How to Water Seedlings Without Killing Them

Seedlings have tiny roots and almost no reserves, so they swing from thirsty to drowning very quickly. Getting watering right is the difference between sturdy transplants and trays of collapsed stems.

Bottom watering is best

Stand trays or cell packs in a shallow tray of water for 10–20 minutes, until the surface of the compost darkens, then lift them out to drain. Benefits:

• Seeds and tiny seedlings are not washed out or flattened.
• The surface stays drier, which discourages fungus gnats and damping off.
• Roots grow downward toward the moisture.

If you water from above, use a mister or a can with a very fine rose until seedlings have their first true leaves.

How often?

When the top 1 cm (½ in) of mix is dry and the tray feels noticeably lighter. That might be daily under strong grow lights or in a warm windowsill, or every two to three days in cool conditions. Check daily rather than watering by the calendar.

Signs you are overwatering

• Stems pinch and collapse at soil level (damping off).
• White fuzzy mould or green algae on the surface.
• Fungus gnats hovering around the trays.
• Pale, slow-growing seedlings in mix that is always dark and wet.

Signs of underwatering

• Seedlings flop but recover within an hour of watering.
• Mix shrinks from the edges of the cell.
• Leaf tips brown and crisp.

Preventing damping off

• Use fresh, sterile seed-starting mix; the seed starting mix calculator shows how much to buy.
• Sow thinly so air moves between seedlings.
• Run a small fan on low for a few hours a day.
• Use room-temperature water and clean trays.

Setting up for success

Good light and spacing make watering easier because sturdy seedlings use water steadily. Plan your trays with the seedling shelf calculator, and test old seed with the germination calculator so you sow the right amount and avoid overcrowded trays.