The fertiliser shelf at a garden centre implies that every plant needs its own bottle, and that more feeding means more flowers. Neither is true. Most garden plants in reasonable soil need little or no feeding at all, and the ones that do — mainly plants in containers and heavy-cropping vegetables — need it at particular times, not constantly. Here is what the numbers on the pack mean and when they actually matter.
N-P-K: the three numbers explained
Every fertiliser sold in the UK states its content of the three major nutrients as an N:P:K ratio — say 7-7-7 or 4-3-8:
- N — nitrogen drives leafy, green growth. Lawns, brassicas and foliage plants want plenty; too much gives lush, soft, floppy growth that aphids adore and frost ruins, often at the expense of flowers.
- P — phosphorus supports root development and energy transfer. Genuine phosphorus deficiency is uncommon in UK garden soils — many are already well supplied — which is why "root-booster" claims deserve scepticism.
- K — potassium (potash) promotes flowering, fruiting and general hardiness. High-potash feeds are the logic behind tomato food, and behind autumn lawn feeds that toughen grass for winter.
A crude but serviceable memory aid: N for leaves, P for roots, K for flowers and fruit. Fertilisers also contain trace elements — magnesium, iron, manganese and others — which matter mainly in containers and on chalky soils, where iron and manganese get locked up and acid-loving plants turn yellow between the leaf veins.
General feeds and what they're for
- Growmore (7-7-7): the classic balanced synthetic granule. Cheap and even-handed; a reasonable spring dressing for vegetable beds. It gives no organic matter, just nutrients.
- Blood, fish and bone: the traditional organic equivalent, roughly balanced with slightly more phosphorus, releasing over weeks as soil organisms break it down. Good general spring feed around fruit, roses and vegetables. Foxes and dogs sometimes dig for it — work it into the surface.
- Chicken manure pellets: organic, relatively high in nitrogen. Useful for leafy crops and as a general border feed; easy to overdo on flowering plants.
- Liquid seaweed: low in the major nutrients but rich in trace elements. Best regarded as a tonic and trace-element top-up rather than a main feed — a useful supplement, not a substitute.
Specific feeds and when they're worth it
- Tomato feed (high potash, roughly 4-3-8 with magnesium): the most useful bottle in the shed, and not just for tomatoes (Solanum lycopersicum). Use it weekly on any container plant grown for flowers or fruit once flowering starts — pelargoniums, fuchsias, chillies, dahlias, hanging baskets.
- Ericaceous feed: formulated for lime-hating plants — rhododendrons, camellias, blueberries (Vaccinium), pieris — with nutrients in forms available at low pH and added iron. Essential for these plants in pots; useful in the ground on neutral or chalky soils where their leaves yellow.
- Rose feed: balanced-to-high potash with magnesium. Genuinely suits roses, applied in March and again after the first flush in June or July — though a generic high-potash feed does much the same job.
- Lawn feeds: spring formulations are high-nitrogen; autumn ones low-nitrogen, high-potash. The seasonal distinction is real — do not use spring feed in October.
- Controlled-release granules (coated prills lasting up to six months): mixed into container compost at planting, they remove the summer feeding chore for a price. Release depends on warmth and moisture, so they under-deliver in a cold wet summer.
Organic versus synthetic, honestly
Synthetic fertilisers are concentrated, precise, fast-acting and cheap per unit of nutrient. They feed the plant directly but do nothing for soil structure or soil life, they are energy-intensive to manufacture (nitrogen especially), and their solubility makes them easy to over-apply, with surplus washing into watercourses.
Organic fertilisers (blood fish and bone, manures, pellets, seaweed) release slowly as soil organisms process them, so they are harder to overdose and they feed the soil food web as well as the plant. Their analysis is lower and vaguer, they act slowly — no good for rescuing a starving pot plant this week — and per unit of nitrogen they cost more.
The honest summary: for open ground, organic matter plus occasional organic fertiliser builds a soil that needs less feeding every year, and that long game is the better one. For containers, where there is no soil food web to speak of, synthetic liquid feeds are effective and proportionate, and no amount of purism changes the chemistry a pot plant experiences. Many good gardeners use both without embarrassment. What is not optional: compost, manure and mulch do more for most gardens than any bottle, because they fix structure and water-holding as well as nutrition.
When feeding is pointless — or actively harmful
This is the section the fertiliser aisle will not tell you:
- Established borders, shrubs and trees in decent soil generally need no fertiliser at all. An annual organic mulch supplies all they require. If an established shrub looks poorly, the cause is far more often water, light, pests, disease or compaction than hunger — feeding a stressed plant does not fix stress.
- Late-season nitrogen is a genuine harm. High-nitrogen feeds after about August push soft new growth that will not ripen before frost, inviting winter damage and disease on roses, shrubs and lawns alike. From late summer, feed with high-potash products or not at all.
- Drought-stressed plants: fertiliser salts in dry soil scorch roots. Water first, feed later, if ever.
- Wildflower areas and most native plants flower best on poor soil; feeding hands victory to grasses and nettles.
- Mediterranean plants — lavender, rosemary, Cistus — grow leggy, soft and short-lived when fed.
- Newly planted trees and shrubs: research consistently shows fertiliser at planting does little or nothing for establishment — water and mulch are what matter. Skip the handful in the hole.
- Feeding to fix yellow leaves without a diagnosis: yellowing has many causes — waterlogging, cold, pH-induced lock-up, magnesium deficiency, simple old age of lower leaves — and only some respond to feed.
Containers: the real feeding job
Pots are where feeding earns its keep. Multi-purpose compost contains enough fertiliser for roughly four to six weeks; after that, the plant is living on what you supply. A simple schedule covers almost everything:
- At planting (March–May): either mix controlled-release granules into the compost, or plan to liquid-feed later — not both at full rate.
- From four to six weeks after planting: start liquid feeding. Balanced feed for foliage plants; high-potash tomato feed for anything flowering or fruiting.
- June to early September: feed weekly for heavy performers (tomatoes, baskets, dahlias), fortnightly for the rest. Water first if compost is dry, then feed — feed onto dry roots scorches.
- From mid-September: taper off and stop. Permanent container shrubs get a final high-potash feed, nothing more until spring.
- Permanent pots (shrubs, trees, ericaceous plants): each spring, scrape off the top 3–5 cm of compost and replace with fresh, plus a slow-release or organic feed. Repot fully every two or three years.
Read the label — and resist the extra scoop
Fertiliser labels state a dose and interval because they mean it. Doubling the scoop does not double growth; it raises the salt concentration around the roots until water is drawn out of them — the browned leaf edges and sudden collapse of an overfed pot plant. If plants look pale and you are sure feeding is the answer, feed at the stated rate more frequently, or use a half-strength feed weekly instead of full strength fortnightly — never a stronger brew. Check the N:P:K on the back rather than the promises on the front: many premium-priced "specialist" feeds are near-identical formulations. Store fertilisers dry and sealed, keep granules off foliage (they scorch where they lodge), and wash hands after use.
The larger truth is that feeding is the smallest part of plant nutrition. Soil enriched each year with compost or manure, mulched and left undug, supplies most plants indefinitely. Buy fewer bottles; make more compost.
Sources and further reading
- Royal Horticultural Society — plant nutrition, fertiliser guidance and container growing advice.
- Garden Organic — organic feeding, green manures and building soil fertility without synthetic inputs.
- gov.uk — regulations on fertiliser labelling and guidance on preventing nutrient run-off into watercourses.