Autumn fertiliser: why no nitrogen in autumn

In autumn a plant needs potassium and phosphorus, not nitrogen. Potassium helps shoots and buds ripen before winter, phosphorus supports the roots and next year’s flower buds, while nitrogen in a mild autumn can trigger new growth that has no time left to turn woody. In Estonia autumn fertiliser goes on from mid-August to the end of September, and the first half of October is the last stretch in which it still does any good.

Why no nitrogen in autumn

Nitrogen is the fuel for shoot and leaf growth. Applied in late summer or during a warm autumn, it wakes buds and produces soft new growth. Those shoots cannot ripen into wood and are the first to be damaged in winter.

To be fair, trials on trees and shrubs show that nitrogen applied after growth has stopped does little harm to winter hardiness on its own. But it does no good in autumn either, and it does carry risks:

• A mild autumn. While it stays warm, nitrogen can start a second flush of growth, and those late shoots go into winter unprepared
• Lawns. Nitrogen-fed grass turns lush and is more prone to snow mould under snow. The RHS explicitly advises against high doses of nitrogen on lawns in late summer and autumn and recommends an autumn lawn feed instead
• Leaching. In Estonia the growing season ends, on average, in late October. Nitrogen the plant no longer takes up is washed away by autumn rain and snowmelt

That is why autumn blends contain no nitrogen at all, or 3–5% at most. Plantena MAX 3-8-20 has 3% in ammonium form, which is too little to start new growth.

Potassium and phosphorus: what they do in autumn

Potassium is the key nutrient in an autumn feed. It regulates the water balance of cells and helps shoots, wood and buds ripen. A plant short of potassium overwinters noticeably worse, and in trials potassium feeding helped more often than not. More than the recommended rate is not better, though: if the soil already has enough potassium, extra will not add hardiness, but it will interfere with magnesium uptake.

Phosphorus feeds the roots and next year’s flower buds. It barely moves in soil: scattered on the surface, it stays in the top centimetre. That is why autumn fertiliser is worked in with light cultivation, and in a planting hole it is mixed into the soil.

How to read the formula on the pack and convert it into grams is explained in “NPK on a fertiliser bag: what the three numbers mean”.

Timing in Estonia: October is the last month

The best window runs from mid-August to the end of September, when shoots have stopped growing but roots are still fully active. In the first half of October feeding still makes sense if the autumn is mild and the soil has not cooled. The reference point is the end of the growing season: on average across Estonia the daily mean temperature drops below +5 °C for good around 28 October. After that roots work more and more slowly, the fertiliser sits in the soil until spring, and some of it is leached out.

A simple rule of thumb: if daytime temperatures are still around +10 °C, the trees are still in leaf and the ground is not frozen in the morning, it is not too late. Once the leaves have fallen and the ground is freezing, close the bag, store it somewhere dry and leave feeding until spring.

How much to apply: rates by plant

The rates below are for Baltic Agro autumn garden fertiliser, which contains no nitrogen. Where the rate for Plantena MAX 3-8-20 differs, it is given separately. For any other product, follow the pack.

• Lawn: 4 kg per 150 m², or about 27 g/m²; Plantena MAX 50–70 g/m². Spread in two passes, lengthways and crossways, because uneven spreading leaves the lawn patchy
• Fruit trees: 80 g/m² of the root area, spread over the whole area under the canopy rather than in a heap by the trunk. If a large tree stands in the lawn, make holes 20–30 cm deep with a steel bar and fill them with fertiliser
• Currants and gooseberries: 80–125 g per bush, or 40–50 g/m²
• Raspberries and strawberries: about 40 g/m² (4 kg per 100 m²)
• Roses: 50–60 g/m². Nitrogen for roses stops in July; phosphorus and potassium go on until the end of September, or early October in a mild autumn
• Conifers and rock garden plants: 40 g/m²
• Rhododendrons: 50–60 g/m²; Plantena MAX 30–50 g/m²
• Ornamental shrubs and hedges: 50–70 g/m²; Plantena MAX 40 g/m²
• Perennials: 30–50 g/m²
• Bulbs at planting: as for perennials, mixed into the soil below the bulbs

As a guide, 1 kg covers about 12 m² under fruit trees or 37 m² of lawn, and 4 kg covers 150 m² of lawn.

How to apply

• On moist soil. The granules have to dissolve or the nutrients never reach the roots. The easiest time is just before rain; if none is coming, water it in
• Work it in. Under shrubs and trees, rake the granules into the top 3–5 cm; on a lawn simply water
• Water conifers before the frost. Winter browning of needles is most often caused by dry autumn soil, winter sun and wind while the ground is frozen. In a dry autumn keep watering conifers until the ground freezes

What not to do

• Do not trust the word “autumn” without the formula: some “autumn” fertilisers on sale are 15-15-15, a general-purpose blend high in nitrogen. In autumn the first number should be 0–5
• Do not use up leftover spring fertiliser, urea or nitrate, and do not dig in fresh manure
• Do not spread fertiliser on frozen ground or snow, as it will run off with the meltwater
• Do not exceed the rate “just to be safe”: excess potassium hinders magnesium uptake
• Do not give potassium chloride to currants, gooseberries, raspberries, strawberries, blueberries or conifers: they are sensitive to chloride and need potassium in sulphate form
• Do not scatter wood ash under blueberries, rhododendrons, heather or conifers: it raises soil pH
• Do not combine feeding with hard pruning: pruning also weakens winter preparation, so prune trees in late winter or early spring

Frequently asked questions

It is already October. Is it too late for autumn fertiliser?

Not in the first half of October, as long as days are around +10 °C and the ground is not freezing in the morning. From late October there is little point: the growing season is over and the fertiliser sits in the soil until spring or is leached out.

Does the garden have to be fed in autumn?

No. Healthy trees and shrubs on fertile soil do fine without an autumn feed. It is worth it on poor and sandy soils, for lawns, for berry bushes after harvest and for plants that grew poorly over the summer.

How is autumn garden fertiliser different from Plantena MAX 3-8-20?

Baltic Agro autumn garden fertiliser has no nitrogen at all, only phosphorus and potassium. Plantena MAX contains 3% ammonium nitrogen plus sulphur, calcium, magnesium and boron; the nitrogen is too little to start growth. The application rates are similar.

Can I feed the whole garden with one fertiliser?

Yes, only the rate changes, from 30 g/m² for perennials to 80 g/m² for fruit trees. The exception is blueberries: they need acid soil and a feeding plan of their own, and ash and lime must not go near them.

From our catalogue

All autumn blends are in the autumn fertilisers section; potassium sulphate and superphosphate on their own are under dry fertilisers.