Is it better to leave grass clippings on the lawn? Yes, sometimes.

Go to Home PageIs it better to leave grass clippings on the lawn?

Every spring the grass starts to grow and for the following summer months we have to cut and keep cutting as it relentlessly tries to get ahead of us and become overgrown. Cutting grass is a chore. The usual routine involves gathering the grass clippings and dumping them in a heap then deciding what to do with this ever-growing pile.

There is a case for asking the question: is it better to leave grass clippings on the lawn? Or, to get the best from it, should we make compost from it while we’re making compost from everything else?

It’s better to leave grass clippings on the lawn if your only other option is to send them to the dump. Grass clippings will feed the lawn when they rot down, releasing valuable nutrients that will generate fresh grass growth. Leaving the grass clippings spread out will also retain moisture during dry times.

What else?

Can you leave grass clippings on lawn? Do grass clippings cause grass to grow?

Is bagging or mulching better for the grass? When should you bag your grass?

Is it illegal to throw away grass clippings? Can bagged grass clippings catch fire?

What is the best height to cut your lawn? How short should you cut the grass?

How often do you have to mow the lawn? How do I get rid of thatch on my lawn?

Can you put grass clippings in compost?


Is it bad to leave grass clippings on the lawn?

If you leave your lawn uncut until late into the growing season rather than cut early, then this may cause problems. Grass that’s allowed to become long and dense will produce an increased bulk of material. Large volumes of cut grass will gather together from the back of the mower and form lumps which will appear all over the lawn.

At first sight these won’t show up because these lumps will be as green as the grass that’s under it. This will change as the cut grass will start to deteriorate. A heap of cut grass will break down as microbial life forms start to work on it. It will go through stages, the first stage will involve mild fermentation which will produce acidic juices.

Is it better to leave grass clippings on the lawn? If it’s a light cut, yes.

This isn’t anything to panic about but the these juices may be concentrated enough to kill the grass that’s immediately beneath the lump of cut grass. The surface of the lump of cut grass will appear pale having been bleached by sunlight but the inner part will be dark because this will be deteriorating.

After about a week the cut grass will have gone through the acidic stage and the it will go into the decomposition stage. This is when it will rot. With the acidity reduced or completely gone, various life forms will move in and start to digest, what is for them, quite a feast. These will be worms, beetles and an assortment of grubs.

The worms will be the biggest player because they will ingest the rotten grass then they will burrow down into the ground of the lawn. They will then deposit the digested material among the roots of the growing plants that are there. The plants will feed on the nutrients of what the worms have provided.

This is such a neat arrangement and we could argue that the best way of managing grass clippings is to cut the grass and just leave it where it falls.

The only downside to doing this is the potential of the bare patches that the acidity will create in the immediate period after cutting.

One way around this would be to go over the lawn straight after it’s been cut and attempt to spread out the grass clippings so that there is a thin and even layer without any lumps. This would significantly reduce the acidity problem and you wouldn’t see any bare patches at all.

The problem would be reduced even further if, after cutting, there was a period of very hot weather. This would dry the cut grass. It could easily turn into hay. If it’s dry enough to do this then there will be no acidic juices to come away from it simply because it will be too dry.

It will still be necessary to spread this out because when the weather changes and it rains, the hay will begin to rot and juices will leach from it. These are all problems which you will encounter if there is a large quantity of cut grass left on the lawn. If there is just a small amount of grass such that you can barely see it when cut, then it won’t pose any problem at all.

If you cut regularly when the grass has only grown an inch, this amount won’t produce much bulk of cut grass. It will disappear into the ground almost without trace. You won’t need to spread anything around because you won’t find anything to spread.

Can you leave grass clippings on lawn?

Grass clippings can be left on the lawn without adverse effect. If you cut the lawn as soon as you see any growth in the early part of the growing season then the volumes of grass involved will be quite small. It will not form large lumps. The clippings will settle on the ground between the grass plants. This will then shrivel and disintegrate away without trace and will, up to a point, feed the soil. It isn’t worth the effort involved to rake up small quantities of grass clippings. Nature will deal with it.

Do grass clippings cause grass to grow?

When grass is cut, the clippings that are generated are bruised and generally smashed to bits. This condition allows for decomposition to begin soon after cutting. Grass clippings rot faster than almost anything else. When any organic material rots it releases nutrients that are of use to other growing plants, including the grass plants that the clippings were cut from.

The only problem with this happening, apart from the acidity caused in the early stages of decomposition, is that any nitrates that are in the ground will be released into the atmosphere and will be lost. This is not a problem as far as any pollution is concerned; air is made up of ⅘ nitrogen. When any fresh organic material begins to decompose on soil the bacteria that breaks it down will release nitrogen that’s in that soil. It’s just a natural and unavoidable part of the process.

So, you need to be aware that if you have a system where you never collect grass clippings when you mow the lawn that this will deplete the lawn soil of nitrates. Just like most other plants the grass on the lawn needs available nitrates for it to grow. You may find it necessary to add artificial nitrate fertilizer to compensate for what is lost.

Is bagging or mulching better for the grass?

The best way to manage grass clippings is to collect by bagging all of it and make compost from it away from the lawn. This way the lawn won’t lose any nitrates. When the grass clippings have rotten down completely into a dark compost, then spread this on the lawn. Well made compost from any source will always be the best plant food that you can get and it will definitely make the grass grow without compromise.

This compared to the mulching approach where we leave the clippings to rot on the lawn surface is much more desirable for the lawn. Although it does take much more work to do, the results are worth the effort.

When should you bag your grass?

If you are going to bag the grass clippings from mowing your lawn, the best time to do it is at the time of cutting. The ideal way of collecting grass clippings is by having a collection box on the lawn mower. This way none of them go on the ground, decompose and release nitrates from the soil.

This is much better than having to rake it up from the ground after it’s been cut. Grass clippings can’t be practically stored and there is no real need to do so. Because grass clippings are green and moist, they will start to decompose soon after they have been cut unless they are so dry that there isn’t enough moisture to trigger the composting process. Dry grass clippings will be hay and this, by its nature, will be preserved.

Is it illegal to throw away grass clippings?

If by throwing away we mean finding somewhere in the middle of nowhere and just dumping it, then this is definitely unacceptable in civilized society. If you do this then expect a penalty. You need to find out from your local waste collection authorities about what they will take. Landfill space is best left for rubbish and general waste that can’t be reclaimed in any way.

Grass lawn clippings are different, they have a value and uses. When you see a large heap of grass clippings straight after mowing, it may well look like a vast quantity that will get in the way. The thing about grass clippings is that the heap will actually shrink. A heap will reduce in size very quickly within a few days.

The process starts by warming up in the middle of the heap as the bacteria start to work on it. If you leave it for a day then dig it over with a fork, you will undoubtedly see steam rise from the middle of the heap. This will look quite dramatic but it isn’t too much to worry about.

Digging it over will accelerate the rotting process and you will notice that the overall mass of material will reduce. This is because a significant amount of the volume at the beginning is water in the form of sap in the grass. The steaming off process will lose most of this and the volume will reduce.

If you really can’t make use of the grass clippings from your own lawn then it’s worth the effort to try and find somebody, a neighbour perhaps, who will take it from you. There must be someone nearby who would be glad of it, I know I would.

Can bagged grass clippings catch fire?

It’s highly unlikely that grass clippings will catch fire. This is a concern that some may have when they see a large heap of grass clippings which was probably cut yesterday and the heap is at the point where steam is appearing from the top. It’s true to say that the middle of the heap will be very hot, probably boiling.

You won’t be able to hold your hand in it for very long without getting scolded. You could probably boil an egg in a good sized heap of lawn clippings when it’s going through the ‘heating up’ stage. The concern that some have is spontaneous combustion. This is very unlikely to happen because the material involved is too green and wet. Spontaneous combustion will happen to grass that has been harvested as hay on farms. This is when the hay has been harvested before it’s fully dried in the field. Often because the weather is closing in and the farmer wants to get the harvest in before the rain arrives.

In the case of harvested hay, the conditions can be perfect for combustion problems. There is enough moisture for the bacteria to start the process, as with a heap of grass clippings, but the dry matter level is at a point where there can be a flash-over that will lead to a fire.

If you have a grass heap that’s steaming and you are at all concerned then the best thing you can do is to dig a big hole in the middle or at least take the top off the heap.When you do this, don’t be surprised if you see clouds of steam. It may look like the makings of a drama but the steam rising is heat being released that’s trying to build up in the heap. After doing this you should see the amount of steam reduce significantly and the whole heap will start to cool down.

What is the best height to cut your lawn?

When there is about 3 inches of new growth, this would be a good time to give the lawn a trim. It’s possible to cut it when it’s higher but if you cut it when it’s at a manageable height it puts less of a strain on your mowing equipment and you are less likely to suffer mower blockages.

How short should you cut the grass?

Aim to leave about 1 to 1½  inches of uncut grass. You may be tempted to cut the lawn as tight as you can so that you won’t need to go through the chore again for a good while but it’s not wise to cut the lawn to tight to the ground. If you have an extended dry period of weather after cutting then the grass may ‘burn’ off in the sun’s heat and you run the risk of losing it with bare patches appearing.

How short should you cut the grass?

Leaving sufficient height of grass will ensure that some moisture will be preserved at ground level, enough to protect the grass plant. You will also have a healthy carpet of lawn to walk on.

How often do you have to mow the lawn?

This will depend on the type of growing season. In a wet year you may have to be ready to mow every other weekend. This should ideally be done in dry periods of weather between showers. It’s always better if you can mow grass in dry weather when the grass is dry. You can mow wet grass but it isn’t as much fun It tends to stick to everything and it’s heavier to handle if you are collecting it and having to empty the mower’s grass container.

In a dry time it’s quite possible to be able to leave the lawn for a month without mowing. In this case there won’t be much grass growth and it may also be better for the lawn if you leave it uncut as this will conserve, what may turn out to be, valuable moisture.

How do I get rid of thatch on my lawn?

This is when there is a build up of organic material that, for one reason or another, doesn’t rot down and disappear. If it won’t go by natural means then you have to intervene. One way is to go at it with a heavy duty rake and scratch it out. Depending on the scale of the lawn thatch problem, this may be more work than is practical.

How do I get rid of thatch on my lawn?

For a heavy problem you will probably need a ‘de-thatcher’ attachment that fits on a garden rotavator. These will pull the mat of dead material up above the grass, leaving the soil surface among the grass plants clear and open. Once the thatch material is out where you can get at it you can then gather it up and use it to make compost.

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Do grass clippings contribute to thatch?

It’s most likely that lawn thatch will be a combination of moss that has built up and dried grass clippings that haven’t become damp enough to rot  away. This could be a good reason for collecting all the clippings as you mow.

Can you put grass clippings in compost?

Grass clippings are ideal for making compost. It’s probably one of the best things you can add to a compost bin. I say this because with grass clippings there is the absolute certainty that they will rot down into compost and do so relatively quickly. You can also be confident that the compost that is produced from grass clippings will be high quality as it will convert into a black crumbly, easy to use, form.

This is why it’s so important to make use of it when you have it. If you aren’t making compost from you grass clippings then you are definitely missing a trick. The process can be much enhanced and accelerated if you add hydrated lime or ground limestone. This will reduce the acidity of the rotting grass. The value of the lime will be carried over into the soil when you use the finished compost in the garden.

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