Mulching your lawn: when it really helps and when it does not
When you mulch, the clippings stay on the lawn instead of going into the grass box. Soil organisms break the leaf pieces down and return the nutrients to the soil. That saves work and fertiliser. Whether it works, however, depends on a few conditions: how often you mow, how fine the clippings are and whether the grass is dry. Here you can read when mulching helps your lawn and when you are better off using the grass box.
In short
- Mulching works if you mow often and never take off more than a third of the leaf length per cut.
- Only mulch dry grass. Wet clippings clump together and stay on the surface.
- Fine clippings do not cause thatch. Clumps and mats are what cause problems.
- The main season is April to July. From mid September, collect the clippings again.
- If you mulch a lot, rake out the lawn regularly and aerate once a year.
What happens when you mulch
Fresh clippings consist mostly of water. The fine leaf pieces trickle down between the blades to the soil. There, earthworms, fungi and bacteria break the material down, within a few days to weeks if the soil is warm and moist. This releases nitrogen, potassium and trace elements, which the lawn takes up again.
For this to work, four things have to fit together:
| Factor | Good for mulching | Bad for mulching |
|---|---|---|
| Mowing interval | short, little clippings per pass | long, a lot of coarse clippings at once |
| Weather | dry grass, warm soil | wet grass, cool soil |
| Cut | sharp blade, fine pieces | blunt blade, torn, long blades of grass |
| Soil | loose, active soil life | compacted, little air in the soil |
The one-third rule
Never take off more than a third of the leaf length per cut. If you mow at 20 millimetres, the next cut is due at 30 millimetres at the latest. This keeps the clippings short enough to disappear between the blades, and the grass is not weakened by the cut.
In the main growing season in May and June, this means mowing every two to three days, and even more often with a very short cut. That sounds like a lot of work, but it is done quickly, because there is only a little to cut each time.
What mulching gives you
- Nutrients: Clippings contain about 3 to 4 percent nitrogen in the dry matter. If you mulch consistently, you can reduce your fertiliser needs by roughly a quarter.
- Soil life: Earthworms and microorganisms are fed regularly. In the long run this improves the soil structure.
- Moisture: The fine residues shade the soil slightly and slow evaporation a little.
- Less work: No emptying the grass box, no disposing of clippings.
Does mulching cause thatch?
This is the most common concern, and in this form it is not correct. Thatch consists mainly of dead stolons, roots and old shoot bases. These plant parts contain a lot of lignin, are tough and break down only slowly. Fine clippings from young leaf pieces, on the other hand, contain hardly any lignin and are gone quickly.
Thatch builds up when more organic material grows than soil life can break down. Typical causes are compacted, poorly aerated soil, a lot of nitrogen and acidic soil. Mulching plays a minor role at most, namely when the clippings are left lying as clumps or mats. That happens with mowing intervals that are too long and with wet grass.
When mulching does harm
With wet grass
Wet clippings stick together, form clumps and stay on top of the sward. The lawn underneath turns yellow, and the damp mass is a good breeding ground for fungi. So when it is wet, mow with the grass box or wait until the grass has dried.
In late autumn
From mid September the soil cools down and soil life slows. The clippings then stay on the surface longer, and together with leaves and autumn wet they create good conditions for snow mould. What helps most against snow mould is a short, dry sward over winter. In autumn, the clippings therefore belong in the grass box. More on this in Lawn diseases.
With weeds and weed grasses
If annual meadow grass (Poa annua) or weeds are flowering, mulching spreads their seed over the lawn. In this phase, collecting is the better choice.
With grass that is too long
After a holiday or a long rainy spell there is too much material on the lawn. Cut it down first in two steps with the grass box, and only then start mulching again.
Mulching through the year
| Period | Recommendation |
|---|---|
| March | grass box. The soil is still cool, and the first cuts also pick up debris from winter. |
| April to July | main season for mulching: the soil is warm and the clippings disappear quickly. |
| August to mid September | with care: only on dry grass and with short intervals. |
| From mid September | grass box. The clippings would stay on the surface for too long. |
Before renovation work
Before aerating, top-dressing and overseeding, mow as short as possible and collect the clippings thoroughly. The spoons then work with less resistance, the sand trickles down to the soil and the seed gets soil contact and light.
Which mower is suitable?
| Mower | Clippings | What to keep in mind |
|---|---|---|
| Reel mower | very fine, clean cut edges | ideal with short intervals. When there is sand on the lawn, and with leaves and twigs in autumn, use a rotary mower so the reel is not damaged. |
| Rotary mower with mulching function | fine, if the blade is sharp | mulching blades or a mulching plug cut the clippings several times. Sharpen the blade regularly. |
| Robotic mower | very fine, because it cuts every day | always mulches. Rake out regularly and aerate once a year to keep the soil loose. |
A reel mower cuts the blades like scissors. The cut edges do not fray, and the short pieces trickle straight down between the blades. More on this in Reel mower: what it does better than a rotary mower.
Mulching correctly: the rules at a glance
- Never take off more than a third of the leaf length per cut.
- Only mulch dry grass.
- Mow with a sharp blade or a sharp reel.
- No visible carpet of clippings should be left behind. If you see clumps or stripes, the grass was too long or too wet.
- From mid September, collect the clippings.
Raking out, aerating, scarifying: what and when?
If you mulch regularly, follow-up care keeps the soil active. The three measures work on different levels:
| Measure | What it does | When |
|---|---|---|
| Raking out (spring tines) | loosens thatch and residues on the surface without cutting into the sward | several times a season, more often with a robotic mower |
| Aerating (spoons) | pulls out soil cores, brings air and water into the root zone and keeps soil life active | once a year, ideally in September |
| Scarifying (blades) | cuts into the sward and tears out compacted thatch | only with heavy thatch, as a renovation measure |
Why loose soil is the foundation is explained in Lawn aeration: when, how often and how.
Frequently asked questions about mulching
How often do I have to mow if I mulch?
Often enough that you never take off more than a third of the leaf length. In May and June that is usually every two to three days, less often in high summer and early autumn.
Do I need a special mulching mower?
No. A reel mower with short intervals produces very fine clippings. On rotary mowers, a mulching blade or a mulching plug helps.
Can I do without fertiliser altogether?
Usually not. Mulching covers part of the demand, but it does not replace targeted feeding, especially in spring and late summer.
What do I do if thatch builds up anyway?
Rake out first. If the thatch is already compacted, scarify. In the long run aeration helps most, because well-aerated soil breaks down organic material faster.
Can I mulch in autumn?
Yes, until mid September on dry grass. After that, collect the clippings so they do not stay on the lawn over winter.
Tools for active soil
- Lawnpecker: aerating with spoons so air and water reach deeper layers.
- REWUKIL: for raking out and against thatch.
- Falkenstein: reel mower with a clean scissor cut.
