When to Feed Cannabis Seedlings (and How Much)
Cannabis seeds and young seedlings need almost no fertilizer. The seed carries its own food for the first 1–2 weeks, and a good potting soil feeds the plant for another 2–3 weeks after that. In soil, start with a quarter-strength dose only when the seedling has 3–4 sets of true leaves; in coco, rockwool and hydro, feed a very weak solution from the start. Feeding too much, too early is one of the most common ways to burn a seedling.
What a seed already contains
Inside the shell, the embryo sits next to a store of food: starches, oils and proteins. This reserve fuels germination, the stem and the two round seed leaves (cotyledons). The cotyledons themselves are a pantry: in the first week or two the seedling draws on them while its first serrated leaves, the true leaves, develop.
So in the first days the seedling doesn't need you to feed it. It needs a light substrate, the right moisture (see watering seedlings) and good light.
The substrate decides when to start feeding
| Substrate | Nutrients inside | When to start feeding | Starting strength |
|---|---|---|---|
| Light seedling mix | Very low | After 1–2 weeks, with 2–3 true leaf sets | ¼ dose (EC 0.4–0.6) |
| Pre-fertilized potting soil | Enough for 3–4 weeks | After 3–4 weeks, or when lower leaves start to pale | ¼–½ dose |
| Coco coir | None | From the first watering | EC 0.4–0.6 with Cal-Mag |
| Rockwool, hydro | None | From the first watering | EC 0.4–0.6 |
| Living / organic soil | Released slowly by soil life | Often not needed for the first month | Water only |
A rich "hot" soil, made for big plants in flowering, is too strong for a seedling. If that is all you have, mix it with a third of perlite or seedling mix for the first pot.
When to start feeding: the signs
- The seed leaves turn yellow and fall off once there are a few sets of true leaves. That's normal: the seedling has used them up.
- The lowest true leaves pale evenly from light green to yellow-green, while the top keeps growing: the plant is asking for nitrogen.
- Growth slows although light, temperature and watering are right.
If the seedling is dark green, healthy and growing every day, it doesn't need anything yet.
What to feed at the seedling stage
- Seedling or "grow" nutrients at reduced strength: start at 25% of the dose on the label and increase slowly over 2–3 weeks.
- Cal-Mag in coco and with RO water: coco binds calcium and magnesium, and RO water contains none.
- Avoid flowering boosters and strong root stimulators with high doses of phosphorus or potassium: they are for later stages.
- Organic options: worm castings mixed into the soil, or a weak worm-castings tea, feed gently with little risk of burn.
Tip: an EC meter costs little and removes the guesswork. Measure the water first, then the solution: the difference is what you are adding.
pH and EC: the two numbers that matter
If the pH is wrong, the roots can't take up nutrients even when they are there. In soil, normal tap water is usually fine for the first weeks; in coco, rockwool and hydro, measure and adjust the pH every time.
| Stage | EC of the feed (mS/cm) | pH soil | pH coco | pH hydro |
|---|---|---|---|---|
| Seedling, first true leaves | 0.4–0.6 (or water only in soil) | 6.0–6.8 | 5.8–6.3 | 5.5–6.2 |
| 3–4 sets of true leaves | 0.6–0.9 | 6.0–6.8 | 5.8–6.3 | 5.5–6.2 |
| Early vegetative growth | 0.9–1.2 | 6.0–6.8 | 5.8–6.3 | 5.5–6.2 |
The EC values include what is already in your water. If your tap water is 0.4, a feed of 0.6 means you are adding very little — which, for a seedling, is fine.
Signs of overfeeding (nutrient burn)
Seedlings react fast: the symptoms show within 1–3 days.
- Burnt tips: the tips of the leaves turn yellow, then brown and crispy.
- Very dark green leaves with tips curling down ("claws"): too much nitrogen.
- Growth stops and new leaves come out small and twisted.
What to do
- Stop feeding.
- In soil, water with plain water (pH adjusted if needed) until 20–30% runs out of the bottom, then let the pot get light again.
- In coco and hydro, give plain water with a little Cal-Mag, or a solution at half the previous EC.
- Restart at a lower dose after 5–7 days, when new leaves look healthy. Burnt leaves don't recover; watch the new growth.
Underfeeding: less common, easy to fix
A seedling that is truly hungry turns evenly pale from the bottom up and grows slowly. If light and temperature are fine and the plant has passed the 3–4 leaf sets, start a light feed. A slightly hungry seedling catches up in a few days; a burnt one loses a week or more.
Organic or mineral nutrients?
For beginners, organic feeding is more forgiving: nutrients are released slowly by soil life, so it is hard to overdo it. Mineral (synthetic) nutrients act immediately and need measuring, but give precise control and are the standard choice in coco and hydro. Both work; what matters is starting low and changing one thing at a time.
FAQ
When should I start feeding cannabis seedlings?
In pre-fertilized soil, after about 3 weeks, when the seedling has 3–4 sets of true leaves. In coco and hydro, from the first watering, at a very low strength.
Should I fertilize seeds during germination?
No. Germinate with plain water. Nutrients in the germination water or substrate can burn the young root.
My seed leaves are turning yellow: is it a deficiency?
Not if the true leaves above are green and growing. Cotyledons yellow and drop off naturally after 2–3 weeks.
Can I use tomato or houseplant fertilizer?
It works in a pinch for the vegetative stage, at a quarter of the dose; cannabis nutrients are simply easier to dose for each stage.
Less is more at the start. Once your seedlings have filled their first pot, read how to transplant them without stress, and how to germinate seeds for your next grow.