a tray create a dense, humid canopy where air cannot circulate at the medium surface. Dead or failed seeds trapped between viable seedlings decompose and become colonisation sites for mould. Reduce your seed density and the problem usually resolves itself on the next tray.

Poor airflow is the third cause. A completely enclosed, still growing space with no air movement encourages mould. You do not need a fan — normal passive airflow in a room is sufficient. If you are growing in a closet or tent, leave the door cracked or add a small USB fan on the lowest setting.

What to Do If Mould Appears

If mould appears on a tray that is otherwise looking healthy and near harvest, you can often still harvest the crop: cut above the mould line and rinse well. Mould at the base of the stems is surface-level and the green shoots above are unaffected. If the mould is widespread and the shoots smell unpleasant, discard the tray — do not eat a heavily moulded crop. Sanitise the tray with a dilute bleach or hydrogen peroxide solution before reusing.

Prevention Checklist

To prevent mould on every future tray: bottom-water only from day 4 onwards; sow at the correct density for your variety; ensure passive airflow in the growing room; keep trays in a location that is warm but not damp; and clean trays thoroughly between crops. These five practices eliminate mould in the vast majority of home growing situations.