Yellow microgreens are not always a failure. Seedlings kept in darkness naturally emerge pale because they have not yet produced much chlorophyll. The important question is whether they green up after receiving adequate light.
Normal yellowing after germination or blackout
Crops coming out from under a cover can look yellow, cream or light green. Once exposed to appropriate light, healthy seedlings often begin greening within hours and continue deepening in color over the next day or two.
Weak or uneven light
If seedlings remain pale, stretched or lean strongly toward one direction, light may be too weak or uneven. A grow light that is too far from the canopy can produce the same problem as a dim window.
Excess moisture and root stress
Roots need oxygen as well as water. Constantly saturated media can stress the crop, reduce root function and increase disease pressure. Keep media moist but allow drainage, especially after roots are established.
Nutrient issues are less common in very short crops
Many fast microgreens complete their first harvest using reserves stored in the seed plus a modest growing medium. Longer or slower crops may show nutrient limitations, but do not jump straight to fertilizer before checking light, moisture and roots.
A quick diagnostic order
- Ask whether the tray just came out of darkness.
- Check whether the whole canopy receives even light.
- Inspect media moisture and drainage.
- Look for stem collapse, odor or spreading fuzzy growth.
- Compare the problem with another tray grown under the same conditions.
Related: Do Microgreens Need Grow Lights? and Do Microgreens Need Fertilizer?.
Sources: University of Maryland Extension; Penn State Extension.
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