Seedling Fertilizer – When to Feed and How Much
Feeding seedlings is mostly about restraint: the right moment, a weak mix, and the willingness to wait until the plant has roots to take it up.
TL;DR
- No feed until true leaves and one potting on.
- Half label strength, weekly to fortnightly, into damp mix.
- Dark glossy leaves and crisp edges mean too much, not too little.
Why the seed mix has almost nothing in it
That is not a defect. A seed starting mix is designed to be light, free-draining and clean, and a nutrient-rich mix in a small cell is a recipe for burnt roots and algae on the surface. The seedling's first weeks are powered by the seed's own reserves; by the time those run out, the plant has true leaves and a root system that can use what you give it.
The practical marker is the second set of leaves and one move into a larger pot. Before that, plain water. After that, a weak feed once a week if the mix has no compost in it.
How much, exactly
Mix at half the rate on the bottle. If the label says 10 ml per liter, use 5. Water the tray first, then apply the feed to damp mix, so the salts dissolve into moisture rather than sitting against dry roots.
If you potted on into a mix that already contains compost or slow-release granules, stop and think before adding liquid feed: that mix is already feeding the plant, and a weekly dose on top is how seedlings get soft and dark. Judge by growth, not by calendar — a plant putting out a new leaf a week is being fed enough.
The two ways this goes wrong
Too early. Feed before true leaves and the plant has no root surface to absorb it; the salts stay in the cell and burn what root there is. The seedling yellows, and the instinct is to feed more.
Too strong. Crisp brown leaf edges in a small cell, with growth that had been fine going soft and dark. Flush with plain water, skip feeding for two weeks, and go weaker next time. If the plant is already tall and soft going into planting out, it will need a longer hardening off — soft growth burns faster than stocky growth.
Once seedlings are in the garden, the rules change and the doses go up; indoors, everything is scaled to a cell the size of a thumb.
Tools for this job
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Balanced liquid plant food
Mix at half label strength for seedlings; a small bottle lasts a season.
Measuring syringe for feed
Half-strength feeding is only repeatable if you can measure small volumes.
Related guides
Frequently asked questions
Do seedlings need fertilizer?
Not until they have true leaves and have been potted on once. A seed carries its own reserves and a seed starting mix is deliberately low in nutrients, so feeding earlier pushes soft growth the plant cannot support.
How strong should the feed be?
Half the strength on the label, or weaker. Seedlings have small root systems and a fraction of the soil volume of a garden plant; full-strength feed in a small cell is a salt hit rather than a meal.
How often should I feed?
Once a week to once a fortnight at half strength, and only after potting on into a mix that has already been fed. If the mix contains compost or slow-release granules, you can skip liquid feed for the first few weeks entirely.
What NPK ratio suits seedlings?
Something balanced and mild — a general liquid feed around 3-1-2 or 2-1-2 diluted to half strength, or fish emulsion. High-nitrogen lawn feed is the wrong tool: it produces tall, soft, dark growth that collapses outdoors.
What does overfeeding look like?
Leaves that are unusually dark and glossy, rapid soft growth, then crisp brown edges as salts build up. In a small cell the fix is to flush the pot with plain water and stop feeding for a fortnight.
Do I feed before or after watering?
Water first, then feed into already-damp mix. Feeding dry mix concentrates salts at the root surface and can burn the finest roots before the plant ever shows a symptom.