How to Fertilize Indoor Herbs and Vegetables (Without Guessing)

Stalled growth a month or two into an indoor garden usually gets blamed on light. Often the real cause is simpler: the potting mix that came with the bag has run out of food, and nobody’s fed the plant since. Feeding is one of the least glamorous parts of growing indoors, and one of the most commonly skipped — this is what the three numbers on the bottle mean, how much to use, and how to tell you’ve gotten it wrong in either direction.

What the three numbers actually mean

Every fertilizer label shows three numbers separated by dashes — NPK — and they’re not arbitrary. Each one is the percentage, by weight, of one nutrient in the bottle or bag.

• N (Nitrogen) — drives leafy, green growth. The nutrient basil, lettuce and other leaf crops care about most.
• P (Phosphorus) — supports root development and, later, flowering and fruit set. Matters more once a plant starts flowering than during early growth.
• K (Potassium) — general vigor, disease resistance and water regulation within the plant. Doesn’t target one visible feature the way N and P do, but a deficiency shows up as weak, stress-prone growth overall.

A “balanced” fertilizer has roughly equal numbers (20-20-20, 10-10-10). A “bloom” or “tomato” formula is deliberately unbalanced toward P and K (10-30-20 is a common pattern) because a flowering or fruiting plant’s needs shift partway through its life.

Why potting mix stops being enough

Bagged potting mix ships with a modest starter charge of nutrients, and that’s genuinely enough for the first four to six weeks — which is exactly why a new indoor garden can look great at first and then quietly stall. Once that starter charge is used up, the plant is entirely dependent on what you feed it. This is the same four-to-six-week window mentioned in the beginner setup checklist; this article is the detail behind that one line. For what’s actually in that starter charge — and how mix composition and container size affect how fast it runs out — see choosing containers and potting mix for an indoor vegetable garden.

Stalled growth after roughly a month, on a plant that’s getting enough light and isn’t sitting in soggy soil, is the single most common sign that feeding — not light — is the actual bottleneck.

How much, and how often, by crop type

The half-strength, every-two-weeks default holds for most of what’s grown on this site, but it isn’t the same for every crop:

Herbs and leafy greens (basil, mint, lettuce, spinach, arugula): light feeders. Half-strength balanced liquid feed every two weeks from about week four is genuinely enough — see the feeding sections in how to grow basil indoors and the lettuce and spinach guides for the same guidance applied crop by crop. More isn’t better here; excess nitrogen on a leaf crop mostly just grows faster leaves with slightly less flavor.

Fruiting vegetables (dwarf tomatoes, peppers): heavier feeders with a real two-stage need. A balanced, nitrogen-forward feed while the plant is still building leaves and stems, then a switch to a lower-nitrogen, higher phosphorus-and-potassium feed once flowering starts — too much nitrogen at that point produces a lush plant with disappointing fruit set. The full breakdown is in growing dwarf tomatoes indoors.

Microgreens: don’t feed them at all. They’re harvested at 7–14 days, entirely on the energy reserve packed into the seed itself — by the time a nutrient deficiency could show up, the tray is already on a plate. See the growing medium comparison for why soil’s small nutrient contribution barely matters at that timescale.

Three real liquid fertilizers, compared

Product	NPK	Type	Best for	Where to buy
Dyna-Gro Foliage-Pro Complete micronutrients	9-3-6	Synthetic, urea-free	An all-purpose leaf-forward feed with a full micronutrient package built in — no separate cal-mag needed.	Search on Amazon (affiliate link, opens in a new tab)
Jack's Classic All Purpose Balanced 20-20-20	20-20-20	Synthetic, water-soluble powder	A true balanced ratio at high enough strength that half-strength dosing goes a long way — the closest match to the site default above.	Search on Amazon (affiliate link, opens in a new tab)
Espoma Indoor! Organic, OMRI listed	2-2-2	Organic (kelp, humic acid, microbes)	Gentle enough to feed every watering rather than every two weeks — a reasonable pick if you'd rather not calculate a half-strength dilution at all.	Search on Amazon (affiliate link, opens in a new tab)

NPK ratios and formulation details from each manufacturer's own product listing, checked September 2026: Dyna-Gro Foliage-Pro 9-3-6 (dyna-gro.com), Jack's Classic All Purpose 20-20-20 (jrpeters.com), Espoma Indoor! 2-2-2 (espoma.com, OMRI Listed). 'Search on Amazon' links use our affiliate tag; we have not confirmed these are the cheapest current listing for each product.

If you want one bottle that covers everything with no extra additives: Dyna-Gro Foliage-Pro’s built-in micronutrient package is the simplest one-product answer for a mixed windowsill of herbs and greens.

If you want the ratio that matches the site’s own half-strength default most closely: Jack’s Classic 20-20-20 is a true 1:1:1 balanced feed at a strength where halving the label dose is straightforward.

If the plants are herbs you’re eating fresh and you’d rather not think about dilution math: Espoma Indoor! is OMRI listed for organic use, labeled safe for culinary herbs at any point, and its label rate (roughly every two to four weeks) is already close to a gentle, low-risk schedule.

Organic vs synthetic: the honest short version

Synthetic fertilizers (Dyna-Gro, Jack’s Classic) deliver nutrients in a form roots absorb immediately — predictable, fast-acting, easy to dose precisely. Organic fertilizers (Espoma, fish emulsion, worm castings) release more slowly as microbes break them down, which makes them more forgiving of a slightly heavy hand but slower to correct an actual deficiency. Neither is “better” in general; a synthetic feed is easier to reason about as a beginner because the numbers on the label are exactly what the plant gets, when it gets it.

Signs you’re underfeeding vs overfeeding

What you see	Likely cause
Older, lower leaves pale or yellowing; growth has stalled	Underfeeding (nitrogen)
Weeks of flat growth despite good light and correct watering	Underfeeding (exhausted potting mix)
Crispy, brown leaf tips and margins on newer growth	Overfeeding (fertilizer burn)
A white or crusty film on the soil surface or pot rim	Salt buildup from overfeeding
Wilting despite visibly moist soil, after a recent heavy feed	Overfeeding (roots scorched, can't take up water)
Lush leaves, few or no flowers on a fruiting plant	Too much nitrogen relative to phosphorus/potassium

Before blaming feeding specifically, rule out the two more common causes on this site: light and watering problems produce overlapping symptoms — yellowing leaves in particular are the single most ambiguous sign in indoor growing, since underwatering, overwatering, cold damage and underfeeding can all produce it. Check soil moisture and light first; if both check out and growth is still off, feeding is the next thing to look at. It’s also worth ruling out pests — mite or aphid damage can look similar to a nutrient problem at a glance.

Fixing fertilizer burn: stop feeding immediately, and flush the pot with plain water — enough to run through the drainage hole several times — to wash excess salts out of the soil. Resume feeding at half the dose you were using, once new growth looks normal again.

Fixing underfeeding: resume or start a normal feeding schedule. Recovery is faster than burn recovery — usually visible within one to two feeds.

Feeding is different in hydroponics

Everything above assumes soil, which buffers a dosing mistake to some degree. In a hydroponic system there’s no soil to buffer anything — the nutrient solution is the plant’s entire diet, so using a soil fertilizer or guessing at concentration is a much bigger risk than it is in a pot. Use the specific nutrient solution your system specifies, mixed to the strength it calls for, not a generic liquid feed. See hydroponics vs soil for indoor growing for why that tradeoff exists, and the hydroponic cost breakdown for what nutrient solution typically costs to keep stocked.

A simple default schedule

• Weeks 1–4: don’t feed. The potting mix is still doing the work.
• Week 4 onward, herbs and leafy greens: half-strength balanced liquid feed every two weeks.
• Week 4 onward, fruiting vegetables: the same, switching to a higher-P, higher-K feed once flowering starts.
• Any time growth stalls with good light and correct watering: that’s the cue to check whether you’ve actually been feeding on schedule, not a sign to add more light.
• Any time you see crispy tips or a white crust on the soil: stop, flush, and come back at half the dose.

Feeding is the part of indoor growing with the least visible daily feedback — you don’t see it the way you see a droopy leaf from underwatering — which is exactly why it’s the step most beginners quietly skip. A recurring reminder on the same timer you’re already using for your grow light is a reasonable way to not forget it.