Autumn fertiliser for the garden: what to feed in September and October
Why nitrogen is wrong in autumn, what phosphorus and potassium actually do, how late feeding still works and how many grams per square metre.
Autumn feeding does not add to this season’s harvest. It decides something else: whether the plant survives the winter and how many flower buds it sets for spring. In Estonia, where winter arrives with thaws and returning frosts, that matters more than summer feeding.
Why nitrogen is wrong in autumn
Nitrogen drives shoot growth. New growth that starts in September has no time to ripen and goes into winter soft — it is the first to freeze, and the branch below it freezes with it. A summer all-purpose fertiliser with a high first number in the formula is therefore wrong for autumn: it makes the plant vulnerable exactly when you need the opposite.
Autumn blends contain either no nitrogen or very little — just enough for the plant to use it up before the cold. Check the NPK figures on the pack: the first number should be small, the third the largest. For example Plantena MAX 3-8-20, or formulations such as 0-3.5-25.
What phosphorus and potassium do
Phosphorus works underground and in the buds. It speeds up fruit ripening, helps set next year’s flower buds and develops the root system — and it is the root that decides how the plant overwinters.
Potassium works inside the cell. It strengthens cell walls and raises the concentration of cell sap, which lowers the freezing point — that is the “antifreeze” role. A plant fed with potassium handles frost, drought and fungal disease better.
How late feeding still works
The best window runs from the second half of August to the end of September. October still makes sense in a warm autumn, but the closer to November the less it does: the plant goes dormant and simply stops taking nutrients from the soil. Fertiliser applied in late October will almost certainly sit unused until spring or wash away with meltwater.
A practical rule: if daytime temperatures are still around +10 °C and the leaves are holding on the tree, you are not late. If the leaves are down and the soil sets overnight, you are — leave it for spring.
How much to apply
Guide rates for granular autumn blends. Always check the pack for the exact rate — concentrations differ.
- Fruit trees — about 80 g per m² of the area under the canopy
- Currants and gooseberries — 80–125 g per bush
- Strawberries — about 40 g per plant
- Raspberries and ornamental shrubs — about 40 g per m²
- Lawn — 50–70 g per m²
Spread granules on moist soil and work them in with shallow cultivation; on a lawn, spread evenly and water afterwards. Applying to dry ground is pointless — the feed never reaches the roots.
Wood ash — yes, but not everywhere
Ash supplies potassium, calcium and trace elements, and as an autumn addition it is good. But ash raises pH sharply, so keep it away from plants that need acid soil: blueberries, rhododendrons, heathers, hydrangeas, conifers. Under apples, plums, currants, gooseberries and on vegetable beds it is fine — a cup to a cup and a half per bush.
What not to do in autumn
- Do not feed fresh manure — that is the same nitrogen, only slower
- Do not combine autumn feeding with hard pruning: both wake the plant up
- Do not spread more than the rate “just in case” — excess potassium blocks magnesium uptake
What to get from the catalogue
Full sections: fertilisers and soil improvers.
