Fall leaves are a great resource for the garden but there are some problems in the way they are used. Should they be left on the lawn? Should you compost infected leaves like maples with black spots. Will too many leaves kill your plants? Do leaves rob soil of nitrogen? Let’s have a look at these and other myths.
Should Leaves be Left on the Lawn?
The simple answer is no, but there are some exceptions.
Cool season lawn grass grows best in spring and fall when temperatures are lower. They need sunlight to grow and leaves can reduce that to such a low level that grass dies. Having leaves on a lawn is no different than covering it with cardboard to kill the grass.
It is best to have no leaves but there are a few exceptions. The odd leaf will not kill the grass because light can get around the sides. Small leaves like the ones from birch trees are not nearly as big a problem as large maple leaves. Small leaves from perennials are also OK. If you can’t see most of the grass, it’s a problem.
Should You Compost Infected Leaves?
Some maples get black spots and it is recommended that you don’t compost these. Leaves with powdery mildew are also a concern. The fear is that if you use such infected leaves in your garden, even after composting, the fungal spores will survive and infect plants in spring.
It is true that surviving fungal spores will infect plants the following year but not all fungi can survive a cold winter.
Although the logic seems sound it really does not matter much in most cases. By fall, virtually every leaf in the garden is infected with some type of fungi, even if you can’t see anything on the leaf. If you don’t use infected leaves you would not use any plant material. Not only are the leaves infected, but fungal spores cover every inch of soil, mulch and leaf debris. They are everywhere.
Fungal spores move long distances by air currents and all the fungi in your neighborhood will end up in your garden. No matter how careful you are, you will have powdery mildew and black spots next year, so you might as well get some benefit from the leaves. Besides, this is what nature does – nobody removes infected leaves from the forest floor or meadow.
Leaves are Good for Flower Beds
Cover your flower beds with whole leaves or even better, shredded leaves. They keep the soil warmer and protect plants. They also keep moisture in the soil which is important, unless you have very wet winters. Perennials, bulbs and tree roots all benefit from a blanket of leaves.
If you have a thick layer of leaves you can remove some in early spring before new growth starts. If the layer is less then 3″ (7cm) just leave them as mulch in spring. They will slowly decompose by mid-summer.
Can Leaves Smother and Kill Plants?
Yes. Provided your winters are not too wet, the leaves will not harm the plants in winter. However, if the layer is too thick, they can prevent short plants from growing properly the following spring and may even kill them. Most plants have no problem poking their way through a thick layer of leaves. Except for very tiny rock garden plants, a layer that is less then 3″ (7cm) will not harm plants.
The exception to this rule are gardens that have a wet winter, or plants that are vey sensitive to water around their crowns in winter. Our winter can be wet, so I keep leaves off on the rock garden because these plants are both small and tend to be sensitive to moisture. The rest of the garden gets leaves.
Do Leaves Rob Soil of Nitrogen?
This is a common belief but it is not true, provided the leaves are sitting on the soil.
Since leaves have a high C:N ratio, they have little nitrogen relative to carbon, and if they are buried in soil they do compete with plants for nitrogen. They should only be used as a mulch. Nature will decompose them and take them underground when appropriate.
Mulching Leaves with a Lawn Mower
Is this a good idea? Raking leaves is better for the environment than mowing them. Gas mowers cause pollution while raking keeps you healthier. You might think battery operated mowers are OK, but remember that the electricity you use needs to be produced somewhere and that has environmental consequences.
Is shredding better than raking? Some of the claims for chopped up leaves are not valid. For example:
- Improves the health of your soil – both shredded and unshredded leaves do that. Shredding does speed up the decomposition process by a few weeks.
- Beneficial for insects, critters and microorganisms – shredding probably does more harm to insects and critters than not shredding.
- Saves money on garbage bags – this has nothing to do with mulching.
- Create mulch for other areas of the garden – leaves work just as well.
- Saves time and effort, since mowing is quicker – this one is true.
Shredding does allow you to leave the leaves on a lawn, provided you don’t have too much material.
The best option is to rake the leaves off the lawn and use them somewhere else in the garden. If you want to reduce the amount of raking, mow the ones that you can leave on the grass, and rake thicker layers for use in the garden or compost bin.
Lawn mowers work better for mulching if you use a mulching blade and cover the exit hole.
You can Mow 6″ of Leaves
There are quite a few sites that repeat the claim that “you can mulch 6″ of leaves at one time and leave them on the lawn”. This is a myth. A mulching mower does not work on 6″ (15 cm) of leaves.
The research said that “if done properly six inches or more of fallen leaves can be chopped by the mower and returned to the soil without causing damage”. However, this needs to be done with a series of frequent mows over a period of time which reduces “up to six inches” of leaves into fine mulch that filters back into the turf. It does not mean you can wait until you have a pile that is 6″ thick and them mow it.
Less Lawn Weeds
Mulched leaves can be left on the lawn provided you do not leave too many. When you are done, you should clearly see the grass.
Michigan University studied the effects of leaving mulched leaves on the lawn and found that after 3 years they needed to add less fertilizer and there were fewer weeds. The chopped leaves cover bare spots in the lawn, keeping light from reaching the soil which reduces weed seed germination. It can dramatically reduce dandelions and crabgrass.
Fall leaves have a fairly high C:N ratio of 30-80 (Compost Science for Gardeners) and there are a couple of different ways to compost them.
- Produce leaf mold. Pile them up and let nature take care of things. This method is easy, produces a nice compost, but it is a slow method.
- Use a traditional compost bin. Provided the leaves are mixed with enough greens, this makes compost fairly quickly. A smaller compost tumbler can also be used but it is a bit slower because it never heats up properly.
- Vermicompost. The worms like fall leaves but you can only compost a small amount this way.
Is composting a better option than just mulching? All of these methods eventually produce the same nutrients and provide about the same amount of carbon, so they all benefit the garden equally well. Faster methods start helping soil sooner – that is really the only difference.
Do You Need Fall Fertilizer?
I found this false claim, “The nitrogen boost from mulched leaves is such that you don’t even have to fertilize in the fall”.
Fall leaves have a low level of nitrogen so they are not going to add any to your lawn. In a non-tropical climate fall leaves sitting on the surface of the soil won’t even decompose for a year or more.
You may be able to reduce fertilizer in future years, but not in the short term. In fact adding nitrogen fertilizer will help the leaves decompose faster.
Other Myths About Fall Leaves
- Micro-organisms that live in the soil beak down organic material such as leaves. That is true, but very little of this happens during winter in a colder climate.
- Worms get in on the action, too. Also true, but not in winter.
- Mulched leaves provide roots with nutrients. Only after the leaves decompose which can take years. Just because you can’t see shredded leaves on the soil, does not mean they have decomposed.
- Mulched leaves will biodegrade and disappear from the lawn by spring. Only in tropical climates.
Leaves left in a zone 5 garden are still there in spring and show no signs of decomposition.