A Simple Fertilising Schedule for Container Vegetables
Container plants exhaust their nutrient supply faster than garden plants because every watering leaches something out of the drainage holes. Feeding is not optional. But the most common mistake is feeding the wrong thing at the wrong time.
Reading the three numbers
Every fertiliser label shows three numbers, N-P-K: nitrogen, phosphorus, potassium, as a percentage by weight.
- Nitrogen (N) drives leaf and stem growth. Leafy crops want a lot of it.
- Phosphorus (P) supports roots, flowers and fruit set.
- Potassium (K) governs fruit quality, disease resistance and water regulation.
| Crop type | Suggested ratio | Example | Feeding pattern |
|---|---|---|---|
| Leafy greens, herbs | High N | 20-5-10 or seaweed feed | Weekly, diluted |
| Fruiting crops, vegetative stage | Balanced | 10-10-10 | Weekly until first flowers |
| Fruiting crops, flowering and fruiting | High K | 5-10-20 or tomato feed | Twice weekly, diluted |
| Root crops | Low N, higher P and K | 5-10-15 | Once at root swell, then fortnightly |
A season in four phases
Phase 1 — planting (weeks 0–4)
Fresh potting mix plus a slow-release granular feed incorporated at planting usually covers this period. Only liquid-feed if the mix was already partly used. Seedlings should get nothing for their first two weeks; their seed leaves carry them and fertiliser at this stage burns roots.
Phase 2 — vegetative growth (weeks 4–8)
Balanced liquid feed, weekly, at half the label rate. Half rate applied often beats full rate applied occasionally — it keeps a steady supply and avoids salt spikes. If growth is pale and slow, this is the nitrogen phase; a seaweed or fish emulsion feed greens plants up within a week.
Phase 3 — flowering and fruiting (weeks 8 onwards)
Switch to a high-potassium feed for tomatoes, peppers, cucumber, courgette and aubergine. Feed twice weekly once fruit is setting. Leafy crops stay on the nitrogen feed throughout — unless they have started to bolt, at which point feeding only accelerates it.
Phase 4 — end of season
Stop feeding fruiting crops about three weeks before you expect to clear them; there is no point pushing growth you will not harvest. Perennial herbs get one last light feed six weeks before the first frost, then nothing until spring, so new growth hardens rather than staying soft.
Diluted feed applied frequently is the safer pattern in containers. A single strong dose on a hot day is the classic cause of fertiliser burn: crispy leaf edges, blackened root tips, and a plant that was healthy yesterday.
Organic versus synthetic
Synthetic feeds are precisely formulated, immediate and cheap. Organic options — seaweed extract, fish emulsion, compost tea, worm castings — release slowly, improve mix structure and are harder to overapply. For containers, a hybrid approach works well: slow-release granules or compost at planting for baseline nutrition, a liquid feed weekly for control. Whatever you pick, a plant showing deficiency needs a fast-acting liquid feed, not a slow amendment.
Reading deficiency symptoms
| Symptom | Likely cause |
|---|---|
| Uniform pale green, older leaves first | Nitrogen deficiency — feed |
| Purple tint on leaves and stems | Phosphorus, often triggered by cold |
| Brown scorched leaf edges | Potassium shortage, or fertiliser burn |
| Yellow leaves, green veins | Iron or magnesium, usually from pH drift |
| Blossom end rot on tomatoes | Calcium uptake failure from uneven watering, not lack of calcium |
That last row matters: blossom end rot is almost always a watering inconsistency problem, and adding calcium rarely fixes it. Even watering does.
Salt build-up
Repeated feeding leaves salts in the mix, visible as a white crust on the surface or on terracotta walls. Flush containers thoroughly once a month in peak season: run water through for two minutes, let it drain completely, then resume normal feeding. If you are reusing mix from last year, flush it before planting — accumulated salts are a common cause of unexplained poor germination.
Common questions
Can I over-fertilise?
Yes, and it is common. Symptoms are crispy brown leaf edges and sudden wilting despite wet mix. Flush the container with plain water for two minutes and resume at half the previous rate.
Should I feed seedlings straight away?
No. Their seed leaves supply everything for the first two weeks, and fertiliser at this stage burns young roots. Begin feeding once the first true leaves are established.
Is homemade compost tea enough on its own?
Not for fruiting crops. It is a good supplement and improves mix biology, but tomatoes and peppers need a formulated high-potassium feed while setting fruit.
Do herbs need feeding?
Soft leafy herbs benefit from a fortnightly dilute feed. Mediterranean herbs such as rosemary, thyme and oregano need almost none and actually grow more strongly and more aromatically in lean mix.
What is the difference between liquid and granular feed?
Granular feeds release slowly over weeks or months and suit a baseline at planting time. Liquid feeds act within days and give you control during the season. Container growing generally needs both: a slow-release base plus a weekly liquid top-up.