Getting the lawn ready for autumn: potassium, the last cut and leaves

There is a morning in October when the lawn stops drying out. The dew sits there until midday, the grass has all but stopped growing, and under the cherry tree there is leaf litter that was not there yesterday. From that morning on, what you do decides how your lawn looks in March.

Winter itself does very little harm to grass. Lawns are ruined by things that happened in autumn, or that failed to happen: too much nitrogen in September, a badly judged final cutting height, leaves left lying for months.

In order: first the overseeding in September, while the soil is still warm, then the potassium feed from mid September · CW 38, then the leaves, and finally the last cut somewhere between CW 44 and CW 46. We are at the end of July right now · CW 31. That leaves you a good four weeks until overseeding and seven until the potassium feed. Plenty of time to get seed and fertiliser without rushing.

Why potassium, and why little nitrogen now

Nitrogen is the fertiliser for the leaf. It makes grass dense and dark green, and that is exactly what you want from April to August. One last slow-release dose in early September is in fact the most valuable feed of the year, because it fuels tillering and roots and brings the spring start forward. What no longer belongs on the lawn from mid September is fast-acting nitrogen: the freshly pushed tissue stays thin-walled and full of water, and it never hardens off. When frost comes, the water freezes between the cells first and pulls moisture out of them until they collapse. The damaged spots are open doors for fungi.

Potassium works differently. It accumulates in the cell sap, and the higher the concentration of dissolved salts inside the cell, the lower its freezing point. It also regulates the stomata, the pores through which the plant loses water on frosty days. That effect has a ceiling, though. If your soil is low in potassium, the autumn feed measurably improves frost hardiness. If it is already well supplied, more potassium buys you nothing, and very heavy doses can encourage snow mould rather than prevent it. The rate on the bag is the upper limit, not the recommendation.

You can spot an autumn lawn fertiliser by the third figure of the NPK ratio being the largest, typically 5 to 5 to 20 or 7 to 3 to 12. The fact that there is nitrogen in there at all is deliberate. What matters is the ratio in favour of potassium, not nitrogen at zero. What you do not reach for is your spring fertiliser with 20 per cent nitrogen, not even as “just half a dose”.

Autumn feeding step by step

1. Pick your window

   Mid September to mid October, roughly CW 38 to CW 42. Uptake carries on while the lawn is still growing and the soil stays above about 5 degrees. By November that is no longer reliable. If you only manage one date, take the last weekend in September.

2. Mow first and clear the clippings

   Short grass lets the granules fall right down to the soil instead of hanging in the layer of blades. Take the clippings off with the collector, otherwise the granules sit on a mat rather than on earth.

3. Spread onto dry blades

   The blades have to be dry, the soil may be moist. Use a spreader and go over the area twice, crossways. Without a spreader, halve the quantity and walk it once lengthways, once across.

4. Water it in

   If no rain is forecast for the next two days, give the area one proper soaking. Granules sitting dry on the sward draw moisture and leave scorch marks.

For how the autumn feed fits into the rest of the year, see the feeding calendar for lawns: three to four feeds a year, and this is the last one.

The last cut: 4 to 5 cm, not a centimetre shorter

There are two opposite mistakes, and both are common. One: shearing the lawn short in November, “so it goes into winter looking tidy”. The other: giving up on mowing altogether and leaving a ten centimetre field standing.

Cut too short, the grass lacks the leaf area it needs for photosynthesis on mild winter days. It then lives off the reserves it would want in March for new growth. After a shaved winter a lawn comes into spring thin and brownish.

Left too long, the blades tip over under the first wet snow and lie flat on top of each other. What forms underneath is a warm, damp cushion with no air movement, and that is exactly the setting Microdochium nivale likes, the pathogen behind snow mould. The fungus does not need snow at all, it infects at anything from zero to twelve degrees given lasting damp.

The target height is 4 to 5 cm. The date is not set by the calendar but by the moment growth stops, so when soil temperature stays below roughly 8 degrees for a few days. In mild river valleys that is often not until mid November · CW 46, in colder foothills already at the end of October · CW 44.

A blunt blade tears the grass open instead of cutting it, and the frayed ends no longer dry off in cool weather. More on that in how to mow a lawn properly.

Get the leaves off before the sward rots

Grass uses light in winter too. Under a closed blanket of leaves it gets none, and because the leaves hold moisture, the sward stops drying off after every foggy morning. Two to three weeks under a wet leaf layer are enough to turn the grass yellow, and after four to six weeks you are left with bare patches.

How fast that happens depends on the tree. Leathery leaves from plane, oak or horse chestnut stick together into an airtight sheet and rot slowly. Big maple leaves cover just as tightly but break down faster. Fine birch leaves trickle down between the blades and cause little trouble.

The rule: go over the lawn once every seven to ten days rather than doing the whole lot in December. A plastic fan rake is kinder than a metal one. It is quicker with the mower: set it one notch higher and drive over the dry leaves with the collector on.

Out on the open lawn that is harmless. Piles that have gathered along edges, under hedges and around shrubs are a different matter. Lift those with a rake first and check whether a hedgehog, a toad or a slow worm has moved in. By October they often have. Leaf blowers with a shredder function are best left in the shed altogether.

Overseeding and aeration belong in early autumn

Late summer is the best sowing window of the year, better than spring. The soil is warm, dew and autumn rain keep it reliably moist, and the summer weeds that grab every gap in spring are no longer germinating. The window is narrow, though: lawn seed germinates from roughly 8 to 10 degrees soil temperature, quickly only above 15, and then needs six to eight weeks to root.

So overseed from the start of September to the end of it at the latest, CW 36 to CW 39, and in mild spots early October at a push. Anything you sow in November either fails to come up or germinates during one mild week and freezes afterwards. The instructions for bare patches are in overseeding a lawn.

Two things decide whether it works. The area has to stay evenly moist for the first two to three weeks, which on dry days means a short watering every day. And seedlings need phosphate to root, which a pure autumn fertiliser or sulphate of potash does not supply. Use a starter feed on the overseeded area and add the potassium only two to three weeks after the seedlings are up.

Aeration, meaning the removal of small soil cores, follows the same logic. It pays off on compacted, loamy areas that hold puddles after rain, and it belongs in September so the holes can still close. Brush in a thin layer of sharp sand afterwards.

Prevent moss rather than fighting it later

Moss is not an attacker, it is a successor. It moves into the spots where the grass has given up, and in the winter half of the year grass gives up a lot: in shade, in waterlogged ground, on acid soil and after too short a cut. If you have a green carpet of moss in March, you missed something in October.

The preventive levers are therefore the same ones as above: potassium instead of nitrogen, leaves off, cutting height 4 to 5 cm. And if puddles stand for days, September aeration is the answer, not moss killer in April.

If the problem comes back every year, a soil test is worth it. Where the critical line sits depends on the soil: on sand around pH 5.5, on loamy sand 6.0, on sandy loam 6.5. You lime when the test says so, not because there is moss. Moss copes perfectly well with neutral soil, and liming on suspicion is the most common wasted effort on a lawn. Lime belongs in early spring, not in the same week as the potassium feed. How to push back white clover and dandelion without chemicals is in clover and dandelion in the lawn.

The timetable in weeks

These are guide values for central Germany. On the coast and in warm river valleys everything shifts a week or two later, at altitude a week or two earlier.

• CW 36 to 39 · September. Overseeding, aeration, still mowing at summer height.
• CW 38 to 42 · mid September to mid October. Spread the autumn fertiliser and water it in, collector on instead of mulching from now on.
• CW 41 to 48 · October to the end of November. Rake the leaves off every seven to ten days.
• CW 44 to 46 · end of October to mid November. Last cut to 4 to 5 cm, as soon as growth has stopped.
• CW 49 to 8 · December to February. Do not walk on it, do not feed, do not mow. Lime and the first raking through wait for early spring, once the soil has dried out.

If you have an area where crocuses or daisies have naturalised, the same rules apply for the last cut. It is only in spring that the crocus corners stay unmown, until the foliage has yellowed and pulled back. Daisies need no such consideration, they take any cut you give them. Everything else is in the checklist for putting the garden to bed.

The lawn is the part of the garden where you are most likely to forget what you did and when. That is why it pays to note the four autumn dates in your garden journal: overseeding, potassium feed, leaf clearing, last cut. After two years you will see whether your timing suits your particular spot.

Common questions about lawn care in autumn

When is the last cut of the year?

Not on a fixed date, but when growth stops, so as soon as soil temperature stays below roughly 8 degrees for several days. In central Germany that is usually the end of October to mid November, CW 44 to CW 46. Cut to 4 to 5 cm, on a dry, frost-free day, with the collector on.

Which fertiliser should I use on the lawn in autumn?

One with a high potassium share and little nitrogen. On the bag you spot it by the NPK ratio having the largest third figure, for example 5 to 5 to 20. Cheap alternative: sulphate of potash at roughly 20 to 30 g/m². Spread it between mid September and mid October, CW 38 to CW 42. A small nitrogen share in an autumn feed is normal and intended, what counts is the ratio. What does not suit the job is spring fertiliser with a high nitrogen share, it pushes soft tissue that the first frost damages.

Do I really have to clear the leaves off the lawn?

Yes, as soon as they form a closed blanket. Grass photosynthesises in winter too and needs light. Under wet leaves it gets none, and the sward stops drying off after fog and dew. After two to three weeks under maple or chestnut leaves the grass is yellow, after four to six weeks bare patches appear. Individual leaves are no problem.

Can I still overseed the lawn in October?

Early October in mild spots yes, from mid October hardly at all. Lawn seed only germinates from roughly 8 to 10 degrees soil temperature and then needs six to eight weeks to root. Whatever comes up in November freezes at the first hard frost. The reliable window is CW 36 to CW 39. If you miss it, wait until mid April.

Why does my lawn have grey, matted patches in spring?

There are usually three different things behind that. Most often the sward has simply suffocated under leaves that were left lying: matted, without light, but with no fungus involved. If the patch is roundish and pale to grey-brown with a whitish-pink edge, it is snow mould, which works at anything from zero to twelve degrees given lasting damp, entirely without snow. A grey-white felt with small brown grains after weeks of snow cover is Typhula blight instead, the grey snow mould. Both are triggered by too much nitrogen in autumn, grass left too long, or leaves left lying. There is little to treat in any of the three cases: rake through, let air in, wait. From roughly 15 degrees the lawn grows over it. Overseed from mid April, once the soil has reached 8 to 10 degrees.

Is it alright to walk on the lawn in winter?

On unfrozen ground yes, on frozen grass no. Frozen blades are brittle and snap under your shoe instead of bending. The breaks die back and stay as brown footprints in the green until April. Hoar frost without snow cover is the worst case. Wait until the lawn has thawed.