The tray looks fine — not dry, not flooded. Three days later the stems have collapsed, the root is black, and there's a smell of rot. The problem is not the amount of water — it's when and how it reached the substrate. The same volume of water applied from above onto seedlings can kill them overnight, while the same water delivered from below through a porous mat gives even moisture without a single drop on a stem.
Quick Glossary
- Bottom watering — water is supplied beneath the tray and rises by capillary action through the substrate from the bottom up; leaves and stems stay dry
- Top watering — water is applied to the substrate surface or directly onto plants from above; used mainly before germination
- Misting — water is delivered as fine droplets or vapour; covers the surface evenly without mechanical damage to seedlings
Why the Watering Method Depends on Growth Stage, Not Crop
Microgreens have three fundamentally different stages with different needs: germination under a cover, sprouting and growth after uncovering, and the final phase before harvest. The method that is correct at stage one is destructive at stage two.
During germination under a cover, humidity in the enclosed space is already high. Top watering is minimal or absent — the initial moisture in the substrate is sufficient for germination in most crops. Excess is the first source of mould before the plant even reaches light.
After the cover is removed, microgreens switch to bottom watering or misting. Stems are already formed — a droplet on a stem or leaf with poor ventilation creates conditions for fungal and bacterial disease.
Bottom Watering: When and How
Bottom watering is the primary method for most crops after germination. Water is poured into an outer tray beneath a perforated or porous inner tray, rises by capillary action through the substrate from the bottom up, and reaches the root from below. The leaf mass stays completely dry.
Two conditions are required for bottom watering to work: a substrate with sufficient capillarity, and contact between the substrate and water from below. Jute mat, coco mat, and agro-fleece draw water well. Foam substrates and some compressed coco slabs may have poor capillary draw, resulting in uneven moisture distribution through the substrate height.
Volume: fill the outer tray to 0.5–1 cm and allow the substrate to absorb. If the water is gone within 20–30 minutes, top up. If water remains — drain the excess. Do not leave standing water under the substrate continuously. Constant contact between the bottom and water without drying out causes root asphyxiation from below.
Top Watering: Only Before and Immediately After Sowing
Top watering has a clearly limited role: moistening the substrate before sowing and, if needed, immediately after sowing so that seeds contact a moist surface. Once white rootlets appear and germination begins, top watering stops.
If moisture must be added from above (substrate dried unevenly, the top layer is lagging) — use a fine-mist sprayer, not a watering can or cup. A drop from a watering can weighs thousands of times more than a seedling — it causes mechanical damage, stem lodging, and delivers spores into damaged tissue.
Misting: Where It's Appropriate and Where It's Excessive
Fine-mist application is useful in two situations: during germination of crops that need humid air above the surface (basil, some amaranths), and during growth in rooms with very low humidity where the substrate surface dries faster than the bottom.
The problem with misting is that it wets everything — including stems and leaves. With insufficient ventilation this is a direct route to microgreen disease: a droplet on a leaf at 20–24°C with no air movement is an ideal environment for Botrytis and bacterial necrosis.
Misting as the sole watering method after germination is only justified with active ventilation and a short cycle (3–5 days to harvest). For crops with longer cycles or in low-airflow conditions — switch to bottom watering.
Three Mistakes That Cost the Most
Leaving water under the tray continuously. "Let it absorb as much as it needs" — logic that leads to a permanently waterlogged bottom substrate layer and root rot. Bottom watering is a cycle: fill, wait for absorption, drain the excess.
Top watering after cotyledons have emerged. Even a single top watering at the seedling stage using a coarse-droplet sprayer breaks stems and leaves droplets in leaf axils. Thin-stemmed crops — radish, mustard, amaranth — are especially vulnerable.
Not accounting for substrate differences. Coco mat and foam substrate require different volumes and frequencies — the former holds more moisture and needs bottom refilling less often; the latter dries faster and more unevenly.
How to Know Watering Is Configured Correctly
- Substrate is evenly moist from bottom to top but not wet to the touch at the surface
- Stems stand upright with no signs of lodging
- Root is white and odourless when the tray is lifted
- No droplets on leaves