Fertilizing Plants — When, With What, and How Not to Overfeed

Fertilizing isn’t “feeding just in case” — it’s replacing only what the plant has actually used up. Too much is as harmful as too little, and overfeeding is far easier than underfeeding. This guide shows which nutrient does what, when to apply it, and how to tell when you’ve overdone it.

NPK — three numbers on every package

Every fertilizer carries three numbers, e.g. “7-3-6”. These are the amounts of the three macronutrients:

• Nitrogen (N) — leaves and shoots. Drives green growth. Too much nitrogen produces lush foliage at the expense of flowers and fruit, and makes the tissue soft and prone to disease and to aphids.
• Phosphorus (P) — roots and flowering. Supports root development and the setting of buds and flowers. “Bloom” fertilizers have a raised phosphorus share.
• Potassium (K) — fruit and resilience. Governs fruit quality, water regulation, and resistance to frost and drought. A fruiting plant like the tomato or the highbush blueberry needs it most during the fruiting phase.

Rule of thumb: leafy plants and lawns like a nitrogen-heavy feed, flowering plants such as the hybrid tea rose want more phosphorus and potassium, and fruiting vegetables want potassium above all.

Mineral or organic fertilizer?

Both are good — they mainly differ in speed.

• Mineral (synthetic) feeds act fast and precisely; you dose a known composition. But they make it easy to overfeed and to salt up the soil.
• Organic feeds (compost, well-rotted manure, meal-based fertilizers) release nutrients slowly, improve soil structure and biology, and make burning the roots practically impossible. The downside is a slower, less predictable effect.

For most gardens, the best approach is an organic base (compost once or twice a year) plus a mineral top-up at the peak of the season, when demand is highest.

When to feed — the growing season, not the calendar

A plant takes up nutrients only while it’s growing. So:

• Spring–summer (growing season): the only time for heavy feeding. Start when new shoots appear, and repeat every 2–4 weeks with a liquid feed or every 6–8 weeks with a slow-release fertilizer.
• Late summer: ease off the nitrogen. Fresh, soft growth won’t harden off before winter and will be killed by frost.
• Autumn–winter: stop feeding dormant plants. Feeding a sleeping plant is the easiest way to salt up the medium and damage the roots. The exception is houseplants that keep growing through winter — feed those rarely and weakly.

The timing for individual plant groups is gathered in the gardener’s calendar.

How to dose — less is safer

The single most important rule in this guide: with mineral fertilizers, stick to the lower end of the dose on the package, and when in doubt, give less. A deficiency is easily corrected with the next feed; an excess has to be flushed out first.

• Water the plant before applying a liquid feed — fertilizing dry soil burns the roots.
• Don’t crank up the concentration “by eye” hoping for a faster effect. Plants won’t absorb more than they need; the rest stays in the soil as salt.
• Keep freshly transplanted and sick plants off fertilizer — damaged roots can’t take up food.

Too much or too little? Reading the symptoms

Overfeeding (excess mineral salt):

• scorched, brown leaf edges and tips — the classic sign of salt build-up;
• a white or yellowish salt crust on the soil surface and around the pot rim;
• sudden wilting despite moist soil (the salt “pulls” water out of the roots).

The fix: flush the medium generously with clean water (several pot volumes); in the ground, stop feeding and simply water.

Deficiency:

• paling, yellowing leaves with still-green veins is chlorosis — most often a shortage of iron or nitrogen. Its causes and treatment are covered in the separate entry on iron chlorosis;
• weak growth, small leaves, and no flowering despite a good spot.

Note: in acid-loving plants, chlorosis usually comes not from a lack of iron in the soil but from the wrong pH, which blocks its uptake — so check your soil pH before reaching for fertilizer.

Plants with special needs

• Acid-lovers — the bigleaf hydrangea, the rhododendron, blueberry, and azaleas — need acidifying fertilizers. Ordinary all-purpose feed raises the pH and triggers chlorosis in them. In hydrangeas, pH and aluminium availability also decide the flower colour (acidic soil = blue, alkaline = pink).
• Orchids — the moth orchid is fed with a special, heavily diluted orchid fertilizer, only during the growing season and only onto moist roots. Ordinary houseplant feed at full strength will scorch them quickly.
• Herbs and leafy plants — moderate nitrogen; overfed, they lose their aroma and turn watery.

Fertilizing works best when it answers the plant’s actual need rather than following a calendar routine. Watch the new growth, leaf colour, and flowering — they tell you whether to feed more or hold back.