Choosing an autumn plant feed in the UK is mostly a question of what to stop using. The bottle that did a good job in June is the wrong one in October, and using it now can leave a plant in worse shape by December than if you had fed it nothing at all. The goal has changed. You are no longer growing anything. You are getting roots established and tissue hardened before the cold arrives.
We make liquid feeds and a seaweed biostimulant in East Kilbride, so this is territory we spend our year in. Here's what still does something useful after the summer peak, and what to leave on the shelf.
Why does autumn plant feed need a different NPK ratio?
The three numbers on a fertiliser bottle are nitrogen, phosphorus and potassium, and they do quite different jobs.
Nitrogen drives leaf and stem growth. That is exactly what you want in May and exactly what you don't want now. Soft, sappy growth pushed out in October has no time to harden off, and the first proper frost turns it to mush. On a shrub that can mean dieback. On a lawn it invites fungal disease into lush growth that never dries out.
Phosphorus supports root development, which is the one process still genuinely running. Roots keep growing after top growth stops, right up until soil temperature drops below roughly 6°C.
Potassium is the autumn nutrient. It regulates water movement in the plant, thickens cell walls and improves cold hardiness. A plant with good potassium status going into winter simply survives frost better.
So the shape you want is low nitrogen, moderate phosphorus, higher potassium. In practice that means an autumn lawn feed rather than a spring one, and for borders a tomato-type high-potash feed used at half strength is closer to right than a general-purpose growth feed.
When is it too late to feed?
Soil temperature decides it, not the date.
Nutrients are taken up in solution by actively growing roots. Once soil at 100mm sits below about 6°C for three consecutive mornings, uptake has effectively stopped and anything you apply washes through into the water table. That's wasteful and it's bad practice.
In central Scotland that point usually arrives in late October. In southern England it can be late November. Measure it rather than guess: a digital probe thermometer pushed 100mm into the ground first thing in the morning, three spots averaged, three mornings running. Our September checklist for Scottish gardens sets out how the whole autumn timetable shifts north of the border.
What should you feed in autumn, and how much?
Lawns. An autumn or winter formulation, applied once between mid-September and late October. These are deliberately low in nitrogen and higher in potassium and often include iron, which hardens the sward and knocks back moss. Do not use a spring and summer lawn feed now.
Borders and shrubs. Most established plants need nothing. If a plant has had a hard year, a high-potash liquid at half the summer rate, watered in once, is plenty.
Containers. Pots leach nutrients faster and hold less reserve, so a single low-nitrogen autumn feed is more justified here than anywhere else.
Newly planted trees and hedging. Water rather than feed. Establishment is limited by moisture, not nutrients.
Where does seaweed fit in?
This is the part we have to be careful about, because it's our own product and the category is full of overclaiming.
A seaweed extract is not a fertiliser. Alga-Sol, our cold-pressed liquid seaweed extract, is deliberately low in NPK. It's a biostimulant, which means it works on the plant's own processes rather than supplying the bulk nutrients. It goes alongside your feeding programme, not instead of it, and anyone who tells you a seaweed extract replaces NPK is selling you something.
What makes it relevant in autumn is what's in it. Cold-press Cell Harvest Technology, which is high-pressure cell-burst extraction with no heat and no alkaline hydrolysis, preserves compounds that harsher processing destroys. The verified analysis runs alginic acid at 3.18%, which supports soil structure and moisture retention, mannitol at 2.01%, which is one of the compounds Ascophyllum nodosum produces to survive freezing and desiccation on the shore, and laminarins at 1.17%, which prime the plant's own defence responses. The cytokinins and auxins that drive root development survive cold pressing and largely do not survive alkaline hydrolysis at pH 9 to 11.
Rates are simple: 1:200 for a lawn every four weeks while there's still growth, 1:200 as a soil drench or foliar for borders, and 1:200 as a root dip when you're planting. pH is 5 to 6, so there's no scorch risk on foliage. The seaweed is hand-harvested on the Argyll Coast and processed in Glasgow within 24 hours of leaving the sea.
What it will not do: rescue a dead plant, replace water, or substitute for potassium. There's more on root systems and cold hardiness in seaweed feed in autumn, and the wider Useful Info guides are in our product guides.
What shouldn't you do?
Don't apply a high-nitrogen feed. It's the single most common autumn mistake, and it causes frost damage rather than preventing it.
Don't feed dry soil. Nutrients move in solution. After the summer we've had, a lot of ground is still dry below the surface, and feeding into it does nothing while risking root scorch. Water first.
Don't double up. Applying a granular autumn lawn feed and a liquid feed in the same fortnight doesn't help. Excess simply leaches.
Don't feed at all if the plant doesn't need it. Established trees, most shrubs and a healthy lawn on decent soil will do fine on nothing. Feeding is a response to a deficiency or a stress, not a seasonal ritual.
Lawn work has its own timing rules, which we've set out separately in scarifying, aerating and the 6°C rule.
Feeding roots rather than leaves
Alga-Sol is a cold-pressed Ascophyllum nodosum biostimulant made in Glasgow, used at 1:200 alongside your normal feeding programme to support root development and stress tolerance going into winter. See Alga-Sol liquid seaweed extract, with free UK delivery.
Frequently asked questions
What is the best NPK ratio for autumn feeding in the UK?
Low nitrogen, moderate phosphorus, higher potassium. Nitrogen pushes soft growth that frost kills, while potassium improves cold hardiness and phosphorus supports the root growth still happening.
When should I stop feeding plants in autumn?
When soil temperature at 100mm sits below about 6°C for three consecutive mornings. Below that, roots are not taking nutrients up and anything applied simply leaches away.
Can I use tomato feed as an autumn plant feed?
At half strength, on a plant that needs it, a high-potash tomato-type feed is closer to the right shape than a general growth feed. It is not a substitute for a proper autumn lawn formulation on turf.
Is seaweed extract a fertiliser?
No. It is a biostimulant, low in NPK by design, used alongside a feeding programme rather than instead of one. It supports the plant's own processes rather than supplying bulk nutrients.
Should I feed if the soil is still dry after the drought?
Water first. Nutrients are only taken up in solution, so feeding dry ground achieves nothing and can scorch roots that are already stressed.
